Natalizumab is a very effective treatment for MS. It reduces the risk of relapses and progression of disability in people with relapsing-remitting MS.

But some patients treated with Natalizumab have contracted progressive multifocal leukoencephalopathy (PML), a rare brain infection that can lead to death or severe disability.

Not everyone treated with Natalizumab gets PML, so it is important to be able to quickly recognize those who are at risk.

To do this, doctors do a blood test for the presence of antibodies against the JC virus (antibodies help the immune system recognize foreign organisms). The presence of anti-JCV antibodies indicates a higher risk of developing PML.

The JC virus is common and usually not dangerous. However, when a patient’s immune response is low – say, because they are being treated with Natalizumab – the virus can reactivate and produce PML.

In a new study carried out in California by Focus Diagnostics, researchers looked at a new test for anti-JC virus antibodies in patients being treated with Natalizumab during clinical trials.

In particular, 71 blood samples of patients on Natalizumab who developed PML were collected at least six months before PML onset, in 2012. The researchers found this test seemed more sensitive and more reproducible than the previous one at identifying people at greater risk of contracting PML.

Focus Diagnostics rolled out the new test, called STRATIFY JCVTM DxSelectTM , in 2013 and recently the data from the study has been published. This test, which has replaced the previous one, is being carried out in a small number of laboratories across the world, which receive the blood samples of Natalizumab patients from other countries.

Even if the new test seems to be more effective and reliable than the previous one, the number of PML patients in which it has been tested is low. More research is needed to predict more accurately the risk of PML in individual cases.

Biogen Idec (the maker of Natalizumab) is collecting more data from clinical trials and collaborations from several MS centres across the world.

In German and Spain, researchers have been studying two new markers of PML. The results need to be further assessed through larger studies.

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