Pretrained transformer framework on pediatric claims data for population specific tasks

The adoption of electronic health records (EHR) has become universal during the past decade, which has afforded in-depth data-based research. By learning from the large amount of healthcare data, various data-driven models have been built to predict future events for different medical tasks, such as auto diagnosis and heart-attack prediction. Although EHR is abundant, the population that satisfies specific criteria for learning population-specific tasks is scarce, making it challenging to train data-hungry deep learning models. This study presents the Claim Pre-Training (Claim-PT) framework, a generic pre-training model that first trains on the entire pediatric claims dataset, followed by a discriminative fine-tuning on each population-specific task. The semantic meaning of medical events can be captured in the pre-training stage, and the effective knowledge transfer is completed through the task-aware fine-tuning stage. The fine-tuning process requires minimal parameter modification without changing the model architecture, which mitigates the data scarcity issue and helps train the deep learning model adequately on small patient cohorts. We conducted experiments on a real-world pediatric dataset with more than one million patient records. Experimental results on two downstream tasks demonstrated the effectiveness of our method: our general task-agnostic pre-training framework outperformed tailored task-specific models, achieving more than 10% higher in model performance as compared to baselines. In addition, our framework showed a potential to transfer learned knowledge from one institution to another, which may pave the way for future healthcare model pre-training across institutions.

The rapid growth of electronic health records (EHR) promotes the use of data-driven modeling to improve care delivery and care management for individual patients. Specifically, novel applications are emerging that use state-of-the-art deep learning approaches such as recurrent neural networks 1 , convolutional neural networks 2 and transformers 3 , to predict future medical events. With sufficient patient samples, these deep learning models can progressively extract relevant features and show promising model performance for various predictive tasks. For example, doctorAI, which yields promising results for predicting subsequent medical visits' diagnosis and medication, was trained on a dataset with more than 200,000 patient records.
One prerequisite for training most deep learning models is the availability of substantial amounts of highquality data 4,5 . Although the large-scale EHR database contains millions of patient records, these records are often not entirely applicable for various reasons, such as a limited number of cases for rare conditions and diseases 6 , restricted access to the entire database due to privacy concerns 7 , difficulty in data cleaning and merging 8 (especially if collected from different institutions 9 ). These limitations hinder the data acquisition process and, therefore, restrict the opportunities to develop data-hungry deep learning models, which may slow down the computational advances in healthcare and impede the improvement in care delivery.
Various methods have been proposed to address the data insufficiency issue, including synthetic data generation and incorporating medical domain knowledge. For example, Lee et al. 10 proposed an autoencoder-based deep generative model to learn and synthesize realistic sequential EHR data; Buczak et al. 11 generate EHR records by utilizing domain-specific knowledge and actual data; Ma et al. 12 proposed the PRIME model to leverage medical knowledge graph for rare medical concepts. However, these existing methods share the following drawback: they do not utilize the large amounts of "non-qualified" EHR records that are excluded during the patient cohort selection process.
Another approach to tackle the data scarcity problem is transfer learning (also known as model pre-training), which aims to learn good representations in an unsupervised manner to boost model performance in the downstream tasks. Transfer learning has been proven effective in a wide range of Natural Language Processing (NLP) tasks [13][14][15][16] and Computer Vision (CV) tasks [17][18][19] . Recent studies also applied transfer learning techniques to the healthcare domain. For example, Li et al. 20 proposed BEHRT to pre-train the Masked Language Model (MLM) on more than one million patient records to capture the semantic meaning of medical codes, BioBERT 21 and Clinical-BERT 22 pre-trained BERT (Bidirectional Encoder Representations from Transformers) 23 on clinical text for clinical NLP tasks. Despite their promising results, most of these studies focus on pre-training models on free-text; therefore, model pre-training on structural claims data, especially pediatric claims data, are neglected.
