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A quantitative determination of physiological levels of all-trans-retinoic acid (vitamin A acid) in human serum has been developed. A double-phase extraction of 3.5 ml of serum followed by a specific and sensitive high performance liquid chromatography procedure allowed measurement of levels down to 1 ng/ml. Serum concentrations in 37 fasting volunteers ranged from 2.7 to 4.2 ng/ml and fitted a normal-Gaussian distributional shape with a mean value of 3.5 ng/ml and SD of 0.4 ng/ml, as demonstrated by the Kolmogorov-Smirnov test.
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Leenheer et al. (1982) studied this question.
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