Serum deoxythymidine kinase gives prognostic information in chronic lymphocytic leukemia

A recently developed deoxythymidine kinase assay, utilizing iodine‐125‐iodo‐deoxyuridine as substrate and capable of detecting enzyme activity in serum from healthy humans, was used in an investigation of sera from 55 untreated patients with chronic lymphocytic leukemia (CLL). When confined to the study, the patients were classified as having progressive or indolent disease and according to Rai stage. The results showed a significant correlation between serum deoxythymidine kinase activity (S‐TK) and disease status, i.e., higher values were found in patients with progressive disease, compared to those with indolent disease. S‐TK also correlated with Rai stage. S‐TK values of more than 40 times the normal value were found in some patients. All patients with S‐TK > 8.4 units had a disease that was or became progressive during the observation period. Within the patient group with indolent disease two groups that differed with regard to prognosis could be distinguished according to their initial S‐TK values. In longitudinal studies of 18 patients with indolent disease, S‐TK was found to exceed 8.4 units only on one occasion during an observation period of up to 68 months. In patients with indolent disease, a transition to progressive disease was parallelled by an increase in S‐TK. Studies of S‐TK levels in 18 patients receiving treatment showed that S‐TK decreased during successful therapy. S‐TK was also found to increase when the disease was reactivated. From these results it is concluded that S‐TK could be used as a prognostic marker for the individual CLL patient. Furthermore, S‐TK seems to be useful for longitudinal follow‐up studies of disease status, both in indolent disease and in progressive disease during treatment.