Measurement of melatonin in blood by radioimmunoassay. Analytical considerations and clinical usefulness.

Melatonin assay has proved to be clinically useful for the diagnosis and investigation of several diseases (e.g., Cushing's syndrome, depressive disorders). We have evaluated the analytical performance of a radioimmunoassay for the determination of melatonin in plasma. The interference of some anticoagulants has been investigated, as well as the preliminary extraction of the hormone using a liquid-liquid and a solid-phase extraction method. The variation coefficients of the assay, within and between runs, were between 3.8% and 9.2% and between 4.1% and 10.5%, respectively. The well-documented circadian rhythm of melatonin secretion was confirmed in our healthy subjects. Preliminary results appear to confirm the significance of melatonin measurement in neurosurgical patients.