The direction of the difference between Canadian and American erythrocyte folate concentrations is dependent on the assay method employed: a comparison of the Canadian Health Measures Survey and National Health and Nutrition Examination Survey

Fortification of select grain products with folic acid and periconceptional supplementation recommendations in Canada and the USA have improved folate status, and have been associated with a reduced risk of neural tube defects. In the present study, we aimed to conduct a comparison of erythrocyte folate concentrations from the 2007-9 Canadian Health Measures Survey (CHMS) and the 2007-8 US National Health and Nutrition Examination Survey (NHANES). Erythrocyte folate concentration was assessed in participants aged 6-79 years (CHMS, n 5248; NHANES, n 7070). To account for different folate assays employed -Immulite 2000 immunoassay (CHMS) and microbiological assay (NHANES) - a conversion equation was generated (n 152 adults) to adjust the CHMS data. t Tests were used to examine country differences. Median Canadian erythrocyte folate concentrations (method-adjusted) were lower than those of Americans (988 and 1100 nmol/l, respectively), but unadjusted median Canadian erythrocyte folate concentrations were higher (1250 nmol/l). The upper 95% CI boundary of the method-adjusted Canadian erythrocyte folate distribution overlapped that of the American erythrocyte folate concentrations, while the lower 95% CI boundary of the method-adjusted Canadian erythrocyte folate data was below the American distribution. In summary, the fact that erythrocyte folate concentrations were either higher or lower in Canadians compared with Americans, depending on whether an adjustment was made to account for assay differences, suggests that caution must be exercised in evaluating erythrocyte folate data from different countries because analytical methods are not readily comparable. Furthermore, we cannot unequivocally conclude that there are true differences in erythrocyte folate concentrations between the Canadian and American populations in the post-fortification era.