Circadian rhythm of serum erythropoietin in multiple myeloma.

The diurnal rhythm in the circulating serum levels of erythropoietin (EPO) were determined in a group of 20 adult clinically-healthy subjects, in a group of 10 patients with myeloma without renal impairment and 10 patients with myeloma and renal failure. Venous blood samples were drawn during the span of a whole day and every 4 hr, starting from midnight, for the measurement of serum EPO levels by radioimmunoassay (RIA). Statistical analysis was carried out by means of the "cosinor" method. Results show that the controls and the myeloma patients without renal insufficiency present significant (P < 0.05) circadian rhythms in serum EPO levels; no rhythm (P < 0.05) was detected in patients with myeloma and renal failure. Patients with myeloma and renal failure have significant (P < 0.05) lower mean daily levels and diurnal fluctuations of EPO than the other groups, whereas the patients with myeloma without renal involvement present higher (P < 0.05) mean daily levels and lower (P < 0.05) diurnal variations of EPO than controls; no differences (P > 0.05) exist between the groups regarding peaks of rhythms. These data confirm the existence of a physiological circadian rhythm in serum EPO concentrations, with maximum in the afternoon, and they suggest that renal failure is an important cause of anemia and loss of EPO circadian rhythm in patients with myeloma.