Although it has some fairly distinct signs and symptoms, HE is a diagnosis of exclusion because numerous other conditions can cause similar symptoms, including metabolic problems (e.g., hypoglycemia, renal failure), cerebral vascular incidents (stroke), drug toxicity (alcohol, narcotic, or benzodiazepine overdose), and infection (encephalitis, meningitis). Thus, patients with signs and symptoms of significant HE, particularly when acute in onset, will typically undergo blood testing (complete blood count, blood glucose, electrolytes, toxicology screen, liver and kidney function tests) and often a head CT (to exclude bleeding or other structural problems). However, treatment, most often with lactulose, will usually also be started while awaiting results of these studies when the diagnosis of HE is suspected.
If present, an elevated blood ammonia level is helpful to confirm a diagnosis of HE, but a normal level does not completely rule out the diagnosis. A blood sample for ammonia testing is best taken from arteries, as they are probably more reliable than samples taken from a vein. The blood sample must be processed with haste in order to obtain accurate results. Following ammonia levels on patients with known HE, particularly in the outpatient setting, is of limited value.
Patients with minimal HE have few if any obvious signs and symptoms of the disorder. The diagnosis currently rests on formal psychiatric testing, although ongoing research is focused on developing clinical tools to simplify and streamline the diagnosis.