The muscles make creatine phosphate from creatine so they can use the phosphate to help make the energy that they need, but when they remove the phosphate, creatine phosphate changes to creatinine. Normally, the creatinine is sent to the kidneys. Thus, a high level of creatinine in the urine can mean abnormalities in the muscles or kidneys.
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The cells in the pancreas, kidneys and liver make creatine from the methionine, arginine and glycine amino acids. The cells in the muscle will then use creatine to make creatine phosphate, as explained in "Harper's Illustrated Biochemistry" by Victor Rodwell, Ph.D., Emeritus Professor of Biochemistry at Purdue University. When the muscles are at rest, creatine is primarily in the creatine phosphate form. When they need energy for muscle contraction, however, they take the phosphate in creatine phosphate and transfer it to ADP, or adenosine diphosphate, to make ATP.
ATP stands for adenosine triphosphate, the molecule which the cells use for energy. Thus, the muscle cells remove phosphate from creatine phosphate to make energy, creating a substance called creatinine. The creatinine will be released by the muscle cells and transported to the kidneys to be excreted from the body within the urine, explains Gerhard Meisenberg, Ph.D. of the Department of Biochemistry at Ross University School of Medicine, in "Principles of Medical Biochemistry."
Creatinine Levels in Urine
According to the University of Maryland Medical Center, the normal range of creatinine in a 24-hour urine sample is 500 to 2,000 mg a day. Since it is the breakdown of creatine which is sent to the kidneys, the lab is actually checking on the level of creatinine in the urine. A high urinary creatinine level can mean a problem in the muscles, since the muscle cells send the creatinine to the kidneys. It may also mean there is a problem in the kidneys; that is, the kidneys are unable to excrete the creatinine.
High Creatinine Urinary Levels
Rhabdomyolysis is the medical term to describe the destruction or breakdown of the skeletal muscle; it can cause a high creatinine level in the urine. It can happen from long-distance running, muscle trauma, electrical shock and as a result of some infections, per Elizabeth Corwin, Ph.D. of the Department of Physiology at Pennsylvania State University in the "Handbook of Pathophysiology." Kidney problems which can cause a high urinary creatinine level include kidney failure, glomerulonephritis and obstructions within the urinary tract. In glomerulonephritis, the capillary network of the kidneys is inflamed.