Acute Kidney Injury in the Outpatient Setting Associates with Risk of End-Stage Renal Disease and Death in Patients with CKD

Current acute kidney injury (AKI) diagnostic criteria are restricted to the inpatient setting. We proposed a new AKI diagnostic algorithm for the outpatient setting and evaluate whether outpatient AKI (AKIOPT) modifies the disease course among patients with chronic kidney disease (CKD) enrolled in the national predialysis registry. AKIOPT was detected when a 50% increase in serum creatinine level or 35% decline in eGFR was observed in the 180-day period prior to enrollment in the predialysis care program. Outcomes were progression to end-stage renal disease (ESRD) and all-cause mortality. Association analyses were performed using multiple Cox regression and coarsened exact matching (CEM) analysis. Among 6,046 patients, 31.5% (1,905 patients) had developed AKIOPT within the 180-day period before enrollment. The adjusted hazard ratios of the 1-year and overall risk of ESRD among patients with preceding AKIOPT compared with those without AKIOPT were 2.61 (95% CI: 2.15–3.18) and 1.97 (1.72–2.26), respectively. For 1-year and overall risk of all-cause mortality, patients with AKIOPT had respectively a 141% (95% CI: 89–209%) and 84% (56–117%) higher risk than those without AKIOPT. This statistical inference remained robust in CEM analysis. We also discovered a complete reversal in the eGFR slope before and after the AKIOPT from −10.61 ± 0.32 to 0.25 ± 0.30 mL/min/1.73 m2 per year; however, the loss of kidney function is not recovered. The new AKIOPT diagnostic algorithm provides prognostic insight in patients with CKD.


Results
The study cohort was composed of a total of 6,046 patients enrolled in the pre-ESRD program, contributing to a total of 13,467.68 person-years of follow-up. The median age at enrollment in the pre-ESRD program was 67. 4 years (IQR: 56.9-76.5 years). The median follow-up times for outcomes of ESRD requiring dialysis and all-cause mortality were 1.68 (IQR: 0.80-3.01) and 1.69 (IQR: 0.81-3.03) years, respectively. Overall, 68.5% (4,141 patients) of the study population did not meet the diagnostic threshold of AKI OPT , whereas the remaining 31.5% (1,905 patients) had developed AKI OPT . Among patients with CKD who had a history of AKI OPT , 80.7% (n = 1573) had stable AKI OPT and nearly 20% (n = 368) had deteriorating AKI OPT (Table 1). Both maximum and minimum serum creatinine level were measured in the inpatient setting for 5% of the study population.
Compared with patients without AKI OPT , those with AKI OPT episodes tended to be older, female, nonsmokers, less educated, and have lower BMI and etiologies related to systemic diseases (e.g., diabetes, hypertension, and CVD) ( Table 1). In addition to medication use for comorbidities that commonly accompany AKI OPT , exposure to nephrotoxic agents such as NSIADs and radiocontrast was more prevalent among CKD patients with AKI OPT . Patients with a history of AKI OPT were also more likely to receive hypouricemic and antigout therapy than were those without AKI OPT (Table 1). At the time of enrollment in the pre-ESRD program, patients with deteriorating CA-AKI had the lowest median eGFR (9.5 vs. 30.8 mL/min/1.73 m 2 in patients without AKI OPT and 22.9 mL/ min/1.73 m 2 in patients with stable AKI OPT ). The median difference and percent change between the maximum and minimum serum creatinine levels were 0.30 mg/dL (IQR: 0.16-0.60) and 17.2% (IQR: 9.8-27.0) and 1.63 mg/ dL (IQR: 0.98-3.00) and 77.1% (IQR: 54.9-122.0), respectively, for patients without and with AKI OPT . Kidney function markers, including serum creatinine, blood urea nitrogen, and urine protein to creatinine ratio, demonstrated a significant increasing trend across the AKI OPT subgroups (No AKI OPT , stable AKI OPT , and deteriorating AKI OPT ). For hemoglobin and serum albumin, corresponding decreasing trends were observed (Table 1).
