Overview Mitani and co-authors’ present a deep-learning algorithm trained with retinal images and participants’ clinical data from the UK Biobank to estimate blood-haemoglobin levels and predict the presence or absence of anaemia (Mitani et al. 2020). A major limitation of the study is the inadequate evaluation of the algorithm. I will show how a naïve classification (i.e. classify everybody as healthy) performs much better than their deep-learning approach, despite their model having AUC of around 80%.