Creatinine

Your muscles steadily turn a little of their energy stores into creatinine every day. Your muscle mass barely shifts day to day, so you produce it at a remarkably steady rate.

That creatinine flows to your kidneys, which filter it into your urine. When the kidneys filter less, it backs up in your blood and the level climbs. Labs fold in your age and sex to estimate your filtration rate, called eGFR.

A higher-than-expected creatinine usually means your kidneys are clearing waste a little less efficiently. That can point to dehydration, reduced blood flow, a blockage in your urinary tract, or kidney disease. Reading it early gives you the most room to act, because kidney issues respond best when you spot them and adjust.

A low creatinine is usually harmless and often just reflects lighter muscle mass, though a very low value can hint at muscle loss or undernutrition. Either way, this is one clue, not a verdict. Read it alongside your eGFR and your trend over time, with your clinician.