The relation of age and sex to serum primary (PBA) and total bile acid (TBA) concentrations was evaluated by an enzymatic fluorometric microassay in healthy infants, children and adults. TBA concentrations were the highest in the 6-dayold group and 1-month-old group and seemed to switch to almost normal adult levels by the age of 4-6 years, which persisted throughout life, while PBA concentrations were predominant over a period of 3 days to 3 months after birth. No sex-related differences were observed from neonates to very old persons for any of serum bile acids. These results show that the bile acid metabolism in the liver and the enterohepatic circulation of bile acids are usually matured in infancy and that aging and sex may insignificantly affect serum bile acid metabolic profiles.age ; sex ; primary bile acids ; total bile acids

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An enzymatic fluorometric microassay using 3a-or 7a-hydroxysteroid dehydrogenase was devised to determine serum total or primary bile acids. The present microassay using 50 ul of a serum sample was run as a two-step method ; the first step is the reaction of substrates with NAD+ and the hydroxysteroid dehydrogenase to produce as much NADH as possible in the incubation system and the second step is that of NADH with resazurin coupled with 1-methoxy phenazinemethosulfate to produce fluorescent resorfin. The assay is specific for 3a-or 7a-hydroxy bile acids, sensitive (a detection limit of 1.5 ,u M with a linear range of 1.5-80 p M), reproducible (CV = less than 5%) and inexpensive. As measured by this microassay, the ratio of serum primary to total bile acids (P/T ratio)

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