CEM analysis revealed that the effects of AKI OPT on the progression to ESRD gradually attenuated in subsequent years (e.g., aHR [1.44, 95% CI: 1.10-1.77] for 1-year mortality to aHR [1.10, 95% CI: 0.99-1.41] for 5-year mortality) following pre-ESRD enrollment; however, its effects on all-cause mortality were stable, ranging from an aHR of 1.7 to 1.9 throughout the follow-up period (Fig. 3). Supplementary Table S2 indicates that the matched variables in CEM between patients with and without AKI OPT were well balanced. In the multiple logistic regression of risk markers associated with the risk of developing AKI OPT , we found female gender, advanced CKD stage, diabetes, CVD, and the utilization of NSAIDs, contrast, and diuretics were significantly associated with AKI OPT (Fig. 4).

Discussion
The history of acute change in kidney function prior to pre-ERSD enrollment is prognostically critical in risk assessment and management in patients with CKD. In the present study, patients with AKI OPT were associated with a higher risk of progression to ESRD and all-cause mortality than were those without a history of AKI OPT . The risk was particularly high among patients with the deteriorating type of AKI OPT . We also found that the loss of kidney function before and during the AKI OPT event could not be completely recovered even with meticulous multidisciplinary care. The study results not only provide insight into how AKI OPT modifies the course of CKD but also emphasize the unmet need for the development of a universal screening-based diagnostic workflow to detect AKI OPT .

Continued
The first systematic evaluation of CA-AKI conducted by Kaufman in 1991 established the basic concept of CA-AKI detection 14 . The main diagnostic scheme is to screen admitted patients for impaired kidney function and then track the history, or the baseline serum creatinine level (the lowest reference serum creatinine level) if available, within the 12 months prior to admission or prospective serum creatinine during the entire hospital course 14 . Before the RIFLE criteria for AKI was proposed in 2004, all research had defined CA-AKI passively in the inpatient setting using Kaufman's approach 6,[15][16][17][18] . For instance, Obialo et al. defined CA-AKI as serum creatinine levels of >2.0 mg/dL at admission without a history of kidney disease 17 . Similarly, Hsu et al. defined the incidence of CA-AKI in a large community-based population according to all available inpatient serum creatinine measurements and then selected reference and index serum creatinine level to estimate the incidence of AKI using diagnostic criteria proposed by Hou et al. 6,7 . After 2010, researchers began to use RIFLE-based criteria to define CA-AKI in the inpatient setting 8,19,20 , except for Talabani et al., who used a fixed cohort covering the outpatient setting 21 . Collectively, a clear trend was observed and indicated that CA-AKI is more prevalent than HA-AKI and has a much lower mortality rate 22 . However, it remains unclear why compared with patients with HA-AKI, patients with CA-AKI tend to be classified in the highest AKI severity group but have a much lower risk of mortality 22 . This observation questions the feasibility of applying RIFLE-based inpatient AKI criteria for patients from the community.
The diagnostic algorithm we proposed changes the paradigm for AKI diagnosis and extends the scope of clinical AKI into the outpatient setting. Based on longitudinal clinical data, we verified our proposed definition; a 50% fluctuation in serum creatinine in the 180-day period prior to the pre-ESRD enrollment significantly modified the course of CKD. More importantly, we discovered that complete recovery of kidney function after AKI OPT is unlikely in patients with CKD. The change in eGFR slope from −10.61 back to 0.25 mL/min/1.73 m 2 represents the acute and reversible nature of kidney injury as the accelerated rate in deterioration of kidney function can be www.nature.com/scientificreports www.nature.com/scientificreports/ recovered; however, loss of kidney function could not be completely regained as the post-AKI eGFR slope was not steep upward enough to recover to the baseline kidney function before the event of AKI OPT (Fig. 1). This finding is in concordance with prior evidence indicating that HA-AKI defined using RIFLE-based criteria increases the risk of de novo CKD and accelerates CKD progression in critically ill patients [23][24][25] and admitted patients 26,27 and that a dose-response relationship exists between AKI stage and CKD progression 28 . However, these inferences are limited to inpatient settings and underestimate the true effect of AKI, particularly CA-AKI, which has yet to be adequately defined in the literature.
