What is the best CT scan protocol with or without contrast for assessing a patient with suspected kidney stones or appendicitis?

CT Imaging Protocol for Kidney Stones and Appendicitis

For suspected kidney stones, obtain CT abdomen and pelvis WITHOUT IV contrast (stone protocol); for suspected appendicitis, obtain CT abdomen and pelvis WITH IV contrast; when both diagnoses are being considered simultaneously, start with non-contrast CT and add contrast-enhanced imaging if appendicitis remains a concern after initial evaluation. 1, 2

Kidney Stone Imaging: Non-Contrast CT is Gold Standard

Non-contrast CT abdomen and pelvis ("stone protocol") is the reference standard for kidney stone evaluation, with 97% sensitivity and 95% specificity. 2, 3

Why Non-Contrast CT for Stones:

• Virtually all renal calculi are radiopaque on CT, allowing accurate detection of even small stones without requiring IV contrast. 2, 3
• IV contrast may obscure small renal stones within the collecting system as the enhancing renal parenchyma can mask stone visualization. 1, 3
• Non-contrast CT provides precise stone measurements and exact anatomical location within the ureter, which is crucial for determining whether observation versus intervention is appropriate. 2
• Secondary signs of obstruction including periureteral inflammation, perinephric inflammation, and ureteral dilatation are readily visible without contrast. 1, 2

Radiation Dose Considerations:

• Low-dose CT protocols (<3 mSv) maintain diagnostic accuracy with sensitivity of 97% and specificity of 95% while minimizing radiation exposure. 1, 2, 3
• Low-dose CT is comparable to standard-dose CT for detecting ureteral stones and visualizing hydronephrosis, with sensitivity, specificity, and accuracy ranging from 92% to 99%, 86% to 93%, and 92% to 98%, respectively. 1
• The sensitivity for stones ≤2 mm in diameter is lower at 68% to 79% with low-dose protocols, but this rarely affects clinical management. 1

Appendicitis Imaging: IV Contrast is Preferred

For suspected appendicitis in non-pregnant adults, CT abdomen and pelvis WITH IV contrast is the recommended initial imaging modality. 1

Why IV Contrast for Appendicitis:

• CT with IV contrast has sensitivities ranging from 90% to 100% and specificities ranging from 94.8% to 100% for diagnosing appendicitis. 1, 4
• IV contrast enhancement improves visualization of appendiceal wall thickening, periappendiceal inflammation, and potential complications such as abscess formation or perforation. 1
• Contrast-enhanced CT can detect alternative diagnoses that may explain right lower quadrant pain, including ovarian pathology, inflammatory bowel disease, or other intra-abdominal processes. 1, 5

Non-Contrast CT Performance for Appendicitis:

• Non-contrast CT for appendicitis is conclusive in only 75% of cases, with sensitivity of 90%, specificity of 96%, PPV of 84.8%, and NPV of 97.4%. 1
• In the 25% of inconclusive non-contrast cases, follow-up contrast-enhanced CT improves sensitivity to 95.6%, specificity to 92.3%, PPV to 73%, and NPV to 99%. 1
• Non-contrast CT has lower sensitivity (0.91) compared to CT with intravenous contrast (0.96), CT with rectal contrast (0.97), or CT with intravenous and oral contrast (0.96). 4

Oral Contrast Considerations:

• Oral contrast is not required for appendicitis diagnosis, as CT without enteral contrast has sensitivities of 90% to 100% and specificities of 94.8% to 100%. 1
• Eliminating oral contrast regimens avoids delays in diagnosis and treatment, which is particularly important given the risk of perforation with delayed management. 1
• In one study of 1,922 patients, CT with IV contrast but without enteral contrast yielded 100% sensitivity and 99.5% specificity for appendicitis, with only 0.2% requiring repeat imaging. 1

Clinical Algorithm When Both Diagnoses Are Considered

Step 1: Initial Clinical Assessment

• If patient history includes urolithiasis and urinalysis is positive for blood, there is 94% sensitivity for predicting renal calculi, favoring non-contrast CT as initial test. 1
• If patient has no history of renal stones and negative urinalysis, 15% will have a non-renal stone diagnosis requiring contrast-enhanced CT for optimal evaluation. 1

Step 2: Initial Imaging Selection

• Start with non-contrast CT abdomen and pelvis (stone protocol) when kidney stones are the primary clinical suspicion. 1, 2
• If non-contrast CT definitively identifies stones explaining the clinical presentation, no additional imaging is needed. 2, 3
• If non-contrast CT is negative for stones but appendicitis remains a diagnostic consideration, proceed to contrast-enhanced CT. 1

Step 3: Problem-Solving Approach

• When non-contrast CT shows equivocal findings for appendicitis (appendix not visualized or borderline findings), add IV contrast to improve diagnostic accuracy. 1
• CT signs of appendicitis on non-contrast imaging include: maximal outer diameter >8.2 mm (sensitivity 88.8%, specificity 93.4%), periappendiceal fat stranding, and absence of intraluminal gas. 1
• When two or more CT signs of appendicitis are present on non-contrast imaging, the odds ratio for appendicitis is 6.8, but contrast-enhanced imaging still provides superior diagnostic confidence. 1

Special Populations and Scenarios

Pregnant Patients:

• Ultrasound is the first-line imaging modality for both kidney stones and appendicitis in pregnant patients to avoid radiation exposure. 1, 3
• MRI abdomen and pelvis without IV contrast is the preferred second-line test if ultrasound is inconclusive in pregnancy. 1

Pediatric Patients:

• Ultrasound is the recommended initial imaging modality for suspected appendicitis in children and adolescents. 1
• If ultrasound is equivocal or nondiagnostic, CT abdomen and pelvis WITH IV contrast or MRI abdomen and pelvis without IV contrast are appropriate next steps. 1
• For kidney stones in children, low-dose non-contrast CT remains the gold standard when imaging is required. 2

Recurrent Stone Formers:

• Limit CT scans to the area of interest or use ultra-low-dose protocols to reduce cumulative radiation exposure in patients with recurrent stone disease. 2, 3

Complicated Presentations:

• For patients with diabetes, immunocompromised status, or lack of response to therapy, CT abdomen and pelvis WITH IV contrast is appropriate to detect complications like renal abscess, emphysematous pyelonephritis, or alternative diagnoses. 2

Common Pitfalls to Avoid

• Do not order CT with IV contrast as first-line imaging for suspected kidney stones, as contrast may obscure small stones and provides no diagnostic advantage. 1, 2, 3
• Do not rely solely on non-contrast CT for appendicitis diagnosis when clinical suspicion is moderate to high, as 25% of cases will be inconclusive and require contrast-enhanced imaging. 1
• Do not order CT abdomen and pelvis without AND with IV contrast (dual-phase) for either condition, as there is no relevant literature supporting this approach and it unnecessarily doubles radiation exposure. 1
• Do not assume that low-dose CT protocols compromise diagnostic accuracy for either kidney stones or appendicitis, as they are non-inferior to standard-dose protocols for both conditions. 1, 2, 3
• Do not delay imaging for oral contrast administration in suspected appendicitis, as IV contrast alone provides excellent diagnostic accuracy and eliminates 1-3 hour delays. 1, 6