Test Quick Guide

West Nile virus (WNV) is a member of the Flavivirus family of viruses, which includes several viruses that can infect humans such as Zika, yellow fever, and dengue. This virus is transmitted to humans primarily by mosquitoes. In rare cases, WNV is spread through blood transfusion, organ transplantation, or from birth parent to infant during pregnancy, delivery, or through breast milk.

Most people infected with WNV do not experience symptoms, but infection can result in mild illness and, in rare circumstances, more severe complications.

Samples used for West Nile virus testing include blood and/or cerebrospinal fluid (CSF). Testing for WNV may be recommended for patients who experience symptoms of infection during mosquito season.

About the Test

Purpose of the test

The purpose of a West Nile virus test is diagnostic, which means that testing is used to determine if a West Nile virus infection is the cause of a patient’s symptoms.

Because West Nile virus has similar symptoms to other Flaviviruses, diagnostic testing can also help differentiate a WNV infection from other Flavivirus infections such as Zika, dengue, and yellow fever.

What does the test measure?

West Nile virus testing looks for evidence of the virus that causes a WNV infection by identifying antibodies or genetic material of the virus.

Antibody testing is used to identify proteins made by the immune system to fight foreign substances such as a virus. In WNV testing, antibody tests look for immunoglobulin M (IgM) antibodies that are only produced after a West Nile virus infection. There are two ways to identify these antibodies:

  • Enzyme-linked immunosorbent assay (MAC-ELISA): MAC-ELISA is the preferred method of diagnostic testing for WNV. This test is able to confirm a WNV infection in patients with symptoms that have risk factors for infection such as potential exposure during mosquito season.
  • Plaque reduction neutralization test (PRNT): An antibody test called plaque reduction neutralization test (PRNT) detects antibodies that inactivate the West Nile virus. This test is primarily used to confirm the results of a MAC-ELISA if there is concern that the initial test result could be caused by an infection with another Flavivirus. A positive PRNT test result in combination with a positive MAC-ELISA test result confirms a WNV infection.

West Nile virus antibody testing is most often performed on a blood sample. If a doctor suspects that a severe WNV infection is affecting the central nervous system, additional antibody testing is conducted using a sample of cerebrospinal fluid, a liquid found in the brain and spinal cord.

Nucleic acid amplification testing (NAAT) identifies genetic material, called RNA, of the West Nile virus in samples of blood or CSF. NAAT is much less common than antibody testing, but may be recommended in several situations, including:

  • When results are needed more quickly
  • If the patient is a blood donor
  • If the patient is immunocompromised
  • If test results are inconclusive due a past Flavivirus infection

When should I get a West Nile virus test?

Testing for West Nile virus is suggested when patients are exhibiting symptoms of a WNV infection during mosquito season.

About 80% of people infected with WNV do not develop symptoms. When symptoms do develop, they often begin within 1 to 14 days after exposure and may last several days or up to a month or longer. Symptoms may include:

  • Fever
  • Headache and muscle aches
  • Sore throat
  • Swollen lymph nodes
  • Loss of appetite
  • Abdominal pain and nausea
  • Vomiting and diarrhea
  • Rash on the chest, back, and arms

A few patients with WNV, about 1 in 150, develop a severe infection that affects the nervous system. This condition, called neuroinvasive disease, results in inflammation of the brain or spinal cord. Symptoms of neuroinvasive disease include:

  • Fever
  • Irritability
  • Confusion and clumsiness
  • Stiff neck or back
  • Sensitivity to light

Finding a West Nile Virus Test

How to get tested

West Nile virus testing is usually ordered by a health care provider. Testing for WNV is most often conducted at a specialized laboratory such as a commercial reference or public health lab.

Can I take the test at home?

West Nile virus testing cannot be performed at home. Samples used for WNV diagnostic testing are collected in a medical facility and sent to a laboratory that performs this specialized testing.

How much does the test cost?

The cost of a West Nile virus test depends on various factors including where you have the test taken, whether other measurements or tests are included, and whether you have medical insurance.

The cost of testing may include an office visit, the fee for drawing your blood, and the laboratory fee for analyzing your sample. These costs may be covered by insurance when the test is prescribed by your physician, but check with your insurance plan for coverage details including whether you are responsible for a deductible or copay.

Taking the West Nile Virus Test

The West Nile virus test uses a blood or cerebrospinal fluid sample collected through a blood draw or lumbar puncture.

Before the test

A blood draw usually does not require any special preparation unless specified by your health care provider. In order to ensure the accuracy of the West Nile virus test, tell your doctor if you are taking any prescriptions or over the counter medications. Also inform your doctor if you have previously been infected with WNV or any other Flavivirus as this may affect your test results.