Claims data, a special kind of EHR, which is mostly used for financial purposes, contains rich clinical information. Such information can reflect the disease progression of patients and offers valuable support for healthcare analysis. Various data-driven models built for different predictive tasks are based on claims data, and, therefore, it makes computational sense to adapt the pre-training framework on claims data. However, pre-training models on claims data have been minimally explored and face unique challenges. First, medical visits are unevenly distributed. The time span between two consecutive medical visits is likely to differ. Second, medical claims contain different types of variables, such as medical codes, claim type, service date, and expenditure. These variables contain clinical meaningful information and need to be modeled collectively. Finally, some rare medical codes, such as brain cancer (ICD-9 191.9), have low frequency even in a large healthcare database and, therefore, suffer from severe sparsity.
Our goal is to learn a universal representation of claims data that can transfer knowledge with little adaptation to a wide range of population-specific tasks. This study took a semi-supervised approach for predictive healthcare tasks using a combination of unsupervised pre-training and supervised fine-tuning. Specifically, we proposed Claim Pre-Training (Claim-PT), a two-stage framework that first uses Next Visit Prediction (NVP) and Categorial Prediction (CP) as objectives to pre-train the initial parameters of the transformer-based neural network. The NVP predictive objective helps capture the relationships between medical claims and the co-occurrence information of medical codes, while the CP predictive objective induces medical domain knowledge to mitigate the rare medical code sparsity issue. Next, the pre-trained model is adapted to a population-specific predictive task, such as asthma exacerbation prediction, using the corresponding selected patient cohort.
We pre-train our model on a large-scale pediatric claims dataset containing more than one million unique patients. We evaluate our framework on two population-specific predictive tasks (i.e., suicide risk prediction and asthma exacerbation prediction) and compare the model performance with discriminatively trained models. Experimental results show that our model can learn generalized patient representation through encoding the sequential medical claims and significantly outperforms baselines. We also evaluate our model's transfer learning ability across institutions. The empirical results confirm that our framework possesses great potential for transfer learning across different healthcare organizations, indicating that organizations with insufficient pediatric claims data can also benefit from our pre-trained model.
We publicly release the pre-trained model along with the population-specific preprocessing steps for claims data on GitHub: https:// github. com/ drxze ng/ Claim-PT. In summary, we make the following contributions: • We train and publicly release Claim-PT, a transformer-based framework trained on a large real-world pediatric claims database. To the best of our knowledge, Claim-PT is the first pre-trained and fine-tuned framework on pediatric claims data, that can deliver a significant performance boost for population-specific predictive tasks. • We demonstrate that our framework can utilize the general claims records for medical knowledge understanding. The pre-trained framework helps to improve downstream population-specific medical predictive tasks and outperforms tailored task-specific baselines. In addition, we show that the pre-trained framework has great potential at knowledge generalization across institutions, paving the way for future care coordination and delivery between healthcare organizations.

Related work
In this section, we first review the related research for transfer learning using claims data. Specifically, we focus on deep learning approaches. Next, we present several medical tasks using claims data and the corresponding predictive models.
Transfer learning using claims data. Transfer learning is an approach where deep learning models are first trained on a large (unlabeled) dataset to learn generalized parameter initialization and perform similar tasks on another dataset. Several state-of-the-art results in the NLP and CV domain are based on transfer learning solutions 24,25 .
Recently, researchers applied transfer learning techniques to the medical domain. Transfer learning enables deep learning models to capture comprehensive contextual semantics, which can benefit the downstream predictive tasks. For example, MED-BERT 26 pre-trained contextualized medical code embeddings on large-scale claims data and illustrate that the pre-trained embeddings can improve model performance on the downstream tasks. Med2vec, proposed by Choi et al. 27 is a skip-gram-based model that can capture the co-occurrence information between medical visits. Med2vec is able to learn semantic meaningful and interpretable medical code embedding, which can benefit the predictive tasks and provide clinical interpretation. BioBERT 21 , is a pre-trained biomedical language model trained on biomedical text instead of claims data, aims at adapting the language model for biomedical corpora.
These studies demonstrate the effectiveness of the pre-train and fine-tune framework with respect to boosting model performance on the downstream predictive tasks, especially when the data size is limited. However, none of the previous research focuses on pediatric claims data. We want to explore whether the pre-train and www.nature.com/scientificreports/ fine-tune paradigm on pediatric claims can benefit downstream predictive tasks, specifically with a populationspecific patient cohort.
