Circadian rhythm of serum erythropoietin in healthy subjects.

Background: The diurnal rhythm in the circulating serum levels of erythropoietin (EPO) was investigated in healthy subjects.

Methods: Venous blood samples were drawn during the span of a whole day every four hours, starting from midnight, for the determination of serum Epo levels by RIA in 40 clinically healthy normocytemic subjects, 27 males and 13 females, aged 30-75 years. Statistical analysis was carried out by means of the "cosinor" method.

Results: Serum EPO presents a significant (p < 0.05) circadian rhythm with higher levels in the afternoon and lower levels in the nightime.

Conclusions: These data confirm the presence of a circadian rhythm in serum EPO levels. The mechanisms behind the circadian rhythmicity are still unknown. For clinical evaluation of the serum EPO values, the time of day for collection of blood samples has to be taken into consideration.