High serum folates and the simplification of red cell folate analysis

Recent substantial increases in clinical blood folate concentrations are no ted. Since red cell folates (RCF) are calculated from whole blood folates ( WBF) by subtraction of the endogenous serum folate (SF) component, the repo rting of clinical RCF results may be delayed because an ever increasing pro portion (15%) of diagnostic SF levels are high (> 20 ng/ml) and need a repe at analysis. We evaluated 'plasma replacement' as a simple preanalytical procedure in wh ich endogenous blood plasma is removed from red cells by washing and substi tuted with 'low-folate' plasma (serum) as an alternative conjugase (gamma-g lutamyl carboxypeptidase) source for folate polyglutamate hydrolysis. Washe d and conventional RCF assays compared well after both manual (n=115, r=0.9 8, 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 reproduc ibility of folate results from washed blood samples was good (CV=< 6%). This novel 'plasma replacement' step halves the cost of a valid RCF assay b y eliminating the need for endogenous SF analysis, and it expedites the rep orting of clinical results.