Variation of serum myoglobin levels in normal individuals. With reference to the use of myoglobin measurements for the detection of women carrying genes for the X-linked muscular dystrophies.

The extent of variation in the serum myoglobin (Mb) levels of normal healthy females were investigated so that the value of Mb measurements for carrier detection could be properly assessed. Factors considered were sex, age, menstrual status, pregnancy, diurnal variation and daily activity. In each instance, Mb levels were compared with those for serum creatine kinase (CK). A significant difference (P less than 0.001) was found between the Mb levels of men and women but no relationship could be found between several measurements of body build and the Mb or CK levels of either sex. An apparent correlation of Mb concentrations with age was found in control groups of both sexes up to 85 years of age, but this was not significant below 60 years of age. No differences in Mb levels were found among premenarchal, menstruating, postmenopausal or hysterectomised women and neither smoking nor the use of oral contraceptives seemed to affect Mb levels. Serum CK levels were very much higher in premenarchal (P less than 0.001) and postmenarchal (P less than 0.05) teenagers than in adult controls. No difference was found between the mean Mb levels of non-pregnant and pregnant (6-40 weeks) women. The mean value for CK activity was significantly lower than normal in the pregnant women (P less than 0.001). A marked circadian rhythm of serum MB levels was found with samples taken at 5 p.m. being approximately 20% lower than those taken at 9 a.m. This unexpected decrease during the day persisted with 48 hours of bedrest suggesting that this rhythm is inherent and unrelated to muscular activity.