A simplified microbiological assay for vitamin B12 estimation, completed on microtitre plates using a colistin sulphate resistant strain of Lactobacillus leichmannii (NCIB 12519), and cryopreserved cultures is described. The new assay correlated well with a more conventional "tube" assay and was not influenced by the presence of antibiotics in serum. Evaluation of assay performance showed excellent interassay and intra-assay precision with quantitative recovery of added cyanocobalamin over a wide range of additions (94.9%-102.1%). The advantages of short incubation time, easy reading, and minimal reagent costs make this assay an attractive option in the routine clinical laboratory and in research.

Summary The results of a large number of nutritional screen requests (n= 871) were compared with corresponding values of erythrocyte indices considered predictive of nutritional deficiencies to determine if such indices could be used in a prospective screening procedure to restrict the number of serum vitamin B12, folate, and ferritin assays. Low mean cell haemoglobins (MCH < 27 pg) were found to be superior to low mean cell volumes (MCV < 77 fl), in predicting low serum ferritin values. The occurrence of deficient ferritin values was 90% when the MCH was very low (MCH < 23 pg). Vitamin B12 or folate deficiency could not be predicted from the MCV. A normal MCV was found in more than 55% of vitamin B12 deficient samples and some 30% of serum B12 deficients (< 150 μg/l) showed no evidence of anaemia (Hb > 12 gm/dl) or macrocytosis (MCV < 100 fl). It would not seem appropriate to use erythrocyte indices alone as a method of selecting samples for further investigation of folate or vitamin B12 status.

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