A doctor requires a patient to have a blood test before taking a Cat scan for abdominal pain. Why?

Many CT scans of the abdomen involve injecting a dye into the vein. This dye "lights up" blood vessels and may make it easier to diagnose certain conditions. the dye can also be toxic to the kidneys. The blood test is a test of kidney function. If abnormal, the radiologist may either not use dye or use a smaller amount so as not to injure the kidney even more. CT scans without dye do not require baseline blood tests beforehand.