A frequently used parameter to evaluate kidney function is the glomerular filtration rate (GFR)[1]. Simply put, the GFR rate is a measure of kidney function and describes the flow rate (milliliters per minute, or mL/min) of filtered fluid through the kidney. The normal range of GFR, adjusted for body surface area, is 100–130 mL/min/1.73 m2. The GFR is adjusted for body surface area since bigger kidneys can filter more fluid, and kidneys get bigger as body surface area gets bigger.
In practice, there are two types of GFR rates[2], depending on how the parameter is determined. The measured GFR (mGFR) is obtained by infusing an external substance that has some good properties for measuring how much fluid the kidney filters. Ideally, the substance would be only eliminated through the kidney, is filtered easily, and is not reabsorbed when it passes through the kidney. One commonly-used substance that checks all the boxes for an mGFR marker is inulin.
The mGFR is the most accurate method to assess renal function. However, it’s also an impractical and expensive method, as the person undergoing the procedure needs to remain in the clinic for several hours. A more convenient and affordable way is to determine the estimated GFR (eGFR). The eGFR is, as the name implies, estimated based on the clearance of a marker that the body makes itself, so nothing needs to be infused. Two common markers are serum creatinine and cystatin C, which can be readily determined from blood samples and fed into equations that give an eGFR value. Due to its simplicity and practicality, the eGFR is also more unreliable, as it often lacks both precision and accuracy.
The 2012 KDIGO guidelines for chronic kidney disease split the difference between convenience and accuracy. They’re fine with using a specific equation to calculate eGFR from serum creatinine levels, but they recommend following up with an eGRF using cystatin C or mGFR to diagnose CKD. They also recommend mGFR when an important clinical decision hinges on an accurate reading.