In the present study, when we applied the KDIGO AKI criteria, only 2,758 patients exhibited consecutive serum creatinine measurements within a 7-day time period. Among them, 699 patients had AKI, and 16.3% of the AKI patients exhibited peak serum creatinine levels in inpatient settings (Data not shown). Therefore, it is not feasible to use RIFLE-based diagnostic criteria to detect AKI in the community. Indeed, the difference between AKI OPT and rapid progressive CKD (when annual eGFR declining rate >5 mL/min/1.73 m 2 ) may be marginal as the two phenotypes shared common risk factors such as nephrotoxic agents, dehydration, or obstructive uropathy 29,30 . It is therefore difficult to differentiate AKI OPT from so called rapid progression of CKD, particularly at the onset of these events. Furthermore, if triggering factors of acute kidney insults are not promptly identified and managed, the reversibility of the AKI and recovery of kidney function will then develop into an irreversible kidney injury leading to the phenotype of rapid progression of CKD with an annual eGFR declining rate persistently >5 mL/min/1.73 m 2 . However, we found a significantly slower progression (from approximately −10 mL/ www.nature.com/scientificreports www.nature.com/scientificreports/ min/1.73 m 2 per year to no progression) after AKI events, which contradicts traditional notions of persistent chronicity. This observation signifies that the phenotype of AKI OPT identified by our proposed criteria is clearly different from that of rapid progression of CKD. The mechanisms underlying the AKI-CKD continuum have been extensively explored in animal models 5 . Maladaptive repair, infiltration of inflammatory cells, stimulation of fibrocysts and myofibrocysts, and tubulointersitital fibrosis have been linked to the development of de novo CKD and CKD progression after AKI 31 . These injurious molecular pathways are triggered in intrarenal microenvironments rich in damage-associated molecular patterns that are sustained by mutually aggravating mechanisms such as hypoxia, reactive oxygen species, or inflammation 32,33 . These mechanistic insights provide conceptual coherence between laboratory and epidemiological findings that supports the causality of AKI-to-CKD progression.
The increased risk of progression to ESRD gradually decreased within 5 years following the pre-ESRD enrollment with the highest risk appearing in the first year (Fig. 3). This finding can be explained by the sudden drop of eGFR before the AKI OPT event and the slow increase of eGFR after the AKI OPT event (Fig. 1). The significant loss of kidney function before AKI OPT event may put patients in advanced CKD stage, which increases their risk of progression to ESRD in the first two years following the event of AKI OPT because regain of kidney function is unlikely in the first two years. Therefore, the first year following AKI OPT is a critical period for clinicians to halt the accelerated progression before patients suffered from persistent uremic symptoms. If patients' dialysis-free status can be maintained in the first two years following AKI OPT , the risk of progression to ESRD would be gradually faded due to better preserved kidney function during the event of AKI OPT or the more pronounced recovery in kidney function after the event of AKI OPT . Our findings regarding the fully adjusted cross-sectional associations between selected clinical factors such as history of CVD and exposures to NSAID or contrast prior to the event of AKI OPT provides useful information on risk markers for development of AKI OPT in real-world practice (Fig. 4). However, more research must be conducted to discover new risk factors or effective prevention for AKI OPT .
