The reasoning
| Patient | First test | Notes |
|---|---|---|
| Adult, non‑pregnant | low‑dose NCCT KUB (IOC) | sensitivity/specificity > 95%; size, site, density, hydronephrosis, alternative diagnoses |
| Pregnancy | US (± transvaginal for distal stones) | MRI urography (no gadolinium) next; low‑dose CT if essential |
| Children | US | — |
| Known radio‑opaque stone follow‑up | KUB radiograph ± US | reduces dose |