The deoxyuridine suppression test performed on phytohaemagglutinin-stimulated peripheral blood cells fails to reflect in vivo vitamin B12 or folate deficiency.

Because phytohaemagglutinin-stimulated lymphocytes acquire folate deficiency in vitro, we have re-examined the claim that the deoxyuridine suppression test (dU-test) based on such cells helps to diagnose megaloblastic states. dU-test results were obtained from the phytohaemagglutinin-stimulated blood cells of 77 patients at 6 concentrations of dU. 21 megaloblastic patients with cobalamin or folate deficiency did not have a mean blood dU-test result significantly higher than that of 21 normoblastic patients at any concentration of dU. Among all patients, however, there was a weak correlation between the blood and marrow dU-test results. Folic acid corrected the blood dU-test results more in the normoblastic than the megaloblastic patients, and a difference between the two groups appeared, but a large overlap persisted. Blood dU-test results tend to be higher in megaloblastic patients, but acquired folate deficiency obscures the distinction from normoblastic patients so that the test is not of diagnostic value.