Understanding the Stratify John Cunningham Virus (JCV) Antibody Test and Progressive Multifocal Leukoencephalopathy (PML) Risk
What is the JC Virus (JCV) and why do I need this test?
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JCV is a common virus that most people get when they were young.
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Most adults have been exposed to it in their lifetime.
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For most people, JCV is not active (latent or sleeping) in the body and never causes problems.
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Sometimes the virus can be active in people taking medications (meds) for MS or other meds that weaken the immune system. It can cause a rare but serious brain infection called PML.
What Is the JCV Antibody Test?
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This is a simple blood test that looks for antibodies of JCV. Antibodies are signs that your body has been exposed to the virus.
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Stratify JCV Antibody uses an Enzyme‑Linked Immunosorbent Assay (ELISA test) that can clearly show if the test result is positive or negative for JCV.
| Your Test Result |
What It Means |
Exposed to JCV? |
Your Risk of PML |
| Negative |
No antibodies found |
You likely have not been exposed to JCV. |
Your PML risk is very low. |
| Positive |
Antibodies found |
You have been exposed to JCV. |
Your doctor will observe your risk closely. |
Do I Need to Repeat the Test?
Yes — Your JCV result can change over time. Most people on MS therapy repeat the test every 6 months or as told by your doctor.
What Happens If My JCV Is Positive?
Testing JCV positive does not mean you have PML now or will get it in the future. It means that you have been exposed to the virus at some point in your life. Most people who test positive keep taking their MS meds safely. You may need extra lab work or visits with the doctor
What is PML?
Risk Factors for PML
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You test positive for JCV antibodies.
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You have been on MS meds for a long time.
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Other meds that weaken the immune system (such as chemotherapy) that you took in the past or are taking now can raise your risk for PML.
Symptoms of PML to Watch For
PML symptoms can seem like a MS relapse, but they are often new symptoms and get worse faster. Symptoms may include:
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- Vision Changes or vision loss
- Confusion or memory problems
- New or worsening weakness, clumsiness, or balance problems.
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- Speech of language problem
- Seizures
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How to Lower your Risk
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Get all the suggested blood tests that check your therapy.
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Go to all your doctor’s appointments.
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Tell your care team if any meds changed, including ones that affect your immune system.
What to Remember
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JCV is common and it does not cause harm to most people.
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The Stratified JCV Antibody test helps your care team choose the safest MS treatment best for you.
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Testing is simple and quick.
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Risk of PML is very low, but it can be a serious infection, so you need to have regular labs and doctor visits.
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Regular blood tests, MRI scans, and clinical visits will help keep your risk low.