Circadian variations of serum myoglobin levels in normal subjects and patients with polymyositis.

Circadian variations in myoglobin levels were determined in 18 normal volunteers and in 10 patients with active polymyositis. In all the controls and in 9 of the 10 patients, serum myoglobin concentrations were highest at 9 am, fell significantly during the day to reach a nadir between 6 and 12 pm, remained constant until midnight, and then rose again at 3 am. The percent differences between the highest and lowest values ranged from 21-66%. This pattern was observed in both untreated patients and patients receiving 6-methyl-prednisolone, which was given either in a single dose at 8 am or in divided doses (8 am and 8 pm). These data demonstrate that the time of sample collection must be taken into account when myoglobin levels are used as a marker of muscle damage.