Intraindividual variation in serum retinol concentrations among participants in the third National Health and Nutrition Examination Survey, 1988-1994

Background: The biological variability in serum retinol concentrations has never been examined in a large sample, and its effect on population distribution estimates and the clinical assessment of vitamin A status is unknown. I Objective: We evaluated the biological CV of serum retinol and examined the effect of the CV on both population distribution estimates and clinical assessments of vitamin A status by using data from the third National Health and Nutrition Examination Survey, 1988-1994. Design: We described the biological CV [(SD/root(x) over bar) x 100] and examined associations between the CV and other factors via multivariate analysis of variance and linear regression. We used linear regression to predict the mean retinol concentration from a single concentration and established 95% CIs for each participant. We estimated the adjusted prevalence of inadequate vitamin A status (retinol < 1.05 mumol/L) on the basis of the CIs.,We estimated an uncertainty range for serum retinol concentrations for which the CIs included the established cutoff. Results: The mean biological CV across all strata was 6.45%. The biological CV varied significantly between racial-ethnic groups (P < 0.05). Prevalence estimates of inadequate serum retinol concentrations were reduced after adjustment for the total variation, with an adjusted overall prevalence of 0.62% compared with an unadjusted prevalence of 2.63%. Conclusions: The actual population prevalence of inadequate vitamin A status may be 75% lower than the estimates previously reported. Confirmation of vitamin A status may be needed for persons in the United States with observed serum retinol concentrations near the recognized cutoff.