Measurement of nitric oxide-related amino acids in serum and plasma: Effects of blood clotting and type of anticoagulant

Background: Analysis of circulatory amino acids is performed in diverse fields of research, but very often without justification or even specification of specimen type. We investigated the impact of coagulation and anticoagulants on amino acid concentrations, with emphasis on amino acids involved in nitric oxide metabolism. Plasma, using either heparin or EDTA as anticoagulant, and serum were collected from 23 apparently healthy subjects. Amino acids were measured with high precision using high-performance liquid chromatographic techniques. Compared to heparin-plasma, the concentrations of almost all amino acids were lower in EDTA-plasma and higher in serum. For EDTA-plasma the mean difference was highest for tryptophan (5.3%). The mean difference between serum and heparin-plasma was much higher for some amino acids, including taurine (42.3%), arginine (36.4%), glutamic acid (16.2%) and serine (5.6%). Differences in amino acid concentrations between EDTA- and heparin-plasma are small and clinically most likely irrelevant. Concentrations of amino acids in serum are higher than in heparin-plasma, which is probably caused by poorly controllable ex vivo release from blood cells during clotting. We advocate using plasma rather than serum and not using different anticoagulants interchangeably in a single study. In addition, we urge editors and reviewers to demand adequate description of specimen type in manuscripts. •Amino acid concentrations are higher in serum than in plasma.•Concentration differences are most pronounced for arginine and taurine.•Differences between heparin- and EDTA-plasma are relatively small.•We advocate the use of plasma rather than serum.•Different anticoagulants should not be used interchangeably.