In one sentence
An estimate of how many milliliters of blood your kidneys filter each minute, scaled to a standard body size, computed from creatinine, from cystatin C, or from both, with your age and sex; it is the kidney figure every laboratory prints and the one VT recomputes with the 2021 equations so that two draws from different laboratories can be compared.
Why it matters
The filtration rate is the kidney's output figure, and its associations are in the renal entry: unrelated to mortality between 75 and 105, rising below 60 (Matsushita 2010). The equations that estimate it were rebuilt in 2021 without the race term the earlier ones carried: in a validation set of 4,050 people with measured filtration, the new creatinine equation using only age and sex was within 30 percent of the measured rate for 85 percent or more of people, and the equations that combined creatinine with cystatin C were more accurate than either marker alone and showed the smallest differences between groups (Inker 2021). Adding cystatin C does more than sharpen the number: across 90,750 people in general-population cohorts, the cystatin-based estimate strengthened the association between the filtration rate and death, and re-sorting a person into a higher or lower band with cystatin C moved their observed outcomes in the same direction (Shlipak 2013). The KDIGO bands laboratories print (G1 at 90 and above, G2 from 60 to 89, G3a from 45 to 59, G3b from 30 to 44, G4 from 15 to 29, G5 below 15) are the working group's (KDIGO 2024).
What moves it
The creatinine estimate: muscle mass, creatine, a meat meal, dehydration, hard training, as the renal entry sets out, and age, which the equation expects. The cystatin estimate: thyroid state, corticosteroids, smoking, body fat, inflammation. Both: the true filtration rate, which falls slowly through life and faster with raised blood pressure and glucose.
How to measure it well
Nothing beyond the panel's rules for the creatinine and cystatin C draws; the estimate is arithmetic. What matters is knowing which equation the laboratory used (its comment usually says) and comparing like with like, which is why VT recomputes.
How Vitality Tracker uses it
The laboratory's eGFR is kept as the laboratory's figure, with its range and flag, never overridden. Beside it VT computes the CKD-EPI 2021 creatinine estimate from the same draw and, when cystatin C exists, the cystatin and the combined estimates, and names which one heads the kidney panel: the combined estimate when both markers exist, the cystatin estimate when the Supplement Tracker holds creatine or the five-segment scale reading shows little muscle, the creatinine estimate otherwise. VT's working optimal (v0.1) for the headline estimate is 90 mL/min/1.73 m² and above, the KDIGO G1 band, shown beneath in smaller type and marked working; the band is shown as the guideline's. A result outside the laboratory's own range prompts the consultation suggestion. The estimate is read as a within-person trend over years.
How far to trust the number
Calculated, with a wide band: within 30 percent of the truth for most people, so a single value cannot separate 85 from 95, and the combined estimate is the narrowest of the three. The figure is standardized to 1.73 m² of body surface, which under-reads very large people and over-reads very small ones.
Sources
- Inker LA, et al. N Engl J Med 2021;385:1737–49. doi:10.1056/NEJMoa2102953
- Shlipak MG, et al. N Engl J Med 2013;369:932–43. doi:10.1056/NEJMoa1214234
- Matsushita K, et al. Lancet 2010;375:2073–81. doi:10.1016/S0140-6736(10)60674-560674-5>)
- KDIGO CKD Work Group. Kidney Int 2024;105(4S):S117–314. doi:10.1016/j.kint.2023.10.018
- Vitality Tracker, Recommended baseline lab panel v0.1; optimal set v0.1 (working).