CT Abdomen and Pelvis with IV Contrast
The most appropriate diagnostic test for this elderly patient is CT abdomen and pelvis with intravenous contrast. This presentation—left lower quadrant pain, fever (38°C), leukocytosis (14.0), anorexia, loose stools, and pain radiating to the back—strongly suggests acute diverticulitis, and CT with IV contrast is the gold standard for diagnosis, complication assessment, and treatment planning in this clinical scenario 1.
Why CT with IV Contrast is the Definitive Choice
CT with IV contrast provides 98-99% diagnostic accuracy for diverticulitis and identifies complications that determine whether the patient needs medical management, interventional drainage, or emergency surgery 1, 2. The American College of Radiology explicitly recommends CT with IV contrast as the initial imaging study for all elderly patients with acute abdominal pain because it confirms diagnosis and distinguishes between complicated and uncomplicated conditions 3.
Key Advantages in This Clinical Context
Superior diagnostic accuracy: CT achieves 98% sensitivity and specificity for diverticulitis, far exceeding clinical assessment alone which has a 34-68% misdiagnosis rate 1, 2.
Complication detection: CT identifies abscess, perforation, extraluminal air, fistula, and obstruction—all of which fundamentally alter management from outpatient antibiotics to interventional drainage or emergency surgery 1.
Risk stratification: CT findings predict treatment failure, need for surgery, and recurrence risk, with features like retroperitoneal abscess, extraluminal air, and longer segments of involved colon indicating worse outcomes 1.
Alternative diagnoses: In elderly patients with atypical presentations, CT identifies other serious pathologies (perforated cancer, ischemic bowel, pancreatitis) in 23-45% of cases 1.
Critical Clinical Considerations in Elderly Patients
Elderly patients present atypically—only 50% have lower quadrant pain and only 17% have fever despite severe diverticulitis 3, 2. Laboratory values can be normal despite severe infection 3. This makes imaging absolutely essential rather than optional, as clinical diagnosis is unreliable in this age group.
The Contrast Question
IV contrast is strongly recommended and should not be withheld due to concerns about renal function 3, 2. While IV contrast improves detection of subtle bowel wall abnormalities and abscesses by distinguishing them from adjacent bowel 1, the benefit of rapid accurate diagnosis outweighs the risk of contrast-induced acute kidney injury, which recent evidence suggests is lower than previously thought 3. If absolute contraindication exists (severe renal disease, anaphylactic contrast allergy), unenhanced CT is acceptable and more accurate than clinical evaluation alone 1, 3.
What CT Will Reveal and How It Changes Management
The CT will show:
Uncomplicated diverticulitis (bowel wall thickening >4mm, inflamed diverticula, pericolic fat stranding): Outpatient oral antibiotics, avoiding 50% of unnecessary admissions 1, 2.
Abscess ≥4 cm: Percutaneous CT-guided drainage plus IV antibiotics 2.
Small abscess <4 cm: IV antibiotics alone for 7 days 2.
Free perforation with peritonitis: Emergency surgical consultation for laparotomy and colonic resection 2.
Extraluminal air <5 cm from affected segment: Medical therapy 1.
Spilled feces or free perforation: Surgical management 1.
Common Pitfalls to Avoid
Do not delay CT waiting for clinical response to empiric antibiotics in an elderly patient with fever and leukocytosis 3, 2. CT influences treatment plans in 65% of elderly patients with acute abdominal pain and changes surgical management in 48% of these cases 3.
Do not assume low inflammatory markers exclude serious pathology—39% of patients with complicated diverticulitis have CRP below 175 mg/L 3, 2.
Do not order colonoscopy acutely—this is contraindicated in acute diverticulitis and should be deferred 6-8 weeks after resolution for age-appropriate colon cancer screening, given his lack of prior screening 1.
Alternative Imaging (Inferior Options)
Ultrasound: While European guidelines support US as first-line 1, it has lower sensitivity in obese patients and distal sigmoid diverticulitis, requires 500 examinations for competency, and has lower specificity than CT 1. Not appropriate as initial test in this high-risk elderly patient.
MRI: Insufficient data to support routine use for diverticulitis, with likely lower accuracy than CT 1. Reserved for pregnant patients or specific contraindications.
Plain radiography: Only useful for detecting free air from perforation or obstruction; does not diagnose diverticulitis 1.