Circadian rhythm of erythropoietin in human serum.

When a sensitive radioimmunoassay for erythropoietin in serum (S-Epo), using labelled recombinant hEpo and antiserum to urinary hEpo, was evaluated for clinical use, a large intra-individual variation was observed during a 24 h period. The possibility of a circadian rhythm with a variation during day time which has to be considered for clinical evaluation of the S-Epo level was investigated in 27 hospitalized adult patients. They had various illnesses, but all had a level of S-Epo at 08.00 or 12.00 hours within the reference range of healthy individuals and normal cortisol levels during the 24 h. A well-marked circadian rhythm of S-Epo was found with the lowest mean level at 08.00 hours, a 42% increase at 16.00 hours and a 60% increase to the highest at 20.00 hours. Twelve (44%) of the 27 patients, with S-Epo levels within the reference range (3.3-13.5 IU/l) at 08.00 or 12.00 hours, had levels above this range at 16.00 hours. For clinical evaluation of the S-Epo level, the time of day for collection of the blood specimen has to be taken into consideration. It is recommended that blood specimens are taken in the morning between 07.30 and 12.00 hours. The mechanisms behind the observed circadian rhythm of S-Epo are unknown.