KUB xray vs CT KUB for renal and ureteric colic

A 21 year old woman presented with vomiting and right flank pain. The pain started at 9am that morning, and had waves of 10/10 severity on a background of 8/10 severity. During a wave, she would be unable to get comfortable at all. Her urine dipstick was positive for blood (+++).

What imaging should be done next?

According to the 2012 iRefer guidelines from the Royal College of Radiologists, CT KUB should be done as the first line investigation, with KUB Xrays reserved for when neither CT nor IVU are indicated e.g. in pregnancy.

iRefer
iRefer

According to the 2012 British Association of Urological Surgeons, everyone gets a KUB Xray and then everyone gets a CT KUB. I do not understand the point of the double exposure, especially given that their own audit standard is CT KUB within 24 hours. What I found most interesting was that the BAUS referred to iRefer in their guidelines as the sole source that this imaging choice was based on.

Another standard I see people following is plain film KUB xrays all round, and only follow up with CT KUB if negative. Although this doesn’t help much with planning interventions, it used to ensure that everyone with stones gets diagnosed and hopefully with less radiation exposure than using CT KUB first line.

With the reduction in the doses used for CT KUBs recently, this may no longer be the case. An abdominal plain xray and a KUB plain xray both exposure the patient to 0.7mSv. To put this in perspective, a chest xray exposes the patient to 0.02mSv. Background radiation is about 2.2mSv in the UK per year. A diagnostic low dose CT KUB can be as low as 3mSv, and follow up with low dose CT KUB as low as 1.2mSv. I put some numbers together, and found that if you assume a sensitivity of 45% for plain film KUB, then if the CT KUB has a dose of 3mSv or lower and about 50% of the patients you investigate actually have stones, then going straight to CT KUB will reduce the total radiation dose compared to plain films followed by CT KUB if negative.

This does assume 100% sensitivity and sensitivity for the CT KUB, and 100% sensitivity for the plain film. I know that’s far from true, but it’s a rough idea of where things may eventually head if the CT KUB dose requirements keep reducing.

To muddy the water, the RCR did an audit looking at imaging done in acute renal colic. They said that CT KUB is the imaging of choice for both the BAUS and the iRefer guidelines, but did not specify whether or not it should be first line. Am I the only one who thinks these two guidelines do not quite agree with each other? And should we be going straight to CT KUB for all first presentation of ureteric stones?

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