A 72-year-old woman is admitted to the hospital with acute onset confusion, disorientation, and visual hallucinations over the past 12 hours. She was previously independent and cognitively intact. On examination, she is agitated, her attention is poor, and she has fluctuating consciousness. Temperature is 38.5°C, heart rate 110/min. Urine dipstick shows leukocyte esterase and nitrites. Which investigation is most appropriate to establish the diagnosis?

Explanation

## Investigation of Choice for Acute Delirium with Suspected Infection **Key Point:** Urine culture and blood cultures are the most appropriate investigations to identify the underlying infectious cause of delirium in this patient. The clinical presentation—acute onset, fluctuating consciousness, poor attention, fever, and urinary tract signs—is classic for delirium secondary to urinary tract infection (UTI) or urosepsis. ### Clinical Features Pointing to Delirium (Not Dementia) | Feature | Delirium | Dementia | |---------|----------|----------| | **Onset** | Acute (hours to days) | Insidious (months to years) | | **Course** | Fluctuating throughout day | Gradual, progressive | | **Consciousness** | Altered (hyperalert or lethargic) | Normal until late stages | | **Attention** | Severely impaired | Relatively preserved early | | **Reversibility** | Often reversible if cause treated | Usually irreversible | | **Fever/Infection** | Common trigger | Not a feature | **High-Yield:** In acute delirium with fever and urinary symptoms, the priority is to identify and treat the underlying cause. Urine culture and blood cultures are diagnostic and guide antibiotic therapy. This is a medical emergency. **Mnemonic:** **I WATCH DEATH** — Common causes of delirium: - **I**nfection (UTI, pneumonia, sepsis) ← **This case** - **W**ithdrawal (alcohol, benzodiazepines) - **A**cute metabolic (hypoglycemia, electrolyte imbalance) - **T**oxins (medications, drugs) - **C**NS pathology (stroke, seizure) - **H**ypoxia - **D**eficiency (B12, thiamine) - **E**ndocrinopathy (thyroid, adrenal) - **A**rrhythmia (cardiac) - **T**emperature (fever, hypothermia) - **H**eavy metals ### Why Other Investigations Are Inappropriate Here **MRI brain:** Indicated if CNS pathology (stroke, bleed, tumor) is suspected, but the clinical picture and fever point to systemic infection. MRI is not urgent in this case. **EEG:** May show diffuse slowing in delirium, but it is not diagnostic and does not identify the cause. It is useful in suspected seizures or non-convulsive status epilepticus. **Lumbar puncture:** Reserved for suspected meningitis or encephalitis. The urinary signs and fever pattern suggest UTI/urosepsis, not CNS infection. LP is contraindicated if increased ICP is suspected and should not delay treatment of presumed sepsis. **Clinical Pearl:** Elderly women are at high risk for UTI-related delirium. The combination of fever, urinary symptoms (leukocyte esterase, nitrites), and acute mental status change is pathognomonic. Cultures (urine and blood) are essential to confirm diagnosis and guide antibiotic selection. [cite:Harrison 21e Ch 297]