This study has several limitations. First, the Health and Welfare Data Science Center (HWDC) of Taiwan did not release the biochemical data through the Health Insurance Medical Information Cloud Inquiry System until 2017; therefore, patients' serum creatinine measurements outside of our hospital were unavailable. Information bias could not be completely excluded; however, a high retention rate among patients in our hospital, which is the largest tertiary medical center in central Taiwan, should have effectively minimized the risk of misclassification. Second, we could not completely exclude the possibility of residual confounding and over-adjustment for variables that could be in the causal pathway. For example, detailed information on environmental factors such as diet, exposure to nephrotoxicants, and physical activity was not available. Third, the study population that were drawn from a pre-ESRD program poses a limitation in terms of generalizability. However, our proposed diagnostic cutoffs for the percent change of serum creatinine and eGFR approximated the 75 th percentile of the overall distribution, which improves generalizability of the proposed AKI OPT algorithm to patients with normal kidney function as within-day variability of serum creatinine above 30% is rarely observed in general population (submitted data) (Supplementary Fig. S2).
In conclusion, we validated an AKI OPT algorithm in the CKD population by demonstrating that this classification could accurately predict the risks of progression to ESRD and all-cause mortality. Our study also revealed that the use of conventional RIFLE-based AKI criteria significantly underestimates the role of AKI in the general www.nature.com/scientificreports www.nature.com/scientificreports/ population. Despite the full recovery of eGFR declining slope after AKI event, the loss of kidney function is not likely recovered, which strengths the causal link between AKI and CKD progression.

Study population.
In 2002, Taiwan's National Health Insurance launched the Project of Integrated Care of CKD, initially targeting patients with an eGFR lower than 60 mL/min/1.73 m 2 ; since 2007, the program has used a multidisciplinary approach to focus on CKD stages 3b-5 34 . This pre-end-stage renal disease (ESRD) program utilizes a multidisciplinary approach (involving nephrologists, renal nursing specialists, pharmacists, healthcare educators and dieticians) to design individualized care plans for a wide range of CKD patients. China Medical University Hospital (CMUH) joined pre-ESRD program in 2003 and has been consecutively enrolling patients who were willing to participate this care program and had a diagnosis of CKD based either on the working diagnoses of nephrologists or in accordance with the criteria outlined in the National Kidney Foundation (NKF)/ KDOQI Guidelines 35 . Up-to-date, the CMUH pre-ESRD program includes more than 11 000 participants with an overall retention rate of 90%. Patients in CKD stage 3b, 4 and 5 were, respectively, followed up at 12, 8 and 4 weeks, or when necessary. Biochemical markers of renal injury including serum creatinine, eGFR, and the spot urine protein-creatinine ratio (PCR) were measured at intervals of no more than 12 weeks 36 . All patients enrolled in the program were followed up until the initiation of maintenance dialysis for ESRD, loss to follow-up, death, or December 31, 2015. ESRD status was verified through active contact and review of electronic medical records (EMRs). Complete mortality data were obtained from the National Death Registry. www.nature.com/scientificreports www.nature.com/scientificreports/ In this study, 6,046 patients aged 20-90 years who remained dialysis-naïve and had records for at least two eGFR measurements before pre-ESRD program enrollment were selected from among 10,277 program participants (the selection process is detailed in Supplementary Fig. S3). The index date was defined as the day of the first AKI event based on our proposed diagnostic criteria of the AKI OPT . All methods in this study were performed in accordance with the relevant guidelines/regulations. The study was approved by the Big Data Center of China Medical University Hospital and the Research Ethical Committee/Institutional Review Board of China Medical University Hospital (CMUH105-REC3-068) and the need to obtain informed consent for the present study was waived by the Research Ethical Committee of China Medical University Hospital.
