Tysabri Screening Test May Be Unreliable

About one-third of multiple sclerosis patients testing negative for antibodies against the JC virus — suggesting that natalizumab (Tysabri) would be relatively safe — were found to have active viremia, researchers said.

The report, published in the June 6 issue of the New England Journal of Medicine, raises the specter that patients with negative JC serology results could be given natalizumab when they may actually be at high risk for progressive multifocal leukoencephalopathy (PML), an often fatal type of brain inflammation.

PML is caused by JC virus becoming active in the brain. Natalizumab appears to contribute to reactivation of latent JC virus infections. MS patients with such infections face a PML risk while on natalizumab ranging from about 0.01% to 0.1% depending on the presence of other risk factors

Eugene O. Major, PhD, of the National Institute of Neurological Disorders and Stroke in Bethesda, Md., and colleagues reported having tested blood samples from 49 patients for anti-JC virus antibodies and for JC virus DNA.

In 26 of the patients, the samples were obtained immediately before starting on natalizumab. Samples from the other 23 were obtained after at least 2 years of periodic natalizumab infusions.

Ten patients in the first group were found to have JC viral DNA in their blood, with four lacking a positive result in anti-JC virus antibody testing. (Seronegativity was defined as antibody titers of less than 2,560 units.)

In the group of 23 patients tested after 2 years of natalizumab treatment, seven were found to be viremic and two were seronegative, the researchers reported.

Overall, they indicated, six of 17 patients (35%) showing JC viremia were seronegative with the antibody test.

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