[Comparative study of the detection of plasma folate with microbial assay and radioimmunoassay].

Objective: To compare two methods (microbial assay and radioimmunoassay) for measuring plasma folate concentrations, and to examine the relationship between plasma folate levels, and alcohol consumption, tobacco use and body mass index, and the risk of hyperhomocysteinemia in China.

Methods: We used a microtiter plate microbial assay and a radioimmunoassay to measure the folate concentration in 88 plasma samples. After comparing the results of these two methods and fitting a regression line, we examined the geographical, seasonal, and gender differences in folate concentration of plasma collected from 2,422 adults in south and north areas in China, and evaluated the association of plasma folate concentration, with alcohol consumption, cigarette smoking, and body mass index, and with the risk of hyperhomocysteinemia, using the data from the two assays.

Results: The data from the two assays had a linear relationship (r=0.879, P=0.000); the regression was Y=0.683X+0.308 (where X and Y were nature logarithmic transformations of plasma folate by microbial assay and radioimmunoassay, respectively); however, the mean plasma folate levels by microbial assay were much higher than those obtained by radioimmunoassay. Both data sets showed similar plasma folate distributions among Chinese adults, associations with other risk factors, and the risk of hyperhomocysteinemia. We estimated that 19.9% of the Southerners and 67.1% of the Northerners had plasma folate concentrations by radioimmunoassay lower than the 6.8 nmol/L used to define plasma folate deficiency.

Conclusion: There is a linear relationship between plasma folate levels determined by microbial assay and radioimmunoassay, but because of the different levels obtained in the two assays, it is difficult to use the microbial assay results to evaluate folate status at this time. The use of 10.5 nmol/L as a cut-off for plasma folate deficiency by microbial assay needs further study.