What is eGFR?

Filtration is the kidneys' central job, pulling waste out of the blood while holding on to water and salts. Kidney function can decline a long way without producing symptoms, so a fall in the estimate is often the first indication, arriving before anything is felt.

Urea is cleared by that same filtration, though protein intake and hydration move it as well. The kidney also holds Potassium, Sodium and Phosphate within narrow bounds. Those three therefore show whether the kidney is holding the blood's salts steady, where eGFR shows how fast it is filtering waste out.

Creatinine comes from muscle breaking down, so muscle mass moves the estimate even when the kidneys have not changed. Recent meat intake, hydration and certain medicines act on creatinine too. Age enters the calculation directly. Diabetes and hypertension act on the kidney tissue itself, damaging it over years.

Kidney filtration usually changes over years rather than weeks. Creatinine does not: it can move within days, taking the estimate with it while the kidneys are unchanged. Yearly measurements are spaced widely enough for the slow direction of kidney function to show through that day-to-day noise. This is Do Health's monitoring schedule, not a general recommendation for everyone.