Field-study screening of blood folate concentrations: specimen stability and finger-stick sampling

We describe optimized procedures for field studies of blood folate concentrations by using finger-stick blood sampling and include relevant studies on blood folate stability. We introduce whole-blood folate adjustment using sample hemoglobin (folate/hemoglobin, nmol/g) as a novel and practical tool yielding accurate and precise results when blood volume or dilution is unknown. Red cell folate concentrations (nmol/L) of 11,887 Americans correlated well with hemoglobin-corrected whole-blood folate concentrations (r2 = 0.993; red cell folate = 0.347 x hemoglobin folate + 1 nmol/L), which supports the approach of using the mean cell hemoglobin concentration (g/L) to interconvert red cell and hemoglobin folate data. Folate concentrations in capillary (finger stick) and venous blood samples from 28 normal donors were similar (P > 0.87), correlating closely (r = 0.98, P < 0.001). Whole-blood samples (collected into K2-EDTA-containing evacuated tubes) in field studies are best stored intact at 4 degrees C until they can be processed and frozen (-20 degrees C). Specific knowledge of blood folate stability is essential in planning and designing field studies.