Reye's syndrome is diagnosed based on a medical history, a physical exam, and lab tests, if needed. In general, a diagnosis of Reye's syndrome is considered if:

• The child has recently had a viral illness such as flu or chickenpox and has taken medication that contains aspirin.
• A change in mental status (such as confusion) is noticed and liver problems are identified.
• There is no evidence to suggest that symptoms are caused by other diseases or conditions, such as kidney failure or problems with metabolism.

Lab tests to diagnose Reye's syndrome include:

• Analysis of blood and urine samples to evaluate liver function and to measure blood sugar (glucose) levels. These tests also help to rule out other medical problems as a cause of symptoms.
• A CT scan of the head to check for an injury or bleeding.
• A spinal tap (lumbar puncture) to check for infection in the spinal fluid and to measure the pressure of the fluid within the spinal column. This test is not specific for diagnosing Reye's syndrome. It is often done to eliminate other causes for the symptoms, such as brain and spinal infections.
• A liver biopsy, in which a sample of liver tissue is removed for examination under a microscope. This is the most accurate test to diagnose Reye's syndrome, although it is not always used because it can cause complications.