Determination of free serotonin and its metabolite 5-HIAA in blood human samples with consideration to pre-analytical factors.

Significant differences have been reported over the years in measuring physiological levels of free circulating serotonin (f5-HT) in platelet-poor plasma (PPP). This work shows that there are crucial pre-analytical factors in sample manipulation that can provoke an artifactual release of 5-HT from platelets, and that, even when the sample is accurately processed to obtain PPP, f5-HT levels are approximately 2.8 times higher than those of f5-HT in blood. An alternative methodology consisting of ex vivo blood microdialysis coupled to high-performance liquid chromatography-electrochemical detection is proposed and validated. It is considered the most accurate technique to measure physiological circulating f5-HT and its metabolite 5-hydroxyindoleacetic acid (f5-HIAA), owing to its sensitivity (limits of quantification of 0.08 ng/mL) and reliability since there is no sample manipulation. The f5-HT and f5-HIAA levels in blood and in PPP were studied in control subjects, hypertensive and end-stage renal disease patients, who have a deregulated serotonergic system. This work reveals that blood is the best matrix to determine f5-HT concentrations, and the clinical relevance of the accuracy of f5-HT determination is discussed.