Our preliminary studies have validated in vitro a sensitive test to measure the functionality of blood LYT - BKV. This test is used to evaluate the concentration and functionality of LYT - BKV present in small volume of blood by measuring their specific proliferation after stimulation with peptide cocktails BKV. Proof of concept of the feasibility of this test was established on a small series of samples and highlights significant differences in lymphocyte anti-BKV different patients.