To summarize our main findings: (1) no significant differences were observed between venous DBS and capillary collected DBS, (2) overall, LH plasma agreed well with the other ways of blood sampling, however when Phe concentrations are below approximately 50 μmol/L, i.e. low for PKU patients and usual for TT1 patients, differences with capillary DBS tended to appear (with Phe concentrations being lower in LH plasma), and (3) Phe and Tyr concentrations tended to be higher in EDTA plasma compared to other specimens. This evidence concerns the gene PLOD1 and phenylketonuria.