Plasma or serum 5-hydroxyindoleacetic acid can be used interchangeably in patients with neuroendocrine tumours.

5-hydroxyindole acetic acid, a metabolite of serotonin, is used in the diagnosis and monitoring of patients with neuroendocrine tumours, in particular patients with small intestinal neuroendocrine tumours associated with the carcinoid syndrome. Analysis of 5-hydroxyindole acetic acid was commonly performed in urine, but blood-based assays are now becoming available. The objective of this study was to assess how 5-hydroxyindole acetic acid compares in plasma and serum as a biochemical marker of neuroendocrine tumours. Twenty-four-hour urine, plasma and serum samples were obtained from 80 patients with neuroendocrine tumours and 30 healthy volunteers. We developed a liquid chromatography tandem mass spectrometry assay for plasma and serum 5-hydroxyindole acetic acid. Comparison was made between them, and their cut-off was determined using a receiver-operating characteristic curve. A close correlation was shown between plasma and serum 5-hydroxyindole acetic acid. At a cut-off of 135 nmol/l, a sensitivity of 91.2% with a specificity of 61.9% was obtained for both compared to the urinary assay. A statistically significant agreement was shown when plasma and serum 5-hydroxyindole acetic acid were compared with the currently used urine assay in patients with neuroendocrine tumours; κ = 0.675 (95% CI 0.49 to 0.86), p  < 0.001 and healthy volunteers; 0.967 (95% CI 0.828 to 0.999), p = <0.001. In conclusion, 5-hydroxyindole acetic acid in plasma and serum were comparable, hence either sample type can be used interchangeably.