High serum folates and the simplification of red cell folate analysis.

Recent substantial increases in clinical blood folate concentrations are noted. Since red cell folates (RCF) are calculated from whole blood folates (WBF) by subtraction of the endogenous serum folate (SF) component, the reporting of clinical RCF results may be delayed because an ever increasing proportion (15%) of diagnostic SF levels are high (> 20 ng/ml) and need a repeat analysis. We evaluated 'plasma replacement' as a simple preanalytical procedure in which endogenous blood plasma is removed from red cells by washing and substituted with 'low-folate' plasma (serum) as an alternative conjugase (gamma-glutamyl carboxypeptidase) source for folate polyglutamate hydrolysis. Washed and conventional RCF assays compared well after both manual (n = 115, r = 0.98, y = 1x + 1.26) and automated washing of red cells (n = 170, r = 0.96, y = 0.96x - 0.73 ng/ml) and were not significantly different. The interassay reproducibility of folate results from washed blood samples was good (CV = < 6%). This novel 'plasma replacement' step halves the cost of a valid RCF assay by eliminating the need for endogenous SF analysis, and it expedites the reporting of clinical results.