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. 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/[bar.x]) 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 [micro]mol/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. KEY WORDS Biological variation, CV, regression, regression to the mean, serum retinol, vitamin A, third National Health and Nutrition Examination Survey, NHANES III