The CKD-EPI formula estimates GFR because it was validated against GFR measured using exogenous filtration markers.7 It incorporates age and sex into a relatively complicated formula. These demographics are known at the time of blood collection so the eGFR is automatically calculated and reported by the laboratory. The units for the automated eGFR are mL/min/1.73 m2 and it is now an accepted method for the classification and monitoring of chronic kidney disease (Table 1).8,9
The initial report describing the CKD-EPI formula did not observe an effect of age or body mass index (BMI) on the accuracy of its prediction.7 However, it should be noted that the initial report was based on a population who were mostly younger than 66 years old with a mean body surface area of 1.90–1.93 m2 and a BMI of 27–28 kg/m2 (mean height 170 cm, mean weight 79–82 kg). The automated eGFR may not therefore apply to patients with different demographics. Since the body surface area for most patients is higher than 1.73 m2, the actual GFR in such a patient will be higher than that reported by the laboratory. The eGFR can be de-indexed (converted to actual mL/min) by multiplying the automated eGFR by the patient’s body surface area (m2) and then dividing by 1.73 (see Box 1).
Measured GFR
The mGFR is determined after giving an exogenous filtration marker, such as 51Cr-EDTA, 125I-iothalamate, DTPA or MAG3. It is the most reliable method of quantifying GFR because these markers are filtered and not substantially secreted into or reabsorbed from the nephron. The mGFR can be indexed by adjusting for a standard body surface area of 1.73 m2.
The mGFR methods require parenteral administration of the exogenous marker and multiple blood and sometimes urine samples over time. The incremental gain from the mGFR above eGFR is uncertain in most cases, but they are used in specialist practice before a unilateral nephrectomy when considering the split GFR in each kidney.
Serum cystatin C-based formulae
Cystatin C is another endogenous solute that can be used to estimate GFR. However, the test is not universally offered by pathology laboratories in Australia at present.
Cystatin C is less influenced by muscle mass, so it may be advantageous in patients at extremes of body weight or those with cirrhosis. An alternative CKD-EPI formula has been developed to incorporate cystatin C.