Creatinine, 24-Hour Urine

Creatinine is a waste product produced by muscles during normal metabolism and filtered by the kidneys into urine. It is measured in blood to assess how well your kidneys are working, since healthy kidneys remove creatinine efficiently.

When elevated: Elevated blood creatinine suggests the kidneys may not be filtering waste effectively, which can indicate reduced kidney function or kidney disease. However, creatinine levels are also influenced by muscle mass, so context matters—a muscular person may have a higher baseline than someone with less muscle. When low: Low creatinine is generally not a clinical concern and does not indicate kidney disease. It may be seen in people with very low muscle mass, malnutrition, or certain chronic conditions, but it is not used diagnostically to detect disease.

24-hour urine creatinine reflects total daily muscle mass and metabolism, making it useful for tracking training volume effects and muscle breakdown patterns. Athletes with high training loads may show elevated excretion; this test can help assess whether recovery and nutrition are adequate to support muscle preservation during intense or prolonged training phases.