The JC virus is a common infection completely unrelated to MS. About half of the population have been infected by the virus and it is normally kept under control by the immune system causing no problems. If the immune system is weakened and the body is less able to fight an infection, the virus can reactivate and cause serious and potentially fatal inflammation and damage to the brain known as progressive multifocal leukoencephalopathy (PML).
When you are infected with the JC virus you develop antibodies to the virus. A test called Stratify JCV can detect the presence and level of these antibodies in a blood sample and help give an estimate of your risk of developing PML.
If you start treatment with Tysabri, you will be given this blood test. If the test is negative it should be repeated throughout treatment with Tysabri as you could become infected at any time. If a previous blood test found low levels of JC virus infection, you should also continue to have further tests as the virus level may increase. It is recommended that blood tests are repeated every six months.
If the test for JC virus is found to be negative (often referred to as JCV negative) the risk of PML is very small (less than 1 in 1,000).
A positive test for JC virus (JCV positive) indicates that you are at a higher risk of developing PML, although for most people the risk is still relatively small. This risk also depends on whether you have previously taken a drug that suppresses the immune system (for example, azathioprine, cyclophosphamide, mitoxantrone or methotrexate), how long you have been taking Tysabri (especially over two years) and the amount of JC virus antibody (titre) in your blood. With different combinations of these factors the risk of developing PML ranges from less than 1 in 1000 up to 12 in 1000.
The test does not indicate if you will or will not get PML. The test indicates your relative level of risk, information which can help you and your MS team make decisions about treatments. If you are identified as being at a higher risk, your health will be carefully monitored for any warning signs of PML.
- Annals of Neurology 2014;76(6):802-12. Full article Anti-JC virus antibody levels in serum or plasma further define risk of natalizumab-associated progressive multifocal leukoencephalopathy.
- Multiple Sclerosis 2012 Feb;18(2):143-52. Summary Risk stratification for progressive multifocal leukoencephalopathy in patients treated with natalizumab.
Last reviewed: December 2016
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