Parents and caregivers who believe a child may have Reye syndrome should seek immediate medical attention. Early identification can help reduce the risk of long-term complications (e.g., brain damage, coma) and death.

The first step in diagnosing Reye syndrome is a thorough medical history. This may include questions by a pediatrician about recent viral illnesses (e.g., colds, the flu, chickenpox) and all medications taken recently (e.g., aspirin or any medication containing salicylic acid) by a child. Parents and other adults may be asked about any unusual changes in the mental alertness of the child.

A physical examination may be performed to look for any other signs of illness. A neurological examination may be performed to rule out disorders of the central nervous system. In addition, the following tests may be performed to diagnose Reye syndrome or to rule out other conditions:

• Liver function tests. These are blood tests that identify levels of certain substances (e.g., liver enzymes, glucose, ammonia) that may indicate dysfunction of the liver, which may indicate the presence of Reye syndrome or another condition.
  
  
• Liver biopsy. A tissue sample (taken by needle inserted through abdomen and into the liver) is analyzed for fat accumulation, a sign of Reye syndrome. This test is also used to rule out other possible diseases that may affect the liver.
  
  
• Spinal tap. Analysis of cerebral spinal fluid (CSF). This may help rule out other conditions, such as meningitis or encephalitis.
  
  
• Imaging tests. These may include computed axial tomography (CAT scan) or magnetic resonance imaging (MRI) of the head to rule out other potential causes of symptoms. A CAT scan may also show brain swelling associated with Reye syndrome.

Reye syndrome may be confused with a variety of rare metabolic disorders. Most of these disorders are hereditary and can be identified through blood tests and urine tests. They are sometimes referred to as mitochondrial diseases because problems involve the mitochondria (energy-making structures within cells). Urine tests may also rule out poisoning or drug overdose as the cause of a patient’s symptoms.