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

Circadian rhythm of myoglobin serum concentrations in healthy subjects demonstrated by cosinor analysis.

In order to evidentiate whether a circadian rhythm of the myoglobin serum concentration is present in humans, we measured serially the myoglobin levels in blood samples of 21 healthy subjects (10 females and 11 males), at bed rest, by means of a sensitive and accurate radioimmunological method. The data obtained were then evaluated according to the single and mean cosinor analysis, to assess the presence of a significant circadian rhythm. Eighteen of the 21 subjects showed a significant circadian rhythm. The calculated equation for the mean cosinor was: Y = 19.12 + 4.09 cos (15t-128.24) and the F-value for the zero amplitude test was 19.9 (p less than 0.001). Our data demonstrate that circadian variations of myoglobin serum concentration are likely to occur in humans. These findings strongly emphasize the need to take the time of sample collection into account when employing serum myoglobin as a marker for muscle damage