Comparison of Three Methods for Measuring Vitamin B12 in Serum: Radioisotopic, Euglena gracilis and Lactobacillus leichmannii

Summary. The vitamin B12 content of 481 sera was estimated by a radioisotopic assay and 478 of the sera were also assayed by the Euglena gracilis method and 396 by the Lactobacillus leichmannii method. The serum vitamin B12 level in normal subjects ranged from 235 to 1470 pg/ml, mean 497 pg/ml (radioisotopic), 170–1144 pg/ml, mean. 460 pg/ml (E. gracilis) and 155–1075 pg/ml, mean 471 pg/ml (L. leichmannii). In general, the radioisotopic assay gave higher results than the other two methods and the E. gracilis assay gave the lowest results. The E. gracilis assay gave the clearest distinction between normal subjects and patients with untreated pernicious anaemia. A large number of post‐gastrectomy and folate deficient patients and occasional patients with other conditions gave subnormal results by one or both of the microbiological assays but normal values by the radioisotopic assay. The serum vitamin B12 level measured by both the E. gracilis and radioisotopic assays rose with folic acid therapy in two folate deficient patients. In one of them, the values by the two assays were initially 100 pg/ml apart and remained this degree apart during the response, while in the other patient the two assays gave almost identical results both before and during the response to folic acid therapy.