Is detection of JC virus DNA in blood a valuable screening test for progressive multifocal leukoencephalopathy in Crohn's disease patients eligible for anti-alpha4 integrin therapy?

We read with interest the article by Verbeeck and colleagues ( Gut 2008; 57 :1393–7) investigating the presence of JC virus DNA in urine and blood as a screening tool for active viral replication in immunosuppressed patients with Crohn’s disease. Based on their data, these authors propose using serial assessments of JC virus DNA in the blood compartment to monitor such patients for the risk of developing progressive multifocal leukoencephalopathy (PML) before and during anti-α4 integrin therapy. They postulated that the presence of JC viraemia may be a risk factor for future progression to PML. Moreover, in agreement with other studies they suggest that the absence of JC virus DNA in blood can be associated …