Comparison of the deoxyuridine suppression test with serum levels of methylmalonic acid and homocysteine in mild cobalamin deficiency

Both the deoxyuridine suppression test (dUST) and the cobalamin-dependent metabolites, methylmalonic acid (MMA) and homocysteine, are valuable tools for identifying clinical cobalamin deficiency. Examination of these metabolic changes in mild or marginal deficiency can provide useful comparisons of diagnostic frequencies and sensitivities and help define the sequence of metabolic changes in early deficiency, These tests were therefore compared directly with each other in 50 patients with low cobalamin levels and few or no obvious signs of deficiency. Serum homocysteine (P = 0.0003) and MMA levels (P = 0.0004) correlated with dUST results. However, the dUST results were abnormal significantly more often (38/50 patients) when matched against levels of homocysteine (25 abnormal results of 50; P = 0.007) or MMA (20/50; P = 0.008). Abnormalities of one or both serum metabolite levels (30/50 patients) occurred almost as often as dUST abnormalities (P = 0.059). Metabolite levels, even when originally 'normal', fell with cobalamin therapy in many cases, The results indicate that both the dUST and serum metabolite levels become abnormal before macrocytic anaemia develops in mild cobalamin deficiency. The dUST appears to be the most frequently abnormal of the tests; metabolite levels appear to rise almost concurrently but they do not become diagnostically abnormal as soon.