Can small renal calculi (kidney stones) be missed on a computed tomography (CT) scan with contrast?

Detection of Small Renal Calculi on CT with Contrast

Yes, small renal calculi can be missed on CT scans with contrast, particularly stones smaller than 3 mm in size. 1

Comparison of Non-Contrast vs. Contrast-Enhanced CT for Stone Detection

Non-Contrast CT: The Gold Standard

• Non-contrast CT is considered the reference standard for kidney stone evaluation with sensitivity as high as 97% 1, 2
• Virtually all renal calculi are radiopaque on non-contrast CT, allowing for accurate detection of even small stones 1
• Non-contrast CT provides rapid acquisition with high spatial resolution and ability for multiplanar reformations 2

Contrast-Enhanced CT: Limitations

• Contrast-enhanced CT in the portal venous phase has only 81% overall sensitivity for detecting all renal stones (≥1 mm) compared to non-contrast CT 1
• The sensitivity improves with increasing stone size:
  • 95% sensitivity for stones ≥3 mm 1
  • 98% sensitivity for stones ≥6 mm 1
• Small stones (<3 mm) are most likely to be missed on contrast-enhanced CT 3
• The enhancing renal parenchyma may obscure small stones within the collecting system 2

Factors Affecting Stone Detection

Stone Characteristics

• Stone size is the most significant factor affecting detection:
  • Stones <3 mm are frequently missed on contrast-enhanced CT 3, 4
  • All stones larger than 5 mm were detected on arterial phase scans in one study 3
• Stone density (attenuation) significantly affects detectability on contrast-enhanced CT:
  • Higher density stones (556 ± 353 HU) are more likely to be detected 3
  • Lower density stones (181 ± 47 HU) are more likely to be missed 3

Technical Factors

• CT technique significantly impacts stone detection:
  • Thin slice thickness (1-1.5 mm) is superior to thick slices (5 mm) 1, 4
  • Section width of 5.0 mm revealed significantly fewer uroliths and nephroliths compared to 1.5 mm or 3.0 mm sections 4
• Phase of enhancement (corticomedullary versus nephrographic) has not been shown to affect stone detection rate 1

Clinical Implications

Potential Consequences of Missed Stones

• Even small stones can cause significant clinical problems:
  • Small stones (as small as 3 mm) can cause calyceal rupture in rare cases 5
  • Missed stones may lead to delayed diagnosis and treatment 5

Recommendations for Optimal Imaging

• For suspected urolithiasis, non-contrast CT remains the imaging modality of choice 1, 2
• If contrast-enhanced CT has already been performed:
  • Be aware that small stones (<3 mm) may be missed 3
  • Consider non-contrast CT if clinical suspicion remains high despite negative contrast-enhanced CT 1
• Low-dose non-contrast CT protocols maintain excellent diagnostic performance (97% sensitivity) while reducing radiation exposure 2

Common Pitfalls to Avoid

• Relying solely on contrast-enhanced CT for definitive exclusion of small renal stones 2
• Using thick-slice (5 mm) reconstructions, which may miss small calculi 4
• Failing to consider stone size when interpreting contrast-enhanced CT results 1
• Not recognizing that ultrasonography has even lower sensitivity for small stones (<3 mm) compared to CT 1, 6

Remember that while contrast-enhanced CT may miss small stones, these small stones (<3 mm) often pass spontaneously and may not require intervention, so the clinical significance of missed small stones should be considered in the context of patient management 1.