Criteria for outpatient acute kidney injury. We tracked all serum creatinine measurements of the patients up to 180 days before pre-ESRD enrollment. Serum creatinine was measured using the Jaffe rate method (kinetic alkaline picrate) at CMUH Central Laboratory using a Beckman UniCel DxC 800 immunoassay system (Beckman Coulter Inc., Brea, CA, USA). Calculations of eGFR were performed using the Chronic Kidney Disease Epidemiology Collaboration creatinine equation 37 . An AKI OPT event was defined as a fluctuation of >50% in serum creatinine or >35% in eGFR in the 180-day period preceding pre-ERSD enrollment. The 180-day time  Table 3. Estimates of the main fixed effects obtained from the growth piecewise mixed-effects modeling. Linear model:  frame was chosen based on prior evidence and represents the potentially longer duration of kidney vulnerability to nephrotoxins in elder patients or patients with CKD, particularly nonsteroidal anti-inflammatory drugs (NSAID) 38,39 . Baseline serum creatinine was defined as the best (lowest) serum creatinine within 180 days prior to the pre-ESRD enrollment. Fluctuations in serum creatinine were calculated as the difference between the maximum and minimum values of serum creatinine divided by the minimum value of serum creatinine. Fluctuations in eGFR were calculated as the difference between the maximum and minimum values of eGFR divided by the maximum value of eGFR. We further classified the AKI episode as deteriorating or stable based on the difference between the last S serum creatinine value in the 180-day period and baseline serum creatinine at the time of pre-ESRD enrollment. If the difference was positive and greater than 0.3 mg/dL, then the AKI OPT was defined as deteriorating AKI, whereas if the difference was less than 0.3 mg/dL, it was categorized as stable AKI OPT . This cutoff was selected empirically based on KDIGO serum creatinine criteria for Stage 1 AKI 11 . An alternative definition using 0 mg/dL as the cut-off was also evaluated (Supplementary Table S3). Detailed information of other variables such as sociodemographic characteristics was provided in Supplementary text.

Statistical analyses. Continuous variables are expressed as medians and interquartile ranges (IQRs) and
were compared using the nonparametric Kruskal-Wallis test, whereas categorical variables are expressed as a frequency (percentage) and were compared using the chi-square test. Associations between AKI status (with and without AKI OPT , stable AKI OPT , and deteriorating AKI OPT ) and the 1-year and overall risks of ESRD requiring dialysis and all-cause mortality were estimated using a multivariable Cox regression analysis. Multivariable Cox regression models were initially adjusted for sociodemographic and lifestyle variables, including age, sex, education (<9, 9-12, 12-16 or >16 years), smoking status (never, former or current) and alcohol consumption (never, former or current), followed by adjustments for comorbidities including diabetes mellitus, hypertension, CVD, primary etiologies of CKD, baseline medications (details provided in Table 1) and the baseline serum creatinine defined by the best (lowest) serum creatinine identified within the diagnostic window of AKI OPT . For outcomes of progression to ESRD, we performed competing risk analysis in accordance with the methods outlined by Fine and Gray to minimize potential bias introduced by a competing risk of death 40 . We also applied coarsened exact matching (CEM) analysis with matching criteria of age, sex, baseline eGFR, diabetes, hypertension, and CVD to specifically adjust for imbalances in baseline kidney function between patients with and without AKI OPT 41 . To compare the eGFR progression change before and after the index episode AKI OPT , we further identified a total of 1,106 patients who had undergone at least three serum creatinine measurements within a 2-year interval before and after the index AKI OPT event. We applied the growth piecewise linear mixed model by incorporating random effects for correlated eGFR measurements on the same patient to understand the effect of AKI OPT events on CKD progression using the following equation 42 :  where δ ij = 1 for the period before the AKI OPT event and δ ij = 0 for the period after the AKI OPT event.
Lastly, we used a multivariable logistic regression model to investigate the risk markers, such as demographic and selected clinical factors, for developing AKI OPT . All statistical analyses were performed in SAS (version 9.4, SAS Institute Inc., Cary, NC, USA) and R (version 3.2.3, R Foundation for Statistical Computing, Vienna, Austria). The 2-sided statistical significance level was set at α = 0.05. Ethical approval. The study was approved by the Research Ethical Committee/Institutional Review Board of China Medical University Hospital (CMUH105-REC3-068).

Data availability
The data that support the findings of this study are available on request from the corresponding author, CCK. The data are not publicly available due to them containing information that could compromise research participant privacy.