Predictive models using claims data. There has been active research in modeling the longitudinal claims data for various predictive tasks. Generally, these studies can be divided into two groups: works that focus on predicting a specific future medical event, such as suicide risk prediction, asthma exacerbation prediction; and works that focus on a broader range of medical events, such as auto diagnosis and chronic disease progression modeling. Various deep learning models have been proposed to model claims data for a specific future medical event prediction. Su et al. 28 proposed a logistic regression model with carefully selected features to predict the suicide risk among children. Xiang et al. 29 predict the risk of asthma exacerbations and explore the potential risk factors involved in the progression of asthma via a time-sensitive attentive neural network. Zeng et al. 30 developed a multi-view framework to predict the future medical expenses for better care delivery and care management. Choi et al. 31 proposed RETAIN to estimate the future heart failure rate with explainable risk factors. For generalpurpose disease progressing models, Zeng et al. 3 proposed a hierarchical transformer-based deep learning model to forecast future medical events. Ma et al. 32 leverage medical domain knowledge to model the sequential medical codes for the next visit medical code prediction.
One of the main challenges in developing these models is the size of the dataset. The datasets used in previous studies usually contain over a hundred thousand patients, which is large enough to train most deep learning networks. However, for many population-specific predictive tasks or institutions without a large data corpus, training a complex deep learning model from scratch is not feasible and therefore requires transfer learning or alternative techniques.

Framework
In this section, we first describe the motivation of our study under the real-world scenario. Then we describe the formal definition of the problem of pre-training in healthcare using claims data. Finally, we illustrate our proposed methods in detail.
Motivation and training procedures. Before the formal description of our framework, we would like to explain its underlying motivation and practical scenario by a high-level illustration in Fig. 1. Figure 1 (top) shows the traditional pipeline of building deep learning models on a predictive task in the medical domain. As one can see, numerous claims records are removed during the cohort selection process following the inclusion www.nature.com/scientificreports/ and exclusion criteria. After the cohort selection process, only patients with specific diseases or satisfy certain criteria can remain as cases or controls, leaving a relatively small data cohort compared to the original database. The small data size makes it difficult to adequately train a deep learning model and leads to suboptimal model performance.
The observation above provides the main intuition and motivation behind our method. In other words, we aim to leverage the patient records that are excluded in the cohort selection process to boost the model performance on population-specific predictive tasks. We achieve this goal through a two-stage pipeline. The first stage is to learn a high-capacity patient representation learning model, i.e., capturing the clinical meaning of medical codes and the temporal information between medical visits. The first stage is complete through training the model on all available patient records. Next, a fine-tuning stage is followed, where we adapt the pre-trained model to a population-specific medical task. In the situation where the selected patient cohort is small, the pre-trained model can transfer valuable medical information from the excluded patient records.
Unsupervised pre-training. Given a patient in the claims data represented as a sequence of medical visits v 1 , v 2 , . . . , v i ordered by service date t. The i-th medical visit v i contains a set of medical codes {c 1 , c 2 , . . . , c j } ⊆ |C| , where |C| refers to the vocabulary size of the medical codes. There are three different types of medical visits, i.e., inpatient visit IP, outpatient visit OP, and pharmacy visit RX.
NVP objective function. We first use a modified language modeling objective, named Next Visit Prediction (NVP), to maximize the likelihood of the medical codes in the next visit, as shown below: where P is the conditional probability modeled by a deep neural network with parameters θ , and softmax function is adopted to predicts the medical codes of the next visit: CP objective function. We design another objective function, named Categorial Prediction (CP), aim to mitigate the sparsity issue of rare medical codes: where v i contains the categorical medical codes, and P is the conditional probability modeled by the same deep neural network as in the previous NVP task: The second objective guide the model to learn visit representation v i that are representative of the corresponding code categories. The CP task injects the ontology knowledge of medical codes into the embedding process and force the visit encoder to extract effective information that could boost the pre-training model performance. Sigmoid function is adopted to predicts the corresponding category of the each visit based on the medical code: The definition of the category is the medical grouper ID of the medical codes. For example, ICD-9 code 493.00 can be categorized as CCS 28 using the Clinical Classification Software (CCS). In our study, we use CCS as the category grouper for diagnosis codes and procedure code, and NDC Directory from the Food and Drug Administration (FDA) category grouper for medication codes. Correctly capturing the categories for each medical code within the visits helps to induce expert knowledge and is one of the basis of all downstream prediction tasks.
