What is the most appropriate initial diagnostic study for a 79-year-old woman with altered mental status, combativeness, and suspected delirium?

Urinalysis is the Most Appropriate Initial Diagnostic Study for This Patient with Delirium

A urinalysis is the most appropriate initial diagnostic study for this 79-year-old woman presenting with acute altered mental status and behavioral changes consistent with delirium.

Clinical Presentation Analysis

This patient presents with classic features of delirium:

• Acute onset of altered mental status (3-day history)
• Fluctuating course (worse at night)
• Inattention and altered level of consciousness (not answering questions)
• Behavioral changes (combativeness, yelling, striking out)
• Out of character behavior (normally pleasant and agreeable)
• Low-grade fever (37.9°C/100.2°F)

Diagnostic Approach for Delirium

Step 1: Initial Laboratory Testing

• Urinalysis should be performed first in this elderly patient 1, 2
  • Urinary tract infection (UTI) is one of the most common causes of delirium in elderly patients
  • The patient's low-grade fever further supports this possibility
  • UTIs in elderly patients often present atypically with altered mental status rather than typical urinary symptoms

Step 2: Additional Basic Laboratory Testing

After urinalysis, these tests should be ordered:

• Complete blood count (to evaluate for infection, anemia)
• Basic metabolic panel (to assess electrolyte abnormalities)
• Liver function tests
• Blood cultures (given the fever)
• Thyroid function tests 2

Step 3: Imaging and Additional Testing

• CT scan of the head would be appropriate after initial laboratory evaluation if no clear cause is identified or if focal neurologic deficits develop 1
  • The yield of neuroimaging in altered mental status is relatively low (11-16.5% in studies) 1
  • Without focal neurologic deficits, the likelihood of finding acute intracranial pathology is lower

Why Urinalysis is the First Choice

1. High yield in elderly patients: UTIs are among the most common causes of delirium in elderly patients, especially those from extended care facilities 1, 2

2. Rapid, non-invasive test: Provides immediate results that can guide treatment decisions

3. Cost-effective approach: Following a hierarchical testing approach prevents unnecessary expensive testing 1

4. Guideline-supported: Clinical guidelines recommend urinalysis as part of the first-tier workup for delirium 1, 2

Why Other Options Are Less Appropriate Initially

• Blood cultures: Appropriate but should follow urinalysis; less likely to provide immediate actionable results
• CT scan of head: Appropriate if initial workup is negative or if focal neurologic deficits develop
• Fecal smear for ova and parasites: Low yield given the acute presentation; chronic diarrhea would warrant this test later
• Lumbar puncture: Too invasive as an initial test without signs of meningitis or after negative initial workup

Common Pitfalls in Delirium Evaluation

1. Attributing behavioral changes to psychiatric causes without adequate medical workup
2. Overreliance on neuroimaging before basic laboratory testing
3. Missing atypical presentations of common infections in elderly patients
4. Failure to recognize medication side effects or interactions

Management Considerations

If UTI is confirmed:

• Initiate appropriate antibiotic therapy
• Provide supportive care and reorientation strategies
• Avoid physical restraints when possible
• Consider low-dose antipsychotics only if necessary for severe agitation 3
• Address underlying chronic diarrhea once acute delirium resolves

This patient's presentation strongly suggests delirium from an infectious cause, with UTI being the most likely culprit given the epidemiology and clinical features. Starting with urinalysis represents the most efficient and evidence-based approach to diagnosis.