What labs should be ordered for a patient presenting with altered mental status?

Laboratory Evaluation for Altered Mental Status

For patients presenting with altered mental status, a comprehensive laboratory workup should include complete blood count, basic metabolic panel, liver function tests, urinalysis, blood cultures (if febrile), thyroid function tests, and ammonia levels (if hepatic encephalopathy is suspected). 1

Primary Laboratory Tests

The following laboratory tests should be ordered for all patients with altered mental status:

• Complete Blood Count (CBC)

  • Identifies infections, anemia, and other hematologic abnormalities
  • Elevated white blood cell count may indicate infection as a cause of AMS

• Basic Metabolic Panel (BMP)

  • Detects electrolyte abnormalities (sodium, potassium, calcium)
  • Identifies renal dysfunction, acid-base disturbances
  • Glucose level to rule out hypoglycemia or hyperglycemia

• Liver Function Tests (LFTs)

  • Evaluates for hepatic encephalopathy
  • Identifies liver dysfunction that may contribute to metabolic encephalopathy

• Urinalysis

  • Screens for urinary tract infections
  • Detects ketones (diabetic ketoacidosis)

• Blood Glucose

  • All protocols across EMS agencies require blood glucose checks 2
  • Treatment typically recommended for adults with glucose <60 mg/dL

Additional Laboratory Tests

Based on clinical suspicion and initial findings:

• Blood Cultures

  • Indicated if patient is febrile or infection is suspected
  • Should be collected before antibiotic administration

• Thyroid Function Tests

  • Particularly important in elderly patients
  • Screens for thyroid disorders that can cause altered mental status

• Ammonia Level

  • Indicated when hepatic encephalopathy is suspected
  • Particularly important in patients with known liver disease 3

• Toxicology Screen

  • Indicated when drug overdose or toxicity is suspected
  • Consider specific drug levels based on history or presentation

Imaging and Additional Studies

While not laboratory tests, these studies are often ordered concurrently:

• Brain Imaging

  • CT head without contrast is usually the first-line imaging study 3
  • MRI brain may be indicated for further evaluation
  • Indicated when no clear cause is identified from laboratory tests, when focal neurologic deficits are present, or when there is a history of trauma 1

• Lumbar Puncture with CSF Analysis

  • Indicated when infection (meningitis, encephalitis) is suspected
  • Should be performed after brain imaging if increased intracranial pressure is a concern

• Electrocardiogram (ECG)

  • Evaluates for cardiac causes of altered mental status
  • Only 7% of cases show diagnostic findings for AMS 4

Diagnostic Yield Considerations

It's important to note that the diagnostic value of laboratory studies varies:

• History of present event (51% yield)
• Past medical history (43% yield)
• Physical examination (41% yield)
• Laboratory studies have lower diagnostic yield (chemistry panel 5%, CBC 1%) 4

Special Populations

Patients with Cirrhosis

For patients with cirrhosis and altered mental status:

• Investigate both hepatic encephalopathy and liver-unrelated causes
• Routine ammonia level testing is not recommended, but low levels should prompt investigation of other etiologies 3

Elderly Patients

In elderly patients with altered mental status:

• More comprehensive laboratory evaluation is warranted
• Consider additional tests for B12 deficiency and thyroid disorders 3

Common Pitfalls to Avoid

• Overreliance on neuroimaging before basic laboratory testing
• Missing atypical presentations of common infections in elderly patients
• Failure to recognize medication side effects or interactions
• Excessive laboratory testing without clinical correlation 1

Remember that while laboratory tests are essential, the history and physical examination remain the most valuable diagnostic tools in evaluating patients with altered mental status.