In our experiments, we adopt the state-of-the-art transformer layer as the building block. The medical codes (i.e., diagnoses, procedures and medications), visit types and service date are first map to latent space via embedding matrices. A max pooling layer is then applied to extract the most salient features within a visit. Next, the transformer block, including a multi-headed self-attention operation, position-wise feedforward layer, and skipconnect normalization layer, is applied to produce an output distribution over the target medical visit. In order to generate patient-level embedding, we also include the demographic information as the first token of every patient sequence using the one-hot encoding technique. The hidden state of this token is used as the aggregate patient representation for patient-level classification tasks, while the hidden state of the sequential visits hidden state is used for visit-level classification tasks. The equations are shown below : Population-specific fine-tuning. After training the framework with the objective functions as mentioned above, we adapt the parameters to the downstream population-specific predictive task. We assume a populationspecific claims dataset that contains both positive subjects (y = 1) and negative subjects (y = −1 ) . The input medical visit sequences and demographic vector are passed through the pre-trained model to obtain the final transformer block's activation pe,ê 1 ,ê 2 , . . . ,ê t . For patient-level classification task, pe is then fed into an added linear output layer with parameters W pe to predict y: Figure 2 illustrate the architecture of our framework. Overall, the only extra parameters we require to train during the fine-tuning stage are W pe , and thus we do not need a large patient cohort to train the complex deep learning models from scratch. As a result, our pre-training and fine-tuning framework are suitable for various population-specific predictive tasks.  www.nature.com/scientificreports/ MIMIC-3. A different EHR dataset, MIMIC-3, is used for another across institution experiment. MIMIC-3 is a publicly available clinical dataset that contains ICU patient records for over seven years of observation. This dataset is very different from the PFK pediatric claims data in that it consists of demographically and diagnostically different patients. For example, the median age is 65 in MIMIC-3, while 10 in PFK. The mortality rate among MIMIC-3 patients is about 10%, while close to zero in PFK. Despite the substantial clinical difference between MIMIC-3 and PFK datasets, they share a similar data structure and therefore suitable for our crossinstitution experiment. For all datasets, each medical visit can be represented as the combination of medical codes (i.e., ICD-9 diagnosis codes, CPT procedure codes, and NDC medications). Table 1 lists the detail statistic about the datasets. Experimental setup. We used 80% of the patients as the training set and 20% as the validation set for model pre-training, and 50% of the patients as the training set and 50% as the validation set for population specific tasks. ClaimPT is trained for up to 1000 epochs (i.e., 1000 iterations over the entire training data). The minibatch size was set to 100. To avoid overfitting, we applied the dropout layer, L-2 norm regulation, and early stopping techniques. The size of the all hidden layer was set to 100 to guarantee sufficient expressive power. The number of transformer layers was set to one, while the number of attention heads was set to four. We adjust the number of transformer layers and attention head for training time analysis, as shown in Fig. 3 (the default of layer and head are set to 4 and 2). We can observe that extra transformer layers and attention heads do not benefit the model performance in the pre-training stage. We used RMSprop as the optimizer with a learning rate equal to 0.001. All models were implemented with Python Tensorflow and trained on a server with eight Nvidia Tesla P100 GPUs.

Results
This section demonstrates the effectiveness of our Claim-PT framework on different downstream populationspecific tasks by comparing it with several baselines. First, we conduct two embedding visualization tasks as sanity checks to validate the model's ability to capture the semantic meaning of patients' medical codes and health conditions. Next, we evaluate the performance of the proposed framework on two real-world medical predictive tasks using claims data. Specifically, we conduct a suicide prediction task and an asthma exacerbation task. The two cases are different in patient-selection criteria and therefore yield different target populations. The criteria and model performance are presented in detail for each of the tasks.
