These results are in line with a dose‐dependent increase in AP50 as measured by erythrocyte haemolysis observed in experiments performed by Biesma et al. and Hiemstra et al. 3, 5In addition, we show that the widely used AP assay based on Seelen et al. 16perfectly identifies partial fD deficiency with clinical significance. This evidence concerns the gene ELP1 and hyperinsulinemic hypoglycemia, familial, 4.