Background: The 24-hour urinary output of 5-hydroxyindoleacetic acid (5-HIAA) is used to monitor disease progression and treatment responses of neuroendocrine neoplasms (NENs). Several conditions are required for 5-HIAA assay, involving urine collection/preservation and food/drug restrictions.
Aim: To evaluate the correlation between 5-HIAA concentration in a spot urine sample and the output in a 24-hour urine collection, and whether spot urine specimens can replace 24-hour collection.
Methods: Patients with NENs or symptoms suggestive of NENs were asked to provide a separate spot urine at the end of the 24-hour urine collection for 5-HIAA assessment. The upper reference limit for 24-hour urinary 5-HIAA was 40 µmol/24h. 5-HIAA measurements in spot urine samples were corrected for variation in urine flow rate by expressing results as a ratio to creatinine concentration.
Results: We included 136 paired urinary samples for 5-HIAA assessment from 111 patients (100 NENs). The correlation between 5-HIAA values measured in 24-hour and spot urines was r=+0.863 (p<0.001), and r=+0.840 (p<0.001) including only NEN patients. Using the 24-hour urinary 5-HIAA as reference method, the AUC on ROC analysis for spot urinary 5-HIAA was 0.948 (95% CI, 0.914–0.983; p<0.001), attaining a sensitivity of 83% and specificity of 95% using 5.3 mol/mmol as cut-off for the spot urine. The AUC among NEN patients alone was 0.945 (95% CI, 0.904–0.987; p<0.001).
Conclusions: The ratio of 5-HIAA to creatinine in a spot urine could replace the measurement of 5-HIAA output in a 24-hour urine collection, especially for follow-up of patients with known elevated 5-HIAA levels.