The dU Suppression Test using 125I‐UdR to Define Biochemical Megaloblastosis

Summary. The deoxyuridine (dU) suppression test measures the de novo pathway to synthesis of DNA‐thymine. It demonstrates that in normoblastic human bone marrow the incorporation of triated thymidine (3H‐TdR) into DNA‐thymine is suppressed to less than 10% of control value by preincubation of bone marrow cells for 1 hr with deoxyuridine (dU), but in megaloblastic marrow cells there is less suppression. In the current study it was demonstrated that 125I‐deoxyuridine (125I‐UdR) incorporation into DNA‐125I‐UdR may substitute for 3H‐TdR incorporation into DNA‐thymine in this test, thus simplifying the procedure by replacing a betaemitter with a gamma‐emitter. Biochemical megaloblastosis was demonstrated or ruled out after incubation of human bone marrow for 3 hr with either tracer. The biochemical lesion present in folate or B12 deficiency can thus be recognized even in the absence of morphological change. The technique resolved questions raised by unclear bone marrow morphology and misleading serum vitamin B12 or folate levels, and is now routinely used in our laboratory on all bone marrow aspirates.