Serum caffeine and paraxanthine as markers for reported caffeine intake in pregnancy.
PURPOSE: Previous studies of maternal caffeine use and pregnancy outcome have relied on self-reported use. Even if these were perfectly accurate, inter-individual differences in caffeine metabolism result in a relatively weak correlation between caffeine intake and serum concentration. The purpose of this study was to determine whether the serum concentration of caffeine or its primary metabolite, paraxanthine, obtained at an unknown time during working hours, is useful to distinguish between pregnant women who report consuming small and large amounts of caffeine. METHODS: We selected from the Birmingham fetal growth study 60 women with normal pregnancy outcomes who reported consuming < or = 0.8 mg/kg/day of caffeine in a 24-hour dietary recall, 60 who consumed 0.81-2.5 mg/kg/day, 60 who consumed 2.51-5.0 mg/kg/day and 59 who consumed > or = 5.01 mg/kg/day. These women had serum drawn for storage during regular clinic hours on the same day as the recall interview. Caffeine and paraxanthine were measured in the stored serum using high performance liquid chromatography. RESULTS: The weighted kappa coefficient between strata of caffeine intake and quartiles of serum paraxanthine was 0.58 among smokers and 0.53 among nonsmokers, versus 0.44 and 0.51, respectively, for quartiles of serum caffeine. The Pearson correlation coefficient between intake and paraxanthine was 0.50 for smokers and 0.53 for nonsmokers, and 0.37 and 0.51, respectively, for serum caffeine. These values are comparable to the correlation between reported smoking and serum cotinine in pregnancy. CONCLUSIONS: The serum concentrations of paraxanthine, and to a lesser degree, caffeine are useful to distinguish between women with varying levels of caffeine intake.