The Cylex ImmuKnow Test (Cylex, Columbia, MD) measures immune cell function (ICF) and is based on the amount of adenosine triphosphate (ATP) released when T cells are stimulated by phytohemagglutinin. This preliminary study sought to determine if ICF measurements can be used to stratify kidney transplant recipients according to the risk for developing BK virus infection. ICF measurements were done in 15 samples from 8 patients with BK viremia, 38 samples from 25 patients with BK viruria, and 243 samples from 148 patients with no BK viruria or viremia. The mean+/-SD amounts of ATP released in these 3 groups were 102.9+/-58.6, 227.2+/-146.4, and 231.8+/-150.8 ng/mL, respectively (P= .002, viremia vs all other samples). Within the viruria group, lower ICF values were associated with higher urinary viral load (P= .037). These results show that a decreased ICF test result correlates with active viral replication in kidney transplant recipients.