A 32-year-old woman presents to the emergency department with acute onset severe left-sided flank pain radiating to the groin, accompanied by nausea and haematuria. Vital signs are stable. Non-contrast CT abdomen and pelvis is being considered. What is the imaging modality of choice for confirming the suspected diagnosis?

## Investigation of Choice for Acute Flank Pain with Haematuria ### Clinical Scenario The presentation of acute flank pain radiating to the groin with haematuria is classic for **acute nephrolithiasis (kidney stone)**. The patient is haemodynamically stable, making urgent imaging necessary to confirm the diagnosis and guide management. ### Why Non-Contrast CT (NCCT) is the Gold Standard **Key Point:** Non-contrast CT abdomen and pelvis is the **gold standard imaging modality** for suspected urolithiasis with a sensitivity of 95–98% and specificity of 96–98% [cite:Harrison 21e Ch 279]. **High-Yield:** NCCT is preferred because: 1. **Detects all stone types** — including radiolucent stones (uric acid, cystine) that may be missed on plain radiography 2. **No contrast needed** — avoids nephrotoxicity risk in patients with renal impairment or dehydration 3. **Fast acquisition** — provides rapid diagnosis in acute pain settings 4. **Identifies complications** — can detect hydronephrosis, perinephric stranding, and alternative diagnoses (AAA, appendicitis, etc.) 5. **High specificity** — stone density, size, and location are clearly visualized ### Why Other Modalities Are Inferior | Modality | Limitation | | --- | --- | | **IVU** | Requires contrast (nephrotoxicity risk); misses radiolucent stones; slower acquisition; largely replaced by NCCT | | **Renal ultrasound** | Operator-dependent; poor sensitivity (~50%) for distal ureteric stones; cannot reliably detect all stone types | | **MRI** | Expensive, time-consuming, and not first-line; stones appear as signal voids making them hard to characterize | **Clinical Pearl:** In a stable patient with classic flank pain and haematuria, NCCT is ordered immediately without prior imaging, as it is both diagnostic and cost-effective. ### Imaging Protocol The standard protocol is a **thin-slice (1–2 mm) non-contrast CT** covering the kidneys through the bladder, acquired in both supine and prone positions if needed to differentiate stone from phlebolith.