This finding is not new; there have been a number of reports on poor intermethod agreement of both the serum and whole-blood folate
binding assays (18,19).
Erythrocyte or whole-blood folate
concentrations represent whole-body folate stores and are used to assess folate status.
Comparison of serum and whole-blood folate
measurements in 12 laboratories: an international study [Abstract].
A FLAIR round-robin study of serum and whole-blood folate
results across 11 laboratories in seven European countries reported overall CVs of 18-41% (1).
However, there was no difference between OC users and controls in the relationship between plasma holoTC and total serum cobalamin expressed as the holoTC/cobalamin ratio, and in plasma MMA, plasma tHcy, serum folate, whole-blood folate
, and serum creatinine (Table 1).
The microbiological assays correlated with immunoassays (Immuno 1; Bayer) for estimation of serum (n = 373; r = 0.91) and whole-blood folate
(n = 253; r = 0.61) and serum cobalamin (n = 380; r = 0.96).
Plasma total vitamin [B.sub.12] and whole-blood folate
were determined by commercial immunoassays (Bayer Corporation) on a Beckman-Coulter instrument.
The authors noted that adjusting the pH of a 10 g/L ascorbic acid diluent from pH 2.7 to pH 4 or 4.25 with 1 mol/L NaOH before EDTA-WB dilution (1 in 10) and incubation produced significantly increased whole-blood folate
(WBF) yields (>10%) from fresh, unfrozen hemolysates.
Results of an international round robin for serum and whole-blood folate
. Clin Chem 1996;42:1689-94.
This exercise, as others before it (8, 9), demonstrated the large intraassay and interassay variation in whole-blood folate
We have expressed the results as whole-blood folate
for reasons of clarity because these values are converted in the same way to erythrocyte folate values by using the packed cell volume.
The gas chromatography-mass spectrometry method used to analyze whole-blood folates
showed greater sensitivity than previous chromatographic methods; however, gas chromatography-mass spectrometry requires complex sample preparation, including chemical derivatization, that introduces new sources of experimental error and increases the probability of folate degradation (25, 26).