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.