Evaluation of a radioprotein-binding assay (RPBA) for folate analysis in berries and milk

This study aimed to optimise a commercial radioprotein-binding assay (RPBA), routinely used for clinical samples, for folate quantification in foods containing mainly 5-CH3-H4folate. The assay was modified using external calibration with 5-CH3-H4folate in a lower concentration range diluted in food extraction buffer, rather than the buffer with human serum albumin (HSA) provided by the kit. We evaluated the modified RBPA on some selected food products; milk, whey powder, rose hips, strawberries and European certified reference materials (CRM 421, 485) and the adjustments did not affect the assay negatively. Performance parameters included control of selectivity, absence of matrix effects, recoveries of 94–113%, precision of 1–6 CV% (intra-assay) and 5–9 CV% (inter-assay). Folate concentrations in berries and milk, obtained by the modified RPBA were also compared with other quantification methods such as HPLC and MA. The optimised RPBA was found to be a quick and inexpensive complement to HPLC methods, reliable for folate quantification in foods such as milk and berries that contain mainly 5-CH3-H4folate.