Folate assays: serum or red cell?

Tests for folate and vitamin B12 deficiency are frequently requested by clinicians in many different specialties. An audit of folate assay methodology was undertaken to establish the number of tests and types of assay performed in different centres, and to analyse the indications for these investigations, with a view to advising on the most appropriate assay for use in the laboratory. A questionnaire was sent to 30 centres, 24 (80%) of which participated in the audit. The types of folate assay performed, number of requests, reference range and method of analysis differed between centres. The major specialty users of the service were general practitioners, general physicians and geriatricians. A detailed analysis of 1,259 consecutive requests for folate assays from a single representative laboratory showed a significant correlation between serum and red cell folate levels (r = 0.49, p < 0.001). However, in patients with low serum folate, there was no correlation with red cell folate in the absence of macrocytosis. The major indication for folate analysis was for haematological abnormalities but 36% of cases were for nonspecific indications. A haematologist with an interest in folate metabolism was invited to moderate the results at an audit meeting of haematologists. The consensus was that the most appropriate screening test for folate deficiency is the serum assay, which can be combined easily with vitamin B12 assay.