Sanity check: embedding visualization. Deep learning models understand the data by projecting the inputs into a latent space, where relevant and salient features for solving the problem at hand are well represented  www.nature.com/scientificreports/ and extracted. In our case, the salient features refer to the semantic meaning of patients' medical codes and health conditions. Therefore, we first conduct two sanity checks to verify whether our framework can successfully capture the semantic information of medical codes and stratify patients according to their health conditions. We can observe reassuring patterns from the two embedding visualization plots in Fig. 4. For instance, patients with the same disease are grouped together, while patients that are diagnosed with different diseases are well-separated. This observation indicates that our framework is able to capture the clinical meaningful information within claims data and learn efficient embeddings.
Suicide prediction. Suicide among young people is one of the severe public concerns 33 , which is the second leading cause of death for children according to the American Academy of Child and Adolescent Psychiatry (AACAP). In 2017, about 1 in 13 adolescents have attempted suicide, and these suicide attempts cause a significant healthcare burden. Thus, it is critical in clinical practice to predict the risk of suicide accurately.
Recently, efforts have been made to apply the machine learning architecture to model the sequential claims data to predict suicide risk 13 . However, the excluding criteria limited the patient population (i.e., only a couple of hundreds of patients have committed suicide in one institution's database) and therefore restricted the model performance due to inadequately training.
This case study examines whether the excluded patient population can help pre-train the deep learning framework and ultimately provide a model performance boost on suicide risk prediction tasks. In particular, we first construct the patient cohort according to the patient selection criteria shown in Fig. 5. 163 patients who have suicide attempts during 2013-2014 are selected as positive subjects, while 326 patients who do not have suicide attempts in the same time span are randomly selected as negative subjects. Next, we develop models with the next-visit prediction window, i.e., the model aims to predict the occurrence of a future suicide attempt that happens in the next encounter. Therefore, the model is trained using the patient's medical visits prior to the first suicide attemp (for positive subjects) or the last recorded medical visit (for negative subjects). All Variables that have been used in the pre-training phase are selected for the suicide prediction task. Finally, we randomly divide the patient cohort into 70% training and 30% testing sets.
All models are trained for twenty epochs to ensure sufficient training. The same experiments are repeated five times, and the average model performances are reported for comparison. We leverage Area Under the receiver operator characteristic Curve (AUC), a widely used evaluation metric of discrimination performance that ranges from 0.5 (random prediction) to 1.0 (perfect prediction), to evaluate the predictive performance for all models. The data preprocessing details can be found on our GitHub page.
From Table 2, we can see that the AUC of the proposed pre-training and fine-tuning framework is higher than that of baselines on the suicide risk prediction task. The Claim-PT variants, Claim-PT Start and Claim-PT Pool, use the pre-trained framework for risk prediction without fine-tuning, and the predicted results achieve similar performance compared to baselines. This indicates that our proposed pre-training objectives is able to capture the semantic meaning of medical codes, and thus the framework can distinguish patients based on their health records. Since the suicide risk prediction task contains less than 500 patients, deep learning models, including Doctor AI, Dipole, and TransE, can not gain sufficient predictive power and thus yield suboptimal performance compared to linear regression. We can also observe that the model performance between deep learning models is similar. The reason is likely because the number of patient records and the categories of medical codes is limited, and developing more complicated models with more parameters does not help to sufficiently train the parameters.