Tell your doctor if you are feeling sick or if there were any changes in symptoms prior to the test. The blood draw may be postponed or expedited depending on the severity of your symptoms.

If you have signs and symptoms of neuroinvasive disease, your doctor may order a spinal tap, also known as a lumbar puncture, to collect a sample of cerebrospinal fluid from your spine. Before a lumbar puncture, you may be asked to stop taking certain medications.

During the test

During a blood draw, a blood sample is taken by inserting a needle into a vein in your arm. An elastic band is first tied around your upper arm to increase the blood pressure in your vein. The site where the needle will be inserted is wiped clean with an antiseptic before the needle is inserted. A blood draw may cause a temporary sting or pinch as the needle is inserted. When completed, the elastic band and the needle are removed. A blood draw normally lasts for less than a minute.

There are several ways to collect cerebrospinal fluid, though lumbar puncture is the most common. During a lumbar puncture you will lie on your side or have the test done sitting up. Your back is then cleaned with antiseptic and a local numbing medicine is injected into your spine. A special needle will then be inserted and the sample collected into several vials. Collecting a sample of CSF takes around 30 minutes. You may feel some discomfort during this procedure, but pain should last only a few seconds.

After the test

When the blood draw is complete, a bandage and/or cotton swab is placed over the needle insertion site to minimize bleeding. You may be asked to keep this in place for an hour or more.

Depending on how much blood was drawn you may be allowed to return to normal activities such as walking and driving after the procedure or you may be asked to stay for a few minutes so that you can be observed. Contact your doctor if you notice persistent pain, bleeding, or signs of infection.

After a lumbar puncture, the needle is removed from the spine, the area is cleaned, and a bandage is placed over the needle site. After this procedure you may be asked to remain lying down for several hours to prevent fluid from leaking around the puncture site.

West Nile Virus Test Results

Receiving test results

Receiving test results for West Nile virus can take several days to weeks after the sample is collected. For those in need of an urgent diagnosis, NAAT results may be available faster.

Patients may receive their results from their health care provider during a follow-up appointment. In some cases, results may be provided over the phone, through the mail, in an online health portal, or through email.

Interpreting test results

Test results for West Nile virus are listed as either positive, negative, or inconclusive.

A positive test result means that antibodies to WNV or genetic material from the virus were found in the test sample. Test results may have varying interpretations depending on what test was conducted:

  • A positive antibody test result using a MAC-ELISA is generally sufficient to diagnose patients with symptoms, especially if the test sample was taken at least 8 days after symptoms began. If test results are in question or the sample was collected within 8 days of symptoms, an additional test sample may be collected two weeks later and tested for neutralizing antibodies. A positive PRNT test following a positive MAC-ELISA can confirm the diagnosis.
  • A positive NAAT test for WNV indicates a high probability of a WNV infection.

A negative test result means that no antibodies to WNV or genetic material from the virus were found in the test sample. The interpretation of negative test results includes:

  • A single negative antibody test result using MAC-ELISA does not rule out a WNV infection. Negative test results may be confirmed by collecting a second sample two weeks later and performing a second MAC-ELISA test or a PRNT test. A negative result from either of these tests rules out a WNV infection.
  • A negative NAAT test result cannot rule out WNV since it cannot detect every WNV infection. Antibody tests may be needed to rule out a WNV infection.

Inconclusive test results means that the test could neither confirm or deny the presence of genetic material or antibodies from the test sample.

Are test results accurate?

West Nile virus testing is used to diagnose or rule out a WNV infection. Medical professionals follow established guidelines in order to ensure the accuracy of test results.

A false negative test result means that the test result is negative even though the patient has a West Nile virus infection. False negative test results may occur if the sample was taken too early in the infection and the body did not have time to develop antibodies.

False positive test results means that the test result is positive even though the patient does not have a West Nile virus infection. False positive results may occur during MAC-ELISA testing due to recent vaccination or infection with another Flavivirus.

Do I need follow-up tests?

The need for follow up testing depends on the type of test conducted. Because initial negative test results do not rule out WNV as the cause of a person’s symptoms, follow-up testing with antibody tests is often required.

Questions for your doctor about test results

Receiving test results for West Nile virus may bring up many questions. Some questions that might be helpful when speaking with your doctor about your results include:

  • What was my test result?
  • Is my test conclusive for West Nile virus or another Flavivirus?
  • Does the test result suggest that I have immunity for West Nile virus?
  • Based on my test results, will I be needing any follow-up tests?
  • What can I do to minimize symptoms while I recover?

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