A 58-year-old man with diabetes mellitus presents to the emergency department with fever (39.2°C), tachycardia (HR 118/min), tachypnea (RR 24/min), and hypotension (BP 92/58 mmHg). Blood cultures have been drawn. Clinical examination reveals a urinary catheter in situ with cloudy urine. Which investigation is most appropriate to confirm the source of sepsis and guide antimicrobial therapy?

## Investigation of Choice in Sepsis with Suspected Urinary Source **Key Point:** In a septic patient with clinical signs pointing to a specific anatomical source, targeted microbiological sampling from that site is essential for organism identification and antimicrobial susceptibility testing. ### Clinical Context This patient presents with: - Sepsis criteria: fever, tachycardia, tachypnea, hypotension - Indwelling urinary catheter (major risk factor for urinary tract infection) - Cloudy urine (suggests pyuria and bacteriuria) ### Why Urine Culture and Urinalysis? **High-Yield:** Urine culture is the **gold standard** for diagnosing urinary tract infection as the source of sepsis. It provides: 1. **Organism identification** — allows targeted antibiotic selection 2. **Quantitative bacteriuria** — ≥10^5 CFU/mL in symptomatic patients confirms UTI 3. **Antimicrobial susceptibility** — guides de-escalation after initial broad-spectrum coverage 4. **Rapid turnaround** — urinalysis (nitrites, leukocyte esterase, WBCs) available within 1 hour **Clinical Pearl:** In catheterized patients, any bacteriuria with symptoms warrants treatment; the threshold is lower than in non-catheterized patients due to increased virulence and biofilm formation on the catheter. ### Diagnostic Approach in Sepsis | Investigation | Indication | Timing | |---|---|---| | **Urine culture + urinalysis** | Suspected UTI/urosepsis (catheter, dysuria, pyuria) | Immediate | | Blood cultures | All septic patients | Before antibiotics | | Chest X-ray | Respiratory symptoms or hypoxia | If indicated | | CT abdomen | Unclear source, peritonitis, obstruction | After stabilization | | Sputum/ETT culture | Pneumonia suspected | If respiratory symptoms | **Mnemonic: SEPSIS Source Identification — **S**kin/soft tissue, **E**ndocarditis (blood cultures), **P**eritonitis (imaging + paracentesis), **S**eptic arthritis (synovial fluid), **I**ntra-abdominal (imaging), **S**pinal/meningitis (CSF)** ### Why Blood Cultures Matter Blood cultures are **mandatory** in all septic patients (already done here) but do not identify the source — they confirm bacteremia. Source identification requires sampling from the suspected site (urine, sputum, CSF, wound, etc.).