Two enzymes, lactic dehydrogenase (LD) and phosphohexose isomerase (PHI), were measured in the plasma of 30 patients with leukemia and compared with the findings in 66 control subjects. Abnormally elevated PHI levels were found in both acute and chronic myelocytic leukemia, but not in lymphocytic leukemia. The plasma LD was increased above normal in acute and chronic myelocytic leukemia, in acute lymphocytic, but not in chronic lymphocytic leukemia. Both enzymes were normal or only slightly raised in three patients with the aleukemic type of the disease. Hemolytic anemia in seven leukemic patients was associated with high plasma LD values in the presence of relatively low PHI levels.

Results of serial enzyme studies from the time of diagnosis until death indicated that both plasma enzymes, but especially the LD, usually reflected changes in the course of the disease-falling during remissions and rising during relapses. In most cases this enzyme paralleled the leukocyte level but occasionally indicated the onset of a relapse or remission before the white cell count had begun to change.

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