Can a 5-6 mm Renal Calculus Show No Posterior Acoustic Shadowing?
Yes, a 5-6 mm renal calculus can fail to demonstrate posterior acoustic shadowing on ultrasound, which is why ultrasound has poor sensitivity (24-57%) for detecting kidney stones and should not be relied upon as the primary diagnostic modality. 1, 2
Why Stones May Not Show Shadowing
Gray-scale ultrasound has an overall sensitivity of only 24-57% for detecting renal calculi when compared to noncontrast CT, meaning that nearly half of stones may be missed entirely or appear without classic features like posterior shadowing 1, 2
The absence of posterior acoustic shadowing does not exclude the presence of a stone, particularly for stones in the 5-6 mm range, which are at the threshold where ultrasound performance becomes increasingly unreliable 2
Stone composition, location within the kidney, and technical factors (such as depth, overlying bowel gas, or patient body habitus) can all prevent the generation of posterior acoustic shadowing even when a stone is present 1
Recommended Diagnostic Approach
Noncontrast CT is the reference standard for detecting renal calculi with 97% sensitivity and should be obtained when clinical suspicion remains high despite negative or equivocal ultrasound findings. 1, 3
The American College of Radiology recommends noncontrast CT as the gold standard, with detection accuracy that decreases only with stones <3 mm, not with stones in the 5-6 mm range 1
Low-dose CT protocols maintain 97% sensitivity while reducing radiation exposure and should be utilized for stone detection 3, 2
For a 5-6 mm stone, noncontrast CT will detect it with 98% accuracy, far superior to ultrasound 1
Enhancing Ultrasound Detection (When CT Cannot Be Used)
The American College of Radiology recommends adding color Doppler with assessment of twinkling artifact for small renal stones (<5 mm), which can achieve sensitivity as high as 99%, though this has a false-positive rate up to 60% 1, 2
For stones at 5-6 mm, the twinkling artifact may improve detection rates, but the high false-positive rate means confirmatory imaging with CT is still advisable when intervention is being considered 1, 2
Common Pitfalls to Avoid
Do not exclude the diagnosis of nephrolithiasis based on absent posterior acoustic shadowing on ultrasound - this is a common error given ultrasound's poor sensitivity for direct stone visualization 1, 2
Do not rely on the presence or absence of hydronephrosis to determine if a stone exists - a 5-6 mm non-obstructing calyceal stone may cause pain without causing hydronephrosis, and the absence of hydronephrosis does not exclude a stone 2, 4
Be aware that ultrasound tends to overestimate stone size, particularly for stones ≤5 mm, so a stone reported as 5-6 mm on ultrasound may actually be smaller 2
If clinical suspicion remains high despite equivocal ultrasound findings, proceed directly to noncontrast CT rather than repeating ultrasound or relying on KUB radiography 1