Asthma exacerbation. We next turn our attention to the asthma exacerbation prediction task. Asthma is a common yet serious health problem among children, costing billions of dollars every year and imposes a significant burden on the healthcare system in the U.S. 38 . Asthma exacerbation is a severe asthma condition and requires medical interventions, resulting in an emergency department visit or hospitalization, which is resource- www.nature.com/scientificreports/ consuming and may even result in death. Therefore, it is practically essential to accurately identify asthma exacerbation in advance for better care delivery and care intervention. A data-driven study to predict asthma exacerbation using claims data is conducted, and various methods are compared with our Claim-PT framework to illustrate its effectiveness. We adopt the cohort selection process from a recent asthma exacerbation prediction study 29 , and the excluding criteria is shown in Fig. 6. As shown in the figure, 572 patients with in-patient asthma visits (i.e., patients admitted to hospital due to asthma diagnosis) from 2013 to 2014 are identified as the positive subjects, while 1097 randomly selected patients without asthma hospitalization are selected as negative subjects. A total of 1669 patients are identified and form the final cohort. The experiment setup is the same as the suicide risk prediction task, and the detailed preprocessing steps are on our GitHub page.
As shown in Table 3, Our proposed framework showed supervisor model performance compared to all baselines. The AUC of LR Sparse is the highest among all baselines, including the two Claim-PT variants. This observation is likely because the prediction of asthma exacerbation depends on the frequency of specific medical codes, such as the previous hospitalization. LR Sparse captures relationships between these medical codes and the prediction outcome, while LR Dense loses granularity information through feature preprocessing steps. Although RNN-based models, Doctor-AI and Dipole, are able to capture the sequential information, they have a complex model structure and thus yield suboptimal performance. The inferior performance of TransE can confirm this surmise. TransE utilizes the state-of-the-art transformer block as encoder and has more parameters, but the AUC score is the lowest among all approaches.
Ablation experiments. In this subsection, an ablation study is conducted to validate the effectiveness of the framework and explore critical clinical information with respect to the predictive results. Specifically, we first compared two zero-shot models with the fine-tuned model to illustrate the necessity and advantages of the finetuning stage. Next, we explain the performance of the proposed model by comparing it with the reduced models that only leverage partial input information. Figures 7 and 8 show the experimental results under AUC evaluation metrics for predicting suicide risk and asthma exacerbation, where a higher AUC score indicates better model performance. From Fig. 7, we can  www.nature.com/scientificreports/ observe that our framework showed superior model performance after fine-tuning compared to the two zero-shot approaches. This observation is expected as the framework focused on optimizing the two general objectives (i.e., NVP and CP) at the pre-training stage. Compared to the zero-shot start method, the zero-shot pool shown higher AUC scores in both tasks. The better model performance is likely because the pooling operation can extract the salient features within the input sequences. We can also confirm that the framework is able to capture Figure 6. The cohort selection process for the study of asthma exacerbation. www.nature.com/scientificreports/ clinical-meaningful information at the pre-training stage as the performances of two zero-shot approaches are better than random guesses. In Fig. 8, the model performance with different input variables is shown. We can observe that the pre-training model can gain sufficient predictive power on the two downstream predictive tasks, adding demographic and utilization information can barely improve the model performance. This observation is consistent with experimental results found in the previously study 30 . The marginal improvement is likely due to the fact that medical codes often contain information that is overlapped with medical utilization information and demographic information. From Fig. 8 (right), we can see that only using demographic information will result in poor model performance in both tasks, especially in predicting asthma exacerbation. This observation matches our intuition as the suicide risk is more likely to correlate with age and gender, while asthma exacerbation probability is less likely associated with age and gender. It is also interesting to observe that adding medical information can significantly boost the model performance while adding utilization information improves marginally. Such an improvement pattern is likely because the medical codes sometimes carry utilization information. Figure 9 helps us understand the model performance concerning the two proposed training objectives (i.e., NVP and CP). We can observe that model with both training objectives shown the highest AUC score. This indicates that both objectives are beneficial to learning efficient patient representation and helps predictive downstream tasks. In addition, we can see that using the NVP training objective only can yield good performance. This is likely because NVP enables our model to encode the clinical meaning of medical codes by capturing their co-occurrence information. The learned clinical meaning might overlap with the domain expert's knowledge (i.e., the medical grouper) and, therefore, only marginally improves the model performance when adding the CP objective.
