GFRs

Glomerular filtration rate (GFR) is the volume of plasma filtered by the kidney’s glomeruli per unit time. In adults it is commonly expressed as milliliters per minute per 1.73 square meters of body surface area (ml/min/1.73 m²). The plural form “GFRs” may refer to rates in different individuals or to a range of values observed in a population.

GFR is the principal indicator of kidney function. It reflects how well the kidneys filter waste and excess fluid from the blood. Because direct measurement is impractical in routine care, GFR is typically estimated (eGFR) from serum biomarkers such as creatinine and, less commonly, cystatin C, using equations such as CKD-EPI or MDRD. Creatinine-based estimates are influenced by muscle mass, diet, and medications, while cystatin C can be affected by thyroid function and inflammation. In some settings, direct measurement with exogenous filtration markers (for example, inulin, iohexol, or iothalamate) provides a precise GFR but is seldom used clinically.

Clinically, GFR is used to assess kidney function, stage chronic kidney disease (CKD), and guide drug dosing. Normal or near-normal GFR is typically ≥90 ml/min/1.73 m²; CKD staging commonly describes progressively lower eGFR values, often with urine albumin measurements to indicate kidney damage. GFR naturally declines with age and can be influenced by sex, body size, and historical race coefficients in some estimation equations, though some modern guidelines remove race from the calculation.

Limitations include reduced accuracy in acute kidney injury, extremes of age, severe muscle wasting, or very high or low body size. GFR remains a central though imperfect measure of renal function used in diagnosis, monitoring, and treatment planning.