In one sentence
How well your kidneys filter, estimated from creatinine, the steady waste product of muscle, together with the salts and the water balance the kidneys keep in order.
Why it matters
The kidney's filters are capillaries, so the kidney figure is a reading of the small vessels as much as of the organ, which is why the panel places it beside blood pressure. In a meta-analysis of general-population cohorts with 105,872 people measured for urine albumin and 1,128,310 measured by dipstick, mortality was unrelated to the estimated filtration rate between 75 and 105 mL/min/1.73 m² and rose below it: compared with 95, the adjusted hazard of death was 1.18 at an eGFR of 60, 1.57 at 45 and 3.14 at 15, and the filtration rate and the urine albumin leak multiplied each other's association (Matsushita 2010). The guideline bands that laboratories print (G1 at 90 and above, G2 from 60 to 89, G3a from 45 to 59, and so on) are the KDIGO working group's, and the band most well adults sit in is G1 or G2 (KDIGO 2024). Creatinine is the raw number behind the estimate and it has a weakness the next entry exists for: it is made by muscle, so it rises with muscle mass and with creatine supplements, and an estimate built on it reads a muscular person's kidneys as worse than they are.
What moves it
Creatinine: muscle mass first of all (in 170 healthy adults, serum and urine creatinine tracked lean mass more closely than body weight, and people training moderately or hard ran about 10 percent higher than sedentary people; Baxmann 2008); creatine supplementation (in 18 men taking about 10 g a day for three months, serum creatinine stayed level while it fell in the placebo group that trained alongside them, with no change in true filtration; Gualano 2008); a large meat meal the evening before, because cooked meat carries creatine; dehydration; hard training within 48 hours; and a few medicines that block its secretion. The estimate falls slowly with age in most people. Sodium follows water: it falls with over-drinking and some medicines and rises with dehydration. Potassium reads high from a squeezed sample or a tight tourniquet, which the laboratory usually comments on, and falls with diuretics, vomiting and diarrhea. Urea rises with protein intake and dehydration.
How to measure it well
A fasting morning draw with the rest of the panel, hydrated as on an ordinary day. No hard training or alcohol for 48 hours and no large meat meal the night before. If you take creatine, keep taking it, record it in VT, and add cystatin C to the draw. Same laboratory each time, and the laboratory's comment usually names the eGFR equation it used.
How Vitality Tracker uses it
Each analyte is stored as reported, with the laboratory's range and flag, never overridden, and the eGFR the laboratory printed is kept as the laboratory's figure; VT's own CKD-EPI 2021 recalculation sits in its own entry beside it. Typical laboratory reference for creatinine is the laboratory's own; for eGFR, VT's working optimal (v0.1) is 90 mL/min/1.73 m² and above, the KDIGO G1 band, shown beneath in smaller type and marked working; sodium, potassium, urea, bicarbonate and chloride carry the laboratory's range alone. When the Supplement Tracker holds creatine, VT shows the creatine note beside the creatinine result and beside PhenoAge, which takes creatinine as an input, and prefers the cystatin-based estimate for the kidney panel. Where calcium is bundled with the electrolytes, VT computes corrected calcium from the same draw. Re-test yearly, or at six months when a supplement, a medicine or heavy training is new, and when anything renally active is started.
How far to trust the number
Laboratory: the creatinine assay is standardized and precise. The estimate built on it is not: in the validation of the 2021 equations, 85 percent or more of estimates fell within 30 percent of the measured filtration rate (Inker 2021), so an eGFR of 85 and one of 95 are the same number, and the figure is read as a within-person trend over years, never as a single value.
Sources
- Matsushita K, et al. (Chronic Kidney Disease Prognosis Consortium). Association of estimated glomerular filtration rate and albuminuria with all-cause and cardiovascular mortality in general population cohorts: a collaborative meta-analysis. Lancet 2010;375:2073–81. doi:10.1016/S0140-6736(10)60674-560674-5>)
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2024 clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney Int 2024;105(4S):S117–314. doi:10.1016/j.kint.2023.10.018
- Baxmann AC, et al. Influence of muscle mass and physical activity on serum and urinary creatinine and serum cystatin C. Clin J Am Soc Nephrol 2008;3:348–54. doi:10.2215/CJN.02870707
- Gualano B, et al. Effects of creatine supplementation on renal function: a randomized, double-blind, placebo-controlled clinical trial. Eur J Appl Physiol 2008;103:33–40. doi:10.1007/s00421-007-0669-3
- Inker LA, et al. New creatinine- and cystatin C-based equations to estimate GFR without race. N Engl J Med 2021;385:1737–49. doi:10.1056/NEJMoa2102953
- Vitality Tracker, Recommended baseline lab panel v0.1; optimal set v0.1 (working).