Assay of serum and whole blood folate by a modified aseptic addition technique

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Abstract
An aseptic addition method for the measurement of serum and red cell folate is described and compared with a technique involving heat extraction. Results are similar, but the aseptic addition method is simpler, gives better reproducibility on duplicate assays, and better agreement between results obtained from two dilutions of the specimens. Several important sources of error were discovered, including incorrect preparation and storage of reference solutions, and interference of blood pigments giving falsely high readings when turbidity was measured by light absorbance. The mean serum folate in 95 nulliparous female nurses and laboratory technologists was 5.1 ng/ml (95% confidence limits 2.39 to 10.72 ng/ml); the mean red cell folate was 216 ng/ml (95% confidence limits 116.8 to 400.2 ng/ml). Five of the subjects were considered to be marginally folate deficient by the criteria of hyperlobulated neutrophil polymorphs, low serum or red cell folate. Fifteen subjects had evidence of iron deficiency, but their folate levels did not differ significantly from the rest of the control group.