Serotonin levels and 1-year mortality in patients with neuroendocrine tumors: a systematic review and meta-analysis

: Aim: Elevated serotonin in patients with neuroendocrine tumors (NETs) may impact heart failure incidence but a quantitative relationship has not been established. Materials & methods: Systematic review and meta-analysis of studies assessing 24-h urinary 5-hydroxyindoleacetic acid (u5-HIAA) and mortality in patients with NETs (2007-2017) with a primary outcome of 1-year mortality risk and 24-h u5-HIAA. Results: We identified 1715 records of which 12 studies including 755 patients (3442 person-years with 376 deaths) were eligible for meta-analysis. Mean u5-HIAA was 149.2 mg/24 h (standard deviation: 96.6) and mortality was 13.0%. The meta-regression equation showed an 11.8% (95% CI: 8.9-17.0%; I2  = 93.0%) increase in 1-year mortality for every ten-unit increase in u5-HIAA. Conclusion: Serotonin measured by its metabolite u5-HIAA is predictive of 1-year all-cause mortality in patients with NETs.