What is the role of blood and urine tests in the diagnosis of encephalitis?

Updated: Aug 07, 2018
  • Author: David S Howes, MD; Chief Editor: Barry E Brenner, MD, PhD, FACEP  more...
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A complete blood count (CBC) with differential should be performed, although findings are often within the normal range.

Serum electrolyte levels are usually normal unless dehydration is present; the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) occurs in 25% of patients with St Louis encephalitis.

The serum glucose level should be determined to rule out confusion due to treatable hypoglycemia and to compare with the cerebrospinal fluid (CSF) glucose value. Low serum results are found in nutritionally deprived patients, while diabetic patients may present with elevated glucose levels compatible with complicating hyperosmolar state or diabetic ketoacidosis.

Blood urea nitrogen (BUN) and creatinine levels are helpful to assess hydration status, and liver function tests should be performed to assess for end-organ dysfunction or the need to adjust antimicrobial therapy dosing regimens.

A lumbar puncture (LP) should be performed on all patients suspected of having a viral encephalitis. A platelet count and coagulation profile are indicated in patients who are chronic alcohol users, have liver disease, and those in whom disseminated intravascular coagulation (DIC) is suspected. The patient may require platelets or fresh frozen plasma (FFP) before LP.

A urinary electrolyte test should be performed if SIADH is suspected. Urine or serum toxicology screening may be indicated in selected patients presenting with a toxic delirium or confusional state.

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