Creatinine Tests Before CT Scans

Q:  I have had my bladder removed because of a cancerous tumor. Since there was a possibility that the cancer may have spread to other areas, my urologist said I should have a CT scan every four months for the next 3 or 4 years. When I got to the Radiology Department, it was made clear that to make sure my kidneys were functioning properly. I was told that if the kidneys were not functioning, the contrast dye used in the scan could cause significant damage.

This is a worrisome thought. If this dye can damage kidneys with problems, can it also damage healthy kidneys? I am 75 years old and to the best of my knowledge, there is nothing in my medical history that would indicate a kidney problem.

A:  Creatinine is a normal breakdown product of muscle tissue and if your kidneys are functioning normally, they will excrete creatinine into your urine and eliminate it from your body - otherwise it will build up in your system and your creatinine level will rise. That's why a creatinine blood test is used to determine if you have normal or impaired kidney function.

The iodinated contrast dye used in the CT scan goes from your blood into your kidneys. If your kidneys are working properly, they will be able to excrete the dye into your urine easily, but if not, the dye can damage one or both of your kidneys. Diabetics are especially prone to kidney damage from the dye.

The general consensus is that patients under the age of 40 with no history of kidney disease do not need a blood creatinine test. If the creatinine is mildly elevated (between 1.3 and 2.0, which indicates abnormal kidney functioning) the radiologist may proceed with the CT scan, but lower the dye dose. If your creatinine level is greater than 2.0, the radiologist will try to avoid giving contrast dye.

Because you are 75, and especially because you have had disease involving your urinary system, for your safety you should have a creatinine test before every CT scan involving dye.

"Dear Doctor" is a compilation of patient questions answered by doctors from the Medical College of Wisconsin.