What lab tests can cause new onset of confusion?

Laboratory Tests That Can Cause New Onset of Confusion

The most important laboratory abnormalities to evaluate in patients with new onset confusion include electrolyte disorders, metabolic derangements, infections, and toxicologic causes, as these are common precipitating factors that can directly lead to delirium and altered mental status.

Common Laboratory Abnormalities Causing Confusion

Metabolic Abnormalities

• Hypoglycemia and hyperglycemia are critical metabolic causes of confusion that should be evaluated with immediate blood glucose testing 1
• Hyponatremia and other electrolyte disorders (including hypocalcemia and hypomagnesemia) can directly cause altered mental status and should be assessed through basic metabolic panels 1
• Uremia from renal failure can lead to confusion and encephalopathy, requiring assessment of BUN and creatinine 1

Infectious Causes

• Sepsis and systemic infections can precipitate confusion and delirium, necessitating complete blood count, blood cultures, and inflammatory markers 1
• Central nervous system infections (meningitis, encephalitis) should be considered with appropriate testing including lumbar puncture when clinically indicated 1
• Urinary tract infections and pneumonia are particularly common infectious causes of delirium in elderly patients 1

Toxicologic and Medication-Related Causes

• Drug levels for medications with narrow therapeutic windows (e.g., digoxin, lithium, anticonvulsants) should be checked when relevant 1
• Comprehensive toxicology screening can identify substance intoxication or withdrawal, including alcohol, benzodiazepines, and opioids 1, 2
• Medication interactions, particularly with central nervous system depressants, can cause confusion and should be evaluated 1, 2

Hepatic Abnormalities

• Liver function tests to evaluate for hepatic encephalopathy, especially in patients with known cirrhosis 1
• Ammonia levels may be checked, though a low ammonia level in confused patients should prompt investigation of other etiologies 1

Special Considerations

Delirium Workup

• Delirium is a medical emergency requiring prompt laboratory evaluation, as mortality may be twice as high if diagnosis is missed 1
• Multiple coexisting precipitating factors are common in delirium, necessitating comprehensive laboratory testing 1
• Laboratory testing should be guided by clinical presentation, but routine screening should include glucose, electrolytes, renal function, liver function, complete blood count, and urinalysis 1

When to Consider Neuroimaging

• Brain imaging (CT or MRI) should be considered when laboratory evaluation does not reveal a clear cause of confusion 1
• First episode of altered mental status, focal neurological signs, seizures, or inadequate response to therapy are indications for neuroimaging 1, 3
• Routine brain imaging in patients with recurrent episodes similar to prior episodes of hepatic encephalopathy is not warranted 1

Approach to Laboratory Testing in Confused Patients

Initial Laboratory Panel

• Basic metabolic panel (sodium, potassium, chloride, bicarbonate, BUN, creatinine, glucose) 1
• Complete blood count with differential 1
• Liver function tests 1
• Urinalysis and urine culture 1
• Blood cultures if infection is suspected 1
• Arterial blood gas if respiratory distress or acid-base disturbance is suspected 1

Second-Line Testing Based on Clinical Suspicion

• Thyroid function tests 1
• Vitamin B12 and folate levels 1
• Toxicology screening and specific drug levels 1, 2
• Inflammatory markers (ESR, CRP) 1
• Ammonia level (particularly in patients with known liver disease) 1
• Lumbar puncture if CNS infection is suspected 1

Common Pitfalls to Avoid

• Failing to check glucose levels promptly in confused patients, as hypoglycemia is a rapidly reversible cause 1
• Relying solely on ammonia levels to diagnose or exclude hepatic encephalopathy 1
• Overlooking medication effects and interactions as causes of confusion 1, 2
• Assuming a single laboratory abnormality explains confusion when multiple factors often contribute 1
• Delaying appropriate neuroimaging when laboratory evaluation is unrevealing 1, 3
• Neglecting to repeat laboratory testing if confusion persists despite initial normal results 1