of 3-hydroxyisovalerate acid, or both have also been reported in several overt cases of biotin deficiency (Carlson et al., 1995; Gillis et al., 1982; Kien et al., 1981; Lagier et al., 1987; Mock et al., 1981, 1985). Gender differences are not apparent in these two indicators.

A low plasma biotin concentration is not a sensitive indicator of inadequate biotin intake. Abnormally decreased plasma biotin was absent in half the subjects fed raw egg white (NI Mock et al., 1997) and in some overt case reports of biotin deficiency (Carlson et al., 1995; Khalidi et al., 1984; Kien et al., 1981; Matsusue et al., 1985; Mock et al., 1981, 1985).

Odd-chain fatty acid composition in plasma lipids may reflect biotin status (Kramer et al., 1984; Liu et al., 1994; Mock et al., 1988a, b), but the sensitivity and clinical utility of this measurement remains to be determined. The accumulation of odd-chain fatty acids is thought to result from propionyl-CoA carboxylase deficiency.

All published studies on biotin nutriture (Zempleni et al., 1997) have used one of three basic types of assays to estimate biotin: bioassays (most studies), avidin-binding assays, or fluorescent derivative assays. Recent modifications of bioassays generally have adequate sensitivity to measure biotin in blood, urine, and foods. For example, the bioassay based on Kloeckera brevis has both excellent sensitivity and metabolite discrimination (Guilarte, 1985). However, the bacterial bioassays (and perhaps the eukaryotic bioassays as well) suffer interference from unrelated substances and variable growth response to biotin metabolites. The acid hydrolysis or protein digestion required to release bound biotin may also release other compounds that support bacterial growth.

There are major discrepancies among the various bioassays and avidin-binding assays concerning the true concentration of biotin in human plasma. Reported mean values range from approximately 0.5 nmol/L to more than 10 nmol/L. The avidin-based radioimmunoassay with high-performance liquid chromatography is among the best current assays (Mock, 1996).