A method for detecting the absence or presence of cells of interest in a liquid sample is provided. The method comprises providing a sample suspected of containing cells of interest that contain an intracellular enzyme with a measurable activity. The sample further comprises an extracellular medium that also includes an extracellular enzyme with the measurable activity. The method further comprises the steps of treating the liquid sample with a reagent that inactivates the measurable activity in the extracellular medium but does not inactivate the measurable activity in the cells of interest, lysing the cells of interest to release the intracellular enzyme, and measuring the measurable activity. Thus, the measurable activity of the intracellular enzyme can be measured without interference from the extracellular enzyme. The invention is particularly useful for treatment of bacterially-infected blood using a detection assay based on adenylate kinase activity.