Correction to: Nature Biomedical Engineering https://doi.org/10.1038/s41551-019-0487-z, published online 23 December 2019.

In the version of this Article originally published, in the sentence beginning “In a validation dataset of 11,388 study participants...”, the terms “fundus-image-only” and “metadata-only” were in the wrong order; instead, the sentence should have read “In a validation dataset of 11,388 study participants from the UK Biobank, the metadata-only, fundus-image-only and combined models predicted haemoglobin concentration (in g dl–1) with mean absolute error values of 0.73 (95% confidence interval: 0.72–0.74), 0.67 (0.66–0.68) and 0.63 (0.62–0.64), respectively, and with areas under the receiver operating characteristic curve (AUC) values of 0.74 (0.71–0.76), 0.87 (0.85–0.89) and 0.88 (0.86–0.89), respectively.” This has now been corrected.