GLOSSARY

Creatinine is a waste product formed by the non-enzymatic, steady breakdown of creatine and creatine phosphate in muscle. Because its production in each person is fairly constant and it is excreted mainly by glomerular filtration, it is one of the most common markers of kidney function.

Features

• Production: about 1–2 percent of the body’s creatine store is converted to creatinine each day; production is proportional to muscle mass.
• Excretion: mainly by filtration, with about 10–15 percent by tubular secretion; it is not reabsorbed.
• Normal blood range: about 0.6–1.2 mg/dL in men and 0.5–1.1 in women (depending on the laboratory).

Relationship with GFR

Creatinine and GFR have an inverse, non-linear relationship: a doubling of creatinine roughly means halving of GFR. However, in early kidney damage, creatinine may stay in the