Analytical and preanalytical validation of a new mass spectrometric serum 5-hydroxyindoleacetic acid assay as neuroendocrine tumor marker

Serum 5-hydroxyindoleacetic acid (5-HIAA) could replace the determination of 24-h urinary 5-HIAA for diagnosis and follow-up of neuroendocrine tumors (NETs). We developed and validated a straightforward liquid chromatography tandem mass spectrometry (LC-MS/MS) assay for serum 5-HIAA. We used serum samples from healthy volunteers (n=136) and patients suspected or followed for NET (n=129). Samples were spiked with 5-HIAA-D2, extracted and quantified by LC-MS/MS. We studied the effects of sample storage, sample device, a meal and diurnal variation on serum 5-HIAA. Furthermore, we established a reference range for serum 5-HIAA and compared our assay with a urinary 5-HIAA HPLC assay and a commercial plasma chromogranin A (CgA) immunoassay. Our LC-MS/MS assay is sensitive (LOQ 5nmol/L), has a wide assay range (5–10,000nmol/L) and short analysis time (7min). 5-HIAA in serum is stable for several days in various temperatures and during five freeze–thaw cycles. We found no diurnal variation (p≥0.20) and a meal had no effect on serum 5-HIAA (p=0.89). We suggest an upper reference limit of 123nmol/L for serum 5-HIAA. The area under curve (AUC) in receiver operator characteristics (ROC) analysis was 0.83 for urinary 5-HIAA, 0.81 for serum 5-HIAA and 0.76 for CgA, respectively. The LC-MS/MS assay for serum 5-HIAA discriminates between healthy individuals and patients with NET and is well suited for the diagnosis and follow-up of NETs. •Serum 5-HIAA LC-MS/MS assay is well suited for diagnosis and follow-up of NETs in a clinical laboratory.•The assay is sensitive (LOQ 5nmol/L) and has a wide assay range (5–10,000nmol/L).•The assay discriminates well between the healthy individuals and patients with serotonin-producing NET.•Serum 5-HIAA was stable for at least seven days at +4°C and at room temperature.•There was no diurnal variation in serum 5-HIAA.