Knowledge transfer across institutes. As we observed from the above experiments, the performance of the deep learning model on clinical events is highly dependent on the scale of the dataset. However, many institutions have not yet collected large-scale datasets or can not access sufficient amounts of data due to privacy issues. In these cases, training deep learning from scratch will result in suboptimal model performance and could easily lead to overfitting. One possible way to mitigate this challenge is to transfer knowledge from an institute with a large number of patient records. In this subsection, we evaluate the ability of our framework to transfer knowledge across institutes. Figure 10 demonstrates the experiments on PFK-2013 and MIMIC-3. In Fig. 10 (left), we can observe a vast improvement of the prediction performance induced by knowledge transfer from pre-training. This improvement  www.nature.com/scientificreports/ is because PFK-2013 shares a similar data distribution as PFK (i.e., they both contain pediatric patient records in the Ohio area), enabling the knowledge to transfer from a large dataset to a small dataset successfully. Due to privacy limitations, we can not access another pediatric claims dataset, but we believe the impact of pre-training on improving model performance is still valid. In Fig. 10 (right), we test our pre-trained framework on the MIMIC-3 dataset to explore the cross-domain adaptation result. As seen in the figure, the improvement is barely noticeable. The observation matches the clinical intuition as the MIMIC-3 and PFK have a significantly different patient population, where the former contains elderly and severely ill patients in the ICU department and the latter contains children that often visit the outpatient department. contains ICU patient records. The pre-training procedure results in less than 5% in the model performance for MIMIC-3, but more than 10% improvement for PFK-2013.

Limitation
This study has several limitations. First of all, the findings and conclusions were made based on the experimental results on a state Medicaid pediatric claims dataset. The results and observations may vary by state, insurer plan, population age distribution, or payer type. Second, we conducted the study under specific data preprocessing rules, such as continuous enrollment eligibility. These enforcements may have distorted the underlying population. Third, this study focuses on claims data only. Compared to EHR data, claims data do not contain important clinical information such as vital signs, medical notes, and visual images. Lastly, the black-box nature of the proposed deep learning framework hinders the users or physicians from interpreting the predictive results. Each of the limitations mentioned above points to a good direction. Our future work will try to address these limitations. We plan to incorporate with other healthcare institutes to extend our study scope with more comprehensive datasets. In addition, we will try to collaborate with physicians to deploy the model in the care organization to provide better guidance for care management. Finally, we are considering leveraging the bootstrapping method and sequential attention mechanism to provide confidence interval estimation and consistent interpretation of the model predictions.

Conclusion
This study proposed Claim Pre-Training (Claim-PT), a task-agnostic pre-training and fine-tuning framework that aims to achieve solid clinical concepts understanding and efficient patient representation learning. By pretraining the transformer-based model on large pediatric claims data with millions of patient records, our model acquires significant medical knowledge, understanding, and ability to process unevenly distributed longitudinal medical claims. The sufficiently pre-trained model can then be leveraged to solve downstream population-specific tasks, which often have a small patient cohort and therefore limit the performance of deep learning models. We conducted three experiments to validate the effectiveness of our Claim-PT framework, i.e., suicide risk prediction task, asthma exacerbation prediction, and auto diagnosis task. The experimental results suggest that our Claim-PT framework can significantly boost model performance on discriminative tasks, achieving more than 10% performance gain. We also demonstrate that Claim-PT has the potential to transfer knowledge from one institution to another institution. In summary, our study investigates the possibility of using unsupervised learning to learn general medical knowledge from claims data. The promising results imply that our pre-train and fine-tune framework can be beneficial in solving downstream population-specific tasks.

Data availability
The data source for this study is claims data from Partner For Kids (PFK). Partner for Kids (PFK) is one of the largest pediatric ACOs for Medicaid enrollees in central and southeastern Ohio. The dataset was obtained from a density sampled study that contains more than 600,000 enrollees' medical claims. In accordance with the Common Rule (45 CFR 46.102[f]) and the policies of Nationwide Children's Institutional Review Board, this study used a limited dataset and was not considered human subjects research and thus not subject to institutional review board approval. Restrictions apply to the availability of these data, and so are not publicly available. More information can be found: https:// partn ersfo rkids. org/. For help getting access to the dataset, please contact: PartnersForKids@NationwideChildrens.org.