The recent introduction of radioassays for 'true' cobalamin, as opposed to cobalamin and its analogues, has resulted in significantly lower levels of cobalamin being found in patients with folate deficiency. In study of 81 patients, cobalamin analogue levels were found to increase and cobalamin to decrease as red cell folates decreased. Cobalamin absorption studies in 15 patients with low cobalamin and folate levels were found to be normal in 10 patients, all of whom demonstrated high levels of analogues relative to true cobalamin. We have found that the mean serum cobalamin increased from 210 ng/l (range 100-380) to 309 (150-470) and analogues fell from 226 ng/l (150-280) to 127 (65-190) in folate deficient patients when treated with folic acid. It appears that cobalamin analogue concentrations are increased in folate deficiency, and that in these patients treatment with folic acid alone may correct both the low cobalamin and the high analogue levels.
