Influence of reporting error on the relation between blood folate concentrations and reported folic acid–containing dietary supplement use among reproductive-aged women in the United States

Abstract

Folic acid intake is the most important predictor of blood folate concentrations among nonpregnant women, but the reporting of folic acid-containing supplement use is subject to error. We assessed the effect of reporting error of supplement use on blood folate concentrations. Data from the third National Health and Nutrition Examination Survey were analyzed. Respondents to that survey were asked twice about supplement use: ie, during the household interview, to recall use in the previous month, and during the physical examination, to recall use in the previous 24 h. To examine the effect of error reporting, we classified women (aged 15-44 y) into 5 groups according to supplement use in the previous month (nonusers, those ingesting < 400 micro g/d, and those ingesting >or= 400 micro g/d) and in the 24 h before the physical examination (yes or no). We expected nonappreciable differences in red blood cell (RBC) folate concentration by status of 24-h recall within the same category of previous-month use because RBC folate reflects long-term average consumption. We calculated covariate-adjusted means of serum and RBC folate concentrations. Among women who reported average daily use of >or= 400 micro g folic acid in the previous month, the adjusted mean RBC folate was 436.5 nmol/L (95% CI: 406.7, 466.3 nmol/L) in those who did not take the supplement in the previous 24 h and 519.7 nmol/L (95% CI: 496.2, 543.2 nmol/L) in those who did do so (P < 0.01). This significant difference indicates apparently erroneous reporting of supplement use in the previous month by some participants. The effect of reporting error on blood folate concentrations is important in interpreting survey results, evaluating health education campaigns, and identifying populations needing special education programs.