Creatinine is a waste product produced by your muscles during the breakdown of a compound called creatine, which helps supply energy to your muscles. The kidneys filter creatinine from the blood and excrete it in urine. Measuring creatinine in urine helps assess kidney function, muscle health, and hydration levels.
Since creatinine is produced and excreted at a relatively constant rate under normal conditions, it is often used as a benchmark to normalize the concentration of other biomarkers in urine tests (e.g., "creatinine-adjusted" values).