Biological variation of retinoids in man

This investigation was undertaken to assess biological variation, especially the within-subject variations of all- trans retinoic acid, 13- cis retinoic acid and retinol in human serum. Diurnal variation and variation over a week, a month and a year were studied in 11 males (aged 21 - 54 years) and 17 females (aged 22 - 63 years), all subjectively healthy. We found no diurnal variation with the exception of all- trans retinoic acid, which had maximal concentrations at noon irrespective of food intake. Seasonal variations were marginal. Both all- trans and 13- cis retinoic acids had fairly high within-subject (13.1% and 12.6%, respectively) and between-subject coefficients of variation (15.9% and 21.0%, respectively), while the within-subject CV of retinol was less (5.6%, with a between-subject CV of 21.1%). Thus, the indices of individuality were <1 for all retinoids. The critical differences between two consecutive samples were <40% for the retinoic acids and <20% for retinol. Women had higher all- trans retinoic acid concentrations in serum (5.1 nmol/L vs. 4.5 nmol/L), lower 13- cis retinoic acid concentrations (4.5 nmol/L vs. 5.5 nmol/L) and lower retinol concentrations in serum (2.1 µmol/L vs. 2.5 µmol/L) than men. Thus, samples for retinoid determinations should be drawn in the morning and evaluated using separate gender reference intervals.