What is Creatinine?

Creatinine leaves the blood by one route only, through the kidneys. With production steady and only that one exit, the amount left in the blood follows the speed at which the kidneys are filtering.

eGFR, the estimated filtration rate, is calculated from creatinine, stripping out the effect of age and sex so that what remains is closer to the kidney's own work. Urea is the kidney's other waste product, though protein intake and dehydration move it as well as the kidneys' filtering does. Creatinine's main input is muscle instead, so a change that shows in both markers at once points to the kidney rather than to a few days' eating and drinking.

Muscle mass changes slowly, so its effect on production shows across months. A meat meal adds creatinine directly, since cooking turns the creatine in muscle into creatinine before it is eaten. Hydration moves the measurement without touching production, concentrating or diluting the blood a sample is drawn from.

A large meat meal, a hard workout or a change in fluid intake can move creatinine within a day or two. Kidney filtration changes over months and years instead. A year between measurements is long enough that those short swings do not line up, so a difference still visible means muscle mass or the kidneys' filtering has genuinely shifted, not that one meal preceded the test. This is Do Health's monitoring schedule, not a general recommendation for everyone.