Unenhanced CT is the imaging modality of choice in renal colic, especially in evaluating cases where the diagnosis may be unclear, for example acute flank pain in the elderly patient. It is rapid, can measure calculi and is able to assess the degree of associated urinary obstruction.

It is recommended that CT should be performed within 24 hours of the onset of symptoms as the renal functions could decline rapidly.[10,12]

The CT KUB shows calculus in the right ureter.

Click on the CT to enlarge.

False negative rates of 2-7% are probably due to a small size of stone. Stone size is important given that it relates to spontaneous passage.

Secondary signs such as hydronephrosis, perinephric oedema and stranding, and periureteral oedema are commonly seen. Perinephric stranding appears after 2 hours and is maximal at 8 hours.

Criticism of using CT

Criticism of using CT in evaluating renal colic has centred around radiation exposure. Although this is true, diagnostic yield is greater and may merit the risk.

Low dose CT protocols have been evaluated and appear to maintain diagnostic accuracy. In American studies the cost of CT appears to average out as equivalent to IVU.

Patients with flank pain

Studies have shown that patients with flank pain, upon whom a CT is ordered, have calculi in approximately 70% of cases (47%, however, have a coexistent diagnosis)

Stones not seen on CT

Over 99% of stones are seen on CT, the exceptions are pure matrix stones associated with drugs such as indinavir and related protease inhibitors

Alternate diagnoses

Studies have shown that the majority of alternate diagnoses are detected by CT, especially pyelonephritis, renal mass, adnexal pathology, diverticulitis and appendicitis

Learning bite

CT is rapid, avoids contrast and facilitates alternative diagnostics, especially the red flags.

None of the imaging options are available 24/7 but CT is likely to be the most easily accessible. Most trusts will not do a routine CT KUB overnight but will do it on the weekend during daylight hours