Ammonia Levels Should Not Be Used as a Surrogate Marker of Levels of Asparaginase Activity in Acute Lymphoblastic Leukemia Patients

To the Editor:
Asparaginase is an enzyme which hydrolyses asparagine into aspartic acid and ammonia. Levels of asparaginase activity are relatively easy to measure, and a trough level >100 U/L is associated with complete asparagine depletion. So, the measurement of levels of asparaginase activity is a very useful tool to monitor asparaginase activities in clinical practice. However, these assays are not available in all countries in contrast to ammonia measurements.
Recently, a Polish study group published ammonia levels of 87 children during acute lymphoblastic leukemia (ALL) induction therapy. They concluded that the ammonia concentration reflects the levels of asparaginase activity and could be useful when asparaginase activity assays are not available. We evaluated if ammonia levels could be used as a surrogate marker of levels of asparaginase activity. [...]