Any disease or condition that affects the glomeruli can decrease the kidneys' ability to clear creatinine and other wastes out of the blood. When this occurs, the blood creatinine level will be increased and the creatinine clearance will be decreased because not as much creatinine is able to be excreted in the urine. http://labtestsonline.org/
A number of diseases and conditions can affect kidney function.
Creatinine Clearance: Test Sample
A decreased creatinine clearance rate may also occur when there is decreased blood flow to the kidneys as may occur with congestive heart failure, obstruction within the kidney, or acute or chronic kidney failure. The less effective the kidney filtration, the greater the decrease in clearance.
Increased creatinine clearance rates may occasionally be seen during pregnancy, exercise, and with diets high in meat, although this test is not typically used to monitor these conditions