Glomerular Filtration Rate (GFR)
The most accurate measure of kidney function is Glomerular Filtration Rate (GFR) which uses the clearance of an external filtration marker during a 24-hour urine collection.
This is time-consuming and difficult to do in practice.
Estimated Creatinine Clearance (CrCl)
The alternative to directly measuring GFR is estimating Creatinine Clearance (CrCl) using a single blood level of creatinine and a mathematical formula.
Using serum creatinine to estimate kidney function has limitations, e.g. serum creatinine levels are dependent on muscle mass and diet, and are not accurate in Acute Kidney Injury (AKI).
There are various formulas, and they consider different variables to estimate kidney function.
Cockcroft and Gault formula
The MHRA gives advice on when to calculate creatinine clearance (CrCl) for medicine dose adjustments, e.g. direct-acting oral anticoagulants (DOACs).
The MHRA advises using the MDCALC Creatinine Clearance Calculator.
The UK Renal Pharmacy Group provides information on the limitations of the Cockcroft and Gault formula.
Estimated GFR (eGFR) formulas
The BNF and UK Renal Pharmacy Group recommend two formulas for calculating eGFR and state the limitations with each:
- Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI)
- Modification of Diet in Renal Disease (MDRD)
Do not use blood cystatin C levels instead of creatinine for estimating Glomerular Filtration Rate (eGFR).
Using both CrCl and eGFR calculation
It may be useful to calculate both CrCl and eGFR in the situations described below.
Where the two values for estimated kidney function are different and result in two difference renal doses for a medicine, consider the clinical impact of the dose difference.
For example, using the Cockcroft and Gault formula for a 80-year-old man weighing 60kg with a plasma creatinine of 120micromol/L will give a calculated creatinine clearance of 37mL/min. Using the CKD-EPI formula, the eGFR is 53mL/min/1.73m2.
Overestimates of eGFR
eGFR may appear better than it actually is in the following populations:
- the elderly
- those on a low protein diet
- those with a muscle wasting disorder (e.g. myasthenia gravis, late-stage muscular dystrophy)
Underestimates of eGFR
eGFR may appear worse than it actually is in the following populations:
- high muscle mass (e.g. high-level sport or body builder)
- high protein diet (e.g. taking protein supplements)
- muscle breakdown (e.g. after heavy exercise, myositis, muscular dystrophy)