In nephrolithiasis, the Hounsfield unit (HU) density threshold on non-contrast CT that distinguishes uric acid stones (potentially amenable to alkalinisation therapy) from calcium oxalate stones is:

• A Uric acid stones typically >900 HU; calcium oxalate stones <500 HU
• B Uric acid stones typically <500 HU; calcium oxalate stones typically >900 HU ✓
• C Both stone types have identical HU values on non-contrast CT
• D HU values cannot distinguish stone composition — stone analysis is required

Explanation

On non-contrast CT, stone density in Hounsfield units provides a clue to composition. Uric acid stones are typically low density (<500 HU) because uric acid contains no calcium. Calcium oxalate stones are dense, typically >900 HU (often 800–1200 HU). This is clinically relevant because pure uric acid stones can be completely dissolved by urinary alkalinisation with potassium citrate (target urine pH 6.5–7). Struvite stones are intermediate. While not perfectly accurate, HU thresholds from CT are a useful non-invasive guide before committing to surgery.

Reference: Bailey & Love's Short Practice of Surgery, 27th ed.

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Written and medically reviewed by the StethoPrep medical team.