GFR

Glomerular filtration rate (GFR) is the rate at which the kidneys filter plasma through the glomeruli, expressed as milliliters per minute per 1.73 square meters of body surface area (ml/min/1.73 m2). It reflects overall kidney function and the filtering capacity of the nephron unit. In routine clinical practice, GFR is not measured directly; instead, it is estimated (eGFR) from serum biomarkers and demographic factors.

Measured GFR can be determined by exogenous filtration markers such as inulin, iothalamate, or iohexol clearance, but these methods are primarily used in research or special clinical situations. The most common clinical approach is to estimate GFR from serum creatinine, optionally combined with cystatin C, using equations such as CKD-EPI or MDRD. Some formulas also rely on patient characteristics like age, sex, and race. eGFR values are reported normalized to a body surface area of 1.73 m2 to allow comparison between individuals.

Normal GFR values for healthy adults typically range from about 90 to 120 ml/min/1.73 m2, with gradual age-related declines. A sustained eGFR below 60 ml/min/1.73 m2 for at least three months is used to define chronic kidney disease (CKD) and to stage it from G1 to G5. GFR estimates guide diagnosis, staging, prognosis, and decisions about medication dosing and timing of interventions.

Limitations include reduced accuracy in acute kidney injury, pregnancy, extremes of body size or muscle mass, and in populations where race-based adjustments are used. eGFR may be less reliable at very high or very low values, and clinicians often consider additional measurements and clinical context when assessing kidney function.