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Seasonal and Biological Variation of Blood Concentrations of Total Cholesterol, Dehydroepiandrosterone Sulfate, Hemoglobin A_1c, IgA, Prolactin, and Free Testosterone in Healthy Women

¹ Referencelaboratory, National Institute of Occupational Health, Lersø Parkallé 105, DK-2100 Copenhagen, Denmark.

² Department of Biostatistics, University of Copenhagen, Blegdamsvej 3, DK-2200 Copenhagen N, Denmark.
^a Author for correspondence. Fax 45-39-16-52-01; e-mail ahg{at}ami.dk

Background: Concentrations of physiological response variables fluctuate over time. The present study describes within-day and seasonal fluctuations for total cholesterol, dehydroepiandrosterone sulfate (DHEA-S), hemoglobin A_1c (HbA_1c), IgA, prolactin, and free testosterone in blood, and estimates within- (CV_i) and between-subject (CV_g) CVs for healthy women. In addition, the index of individuality, prediction intervals, and power calculations were derived.

Methods: A total of 21 healthy female subjects participated in the study. Using a random effects analysis of variance, we estimated CV_g and total within-subject variation (CV_ti), i.e., the combined within-subject and analytical variation, from logarithmically transformed data. Analytical variation was subtracted from CV_ti to give CV_i. CV_i was estimated from samples taken monthly during 1 year (CV_iy), weekly during 1 month (CV_im), and six times within 1 day (CV_id).

Results: A cyclic seasonal variation was demonstrated for total cholesterol, DHEA-S, HbA_1c, prolactin, and free testosterone. Within-day variation was shown for prolactin and free testosterone. The overall mean values for the group and the variability (CV_iy and CV_g) were: 5.1 mmol/L, 5.5%, and 5.0% for total cholesterol; 6.6 µmol/L, 7.1%, and 21% for DHEA-S; 4.3%, 2.6%, and 3.3% for HbA_1c/hemoglobin_total; 2.1 g/L, 5.9%, and 13% for IgA; 136 mIU/L, 23%, and 27% for prolactin; and 5.4 pmol/L, 21%, and 29% for free testosterone.

Conclusions: Collecting samples at specific hours of the day or times of the year may reduce high biological variation. Alternatively, the number of individuals may be increased and a paired study design chosen to obtain adequate statistical power.

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