The usual method of diagnosis is through detection of antibodies against the virus, produced by the person infected as part of the immune response. The test is known as an IgM antibody-capture assay and can be performed on spinal fluid and serum - usually within one week of the onset of illness. Further testing of serum samples is recommended to confirm a diagnosis of WNV using virus-neutralisation tests. The virus or its genetic material (RNA) can also be detected in serum, cerebrospinal fluid, or other tissues using a method called PCR. However, this method is not generally recommended in the clinical setting because of its low sensitivity.
Diagnosis usually is made through review of the patient's symptoms and exposure (for example, history of travel to an endemic area, such as the U.S.A) and confirmed by testing the patient's blood and spinal fluid at a reference or public health laboratory. In the USA, nucleic acid tests and antibody testing is also used to screen all blood donors for WNV. The Australian Red Cross does not routinely screen for WNV but will utilise only the plasma portion of blood donations from travelers who have recently visited endemic areas for 8 weeks after return to Australia to minimise any risk of transmission.