For what reasons might creatinine levels be high in the urine?

Chronic kidney dis.: The most common reason is chronic renal disease. If your serum creatinine is high, you likely have the answer.

Urine creatinine: Our corresondent is talking about urine creatinine. This may be high because the urine is concentrated (i.e., the person drank little water) or the person is well-muscled. It's not a cause of concern.