Plasma methylmalonic acid in relation to serum cobalamin and plasma homocysteine in a psychogeriatric population and the effect of cobalamin treatment.

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Cobalamin deficiency seems to be a relatively common condition in psychogeriatric patients. To elucidate the diagnostic possibility of cobalamin deficiency we have in this study analysed three markers for cobalamin deficiency, plasma methylmalonic acid, plasma homocysteine and serum cobalamin, in 96 psychogeriatric patients. Patients were divided into four groups according to serum cobalamins above or below 150 pmol/l and normal (< 19.9 mumol/l) or increased plasma homocysteine. The upper reference limit (95th percentile) for plasma methylmalonic acid in 100 healthy subjects was established to 0.42 mumol/l. The mean value of methylmalonic acid was increased only in the group of patients with serum cobalamin below 150 pmol/l and increased plasma homocysteine compared to the other groups. In this group six (46%) out of 13 patients exhibited increased plasma methylmalonic acid, whereas in the other groups the frequency of increased plasma methylmalonic acid only varied from 10 to 13%. During cobalamin supplementation the most pronounced decrease of plasma methylmalonic acid also occurred in the group of patients with low serum cobalamin levels and increased plasma homocysteine. Only 39% of the initial mean value for plasma methylmalonic acid was noted after 7-10 days of cobalamin administration.