Red cell folate assays: some answers to current problems with radioassay variability.

Red cell folate specimens were added to the Quality Assurance Program (QAP) of the Royal College of Pathologists of Australasia in 1986. The interlaboratory variation in the results of these red cell folate surveys has been persistently unsatisfactory. This study reports an investigation into the factors contributing to the wide variation of results reported in QAP surveys. Survey results were studied, replies to a questionnaire regarding methods sent to all participants were assessed and some new experimental studies were performed. Factors contributing to the poor QAP results include variation in dilution and diluent, calculation errors, failure to freeze the hemolysate prior to assay and to follow the kit manufactorer's instructions, stability of dithiothreitol, and variations in kit methods, especially in the release of bound folate by "boil" and "no-boil" methods. Photodecomposition and the form and concentration of ascorbate may also be significant. Adequate preparation of the hemolysate sample should ensure release of all protein-bound red cell folate with the reduction of all folates to a single form. Kit manufactures, users and the QAP committee can all contribute in all attempt to rectify current sources of error.