Laboratory Testing for Delirium
Order a focused laboratory panel including complete blood count, comprehensive metabolic panel, urinalysis, blood glucose, and thyroid function tests, guided by clinical presentation rather than as an extensive routine battery. 1
First-Tier Essential Laboratory Tests
The 2025 Alzheimer's Association guidelines and American College of Emergency Physicians recommend the following core laboratory investigations for delirium workup 2, 1:
- Complete blood count (CBC) to assess for infection, anemia, or hematologic abnormalities 2, 1
- Complete metabolic panel including electrolytes, renal function (BUN, creatinine), liver function tests, calcium, and glucose 2, 1
- Urinalysis and urine culture as urinary tract infection is one of the most frequent precipitating causes 1
- Thyroid-stimulating hormone (TSH) to evaluate thyroid dysfunction 2, 1
- Vitamin B12 level to assess for deficiency 2
Additional Laboratory Tests Based on Clinical Suspicion
When specific etiologies are suspected based on history and examination, consider 2:
- Ammonia level if hepatic encephalopathy is suspected 2
- Blood cultures if sepsis is a concern 1
- Arterial blood gas for suspected hypoxia or acid-base disturbances 1
- Toxicology screen if substance intoxication or withdrawal is suspected 1
- Alcohol level and assessment for withdrawal syndrome 1
- Medication levels (e.g., digoxin, anticonvulsants) if toxicity is suspected 1
Specialized Testing for Atypical or Refractory Cases
For patients with unexplained delirium despite initial workup, or those with atypical presentations, the 2025 Alzheimer's Association guidelines recommend considering 2:
- Folate level 2
- Methylmalonic acid (MMA) for more sensitive B12 deficiency assessment 2
- Homocysteine 2
- Hemoglobin A1c (HgbA1c) 2
- Rapid plasma reagin (RPR) or VDRL for neurosyphilis 2
- HIV testing 2
- Lyme antibody in endemic areas 2
- Antinuclear antibody (ANA) and other autoimmune markers if autoimmune encephalitis is suspected 2
Ancillary Diagnostic Studies
Beyond laboratory testing, the following should be considered 1:
- Electrocardiogram to assess for myocardial ischemia or arrhythmias 1
- Chest radiography to evaluate for pneumonia, as this is one of the most common precipitating infections 1
Critical Clinical Approach
Laboratory testing should be selective and guided by clinical evaluation rather than performed as an extensive routine battery. 1 The key is to identify reversible underlying causes while avoiding unnecessary testing that delays treatment 1, 3.
Common Pitfalls to Avoid:
- Do not order extensive panels reflexively without clinical indication, as this leads to false positives and delays appropriate management 1
- Do not attribute symptoms to pre-existing dementia without investigating for acute reversible causes, as delirium commonly occurs superimposed on dementia 1
- Do not miss hypoactive delirium, which is frequently mistaken for depression or fatigue and represents the most common missed diagnosis 1
- Do not forget to review all medications, particularly recent additions or dose changes, with special attention to anticholinergic burden, vasodilators, diuretics, antipsychotics, and sedative/hypnotics 1
Timing and Monitoring:
Delirium assessment should be performed every 8-12 hours (at least once per shift) using validated tools like the Confusion Assessment Method (CAM) or CAM-ICU, as mental status fluctuates substantially throughout the day 2, 1.