## Investigation of Choice for Acute Renal Colic **Key Point:** Non-contrast CT (NCCT) abdomen and pelvis is the gold standard investigation for suspected urolithiasis. It has 95–99% sensitivity and specificity for stone detection, regardless of stone composition, and provides critical anatomical detail. ### Why NCCT is Superior | Feature | NCCT | IVU | Ultrasound | KUB | |---------|------|-----|-----------|-----| | Sensitivity for stones | 95–99% | 85–90% | 85–95% | 45–60% (radiopaque only) | | Detects radiolucent stones | Yes | Yes | Yes | No | | Assesses obstruction severity | Yes | Yes | Limited | No | | Stone composition | Yes (Hounsfield units) | No | No | No | | Radiation dose | Moderate | Moderate–high | None | Low | | Speed | Fast | Slow | Fast | Fast | **High-Yield:** NCCT is the **"triple-negative" test** — non-contrast, non-invasive, no iodinated contrast needed. It avoids contrast nephropathy risk and is safe in renal impairment. ### Clinical Pearl Hounsfield unit (HU) measurement on NCCT helps predict stone composition: - **Calcium oxalate/phosphate:** 800–2500 HU (radiopaque, visible on KUB) - **Uric acid:** 300–600 HU (radiolucent) - **Struvite:** 200–400 HU (radiolucent) - **Cystine:** 600–1600 HU (faintly radiopaque) **Mnemonic:** **STONE** for NCCT advantages: - **S** — Sensitivity highest (95–99%) - **T** — Time fast (no waiting for contrast) - **O** — Obstruction detail clear - **N** — No contrast needed - **E** — Excellent for composition ### When to Use Alternatives - **IVU:** Historical gold standard; now reserved for anatomical assessment in recurrent stones or when NCCT unavailable. - **Ultrasound:** First-line in pregnancy (no radiation); limited sensitivity for distal ureteric stones. - **KUB:** Only detects radiopaque stones (~85% of stones); misses uric acid and cystine stones; poor specificity. [cite:Harrison 21e Ch 279]
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.