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

S ummary . 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 ( 3 H‐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 125 I‐deoxyuridine ( 125 I‐UdR) incorporation into DNA‐ 125 I‐UdR may substitute for 3 H‐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 B 12 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 B 12 or folate levels, and is now routinely used in our laboratory on all bone marrow aspirates.