Diagnosis

Besides performing a physical exam, your doctor can confirm the presence of West Nile virus or a West Nile-related illness, such as meningitis or encephalitis, by performing one of the following tests:

• Lab tests. If you're infected, a blood test may show a rising level of antibodies to the West Nile virus. Antibodies are immune system proteins that attack foreign substances, such as viruses. A blood test may not show antibodies at first; another test may need to be done a few weeks later to show the rising level of antibodies.
• Spinal tap (lumbar puncture). The most common way to diagnose meningitis is to analyze the cerebrospinal fluid surrounding your brain and spinal cord. A needle inserted between the lower vertebrae of your spine is used to remove a sample of fluid for analysis in a lab. The fluid sample may show an elevated white blood cell count — a signal that your immune system is fighting an infection — and antibodies to the West Nile virus. If the sample doesn't show antibodies, another test may be done a few weeks later.
• Brain tests. In some cases, doctors may order electroencephalography (EEG) — a procedure that measures your brain's activity — or an MRI scan to help detect brain inflammation.