What labs should be ordered in the emergency department (ED) for a patient presenting with a seizure?

Laboratory Testing for Seizures in the Emergency Department

For otherwise healthy adults with new-onset seizures who have returned to baseline neurologic status, order serum glucose and sodium levels, and obtain a pregnancy test for women of childbearing age. 1

Core Laboratory Tests (Level B Recommendations)

Serum glucose and sodium are the only routinely indicated tests for all adult seizure patients, as these represent the most common metabolic abnormalities associated with seizures. 1, 2

• Serum glucose: Hypoglycemia is one of the few metabolic causes that may not be clinically apparent, though prospective studies found only 1-2 unexpected cases of hypoglycemia in 136-163 patients. 3, 4
• Serum sodium: Hyponatremia is typically predictable from clinical history, with only 1 case of unsuspected hyponatremia (psychogenic water ingestion) found in 98 prospectively studied patients. 1, 5
• Pregnancy test: Required for all women of childbearing age, as pregnancy affects testing decisions, disposition, and antiepileptic drug therapy initiation. 1, 2

The evidence is clear that extensive routine laboratory testing has extremely low yield. In a prospective study of 136 patients, only 2 cases of metabolic abnormalities (both hypoglycemia) were not suspected based on history and physical examination alone. 4

Additional Testing Based on Specific Clinical Scenarios

For Immunocompromised Patients

• Lumbar puncture (after head CT scan) is indicated due to higher rates of CNS infections presenting as seizures. 1, 3
• Complete metabolic panel should be obtained. 3

For Patients with Specific Risk Factors

• Calcium and magnesium: Only check in patients with known renal failure, cancer, malnutrition, or those taking diuretics—not routinely. 1, 2, 5
• Magnesium: Specifically indicated for suspected alcohol-related seizures due to common hypomagnesemia in this population. 2, 3
• Drug/toxicology screen: Consider in first-time seizures with suspected substance use, though no prospective studies demonstrate benefit of routine use. 1, 3

For Patients with Fever or Altered Mental Status

• Lumbar puncture after head CT if fever, meningeal signs, or persistent altered consciousness is present. 1, 5
• Complete blood count to evaluate for infection. 2

Critical Pitfalls to Avoid

Do not order extensive electrolyte panels routinely. There are no prospective studies supporting routine measurement of calcium, magnesium, or phosphate in otherwise healthy patients. 1 Studies consistently show that when metabolic abnormalities are found, they are almost always predictable from history and physical examination. 3, 4

Alcohol withdrawal seizures should be a diagnosis of exclusion, especially in first-time seizures, as structural lesions are common even in this population. 2, 3

Laboratory testing does not replace neuroimaging. Head CT is indicated for most new-onset seizure patients based on age >40 years, focal deficits, immunocompromise, trauma history, anticoagulation, or malignancy—regardless of normal laboratory values. 1

Algorithm for Laboratory Ordering

All Adult Seizure Patients:

• Serum glucose 1
• Serum sodium 1
• Pregnancy test (women of childbearing age) 1

Add Based on Clinical Features:

• Immunocompromised, fever, or meningeal signs: Complete metabolic panel, CBC, lumbar puncture after CT 1, 2, 3
• Known renal disease, malnutrition, or diuretic use: Calcium, magnesium, phosphate 1, 2
• Suspected alcohol-related seizure: Magnesium 2, 3
• Suspected substance use: Toxicology screen 1, 3
• Persistent altered consciousness: Consider arterial blood gas if respiratory compromise present 5

The key principle is that history and physical examination should guide additional testing beyond glucose, sodium, and pregnancy testing. Routine "seizure panels" are not evidence-based and represent unnecessary cost without clinical benefit. 4