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We describe an isocratic liquid-chromatographic method with amperometric detection for determination of serotonin by rapid sample preparation. Platelet-poor plasma and platelets were injected after a single deproteinization step with perchloric acid. Addition of sodium borohydride to whole blood avoids oxidation of serotonin during the deproteinization step without any chromatographic interferences. We purified urinary serotonin by two successive cationic and anionic extraction steps. After urine dilution, urinary 5-hydroxyindoleacetic acid (5-HIAA) was measured under the same chromatographic conditions. Platelet serotonin concentrations correlated more closely with whole-blood serotonin concentrations after correction for platelet number than with concentrations expressed in nmol/L. This suggests that whole-blood serotonin measurements should be corrected for platelet count to eliminate the variability of circulating platelets. Combined determination of serotonin in whole blood and urine and of 5-HIAA in urine provides a useful tool for detecting and monitoring carcinoid tumors.

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