eGFR - estimated Creatinine Clearance

The eGFR is used to screen for and detect early kidney damage and to monitor kidney function. This is now routinely performed every time a serum creatinine concentration is requested to evaluate the status of the kidneys in those with, or at risk of, kidney damage such as individuals with diabetes or hypertension, where early intervention can prevent further damage. In the UK, NICE Guidance (CG182) states: ​“Whenever a request for serum creatinine measurement is made, clinical laboratories should report an estimate of GFR (eGFR)”.

Creatinine Clearance is a more accurate way to detect changes in kidney status than measurement of serum urea and creatinine alone, which are easier to do but cannot pick up early damage to the kidneys. eGFR or calculated creatinine clearance are based on serum creatinine, an easily performed and commonly used laboratory test. The calculation of eGFR which has been adopted in the UK uses a formula which uses the concentration of creatinine in blood, the age and sex of the patient and their ethnic origin.

The eGFR can be determined, with no extra testing, at the same time that a blood sample is sent for a creatinine measurement. Since April 2006 eGFR has been routinely offered by the majority of UK laboratories. It was introduced to improve the detection of early kidney dysfunction so that measures can be taken to halt or at least, slow progression to more severe kidney damage. eGFR is suitable for most patients who are 18 or more years old, not pregnant or malnourished, do not have reduced muscle mass e.g. amputees, and do not have acute kidney injury. GPs and UK laboratories are now using eGFR to look for the presence of kidney dysfunction in various ​‘at risk groups’ including people with diabetes, blood vessel disease, heart problems, high blood pressure, obstructions to urinary flow and in patients taking some commonly prescribed drugs including diuretics (water tablets) and a variety of drugs used to treat high blood pressure.

eGFR or Calculated Creatinine Clearance is a simple test to detect early changes in kidney function. A normal result means that kidney disease is less likely while a low value suggests that some kidney dysfunction is present. Creatinine Clearance results are usually evaluated in the same way. Sometimes, in very early kidney dysfunction (especially when the kidneys are damaged by diabetes), measured or estimated creatinine clearance may actually be high, indicating that the kidneys are working harder than normal.

CKD stages according to eGFR and albumin to creatinine ratio (ACR) (Source) NICE guidelines (CG182)

The actual amount of creatinine that a person produces and excretes is affected by their muscle mass and by the amount of protein in their diet. Men tend to have higher creatinine concentrations in their bloodstream than women or children.

A person’s GFR will decrease with age and can increase during pregnancy.

The calculation for eGFR is intended to be used when kidney function, and creatinine production, is stable. If serum creatinine is measured when the kidney function is changing rapidly, such as with acute kidney injury, then it will not give a useful estimate of the filtration rate. A slightly different equation should be used to calculate the eGFR for those under the age of 18. eGFR may not be as useful for those who differ from normal creatinine concentrations. This may include people who have significantly more muscle (such as a body builder) or less muscle (such as a muscle-wasting disease) than the norm, those who are extremely obese, malnourished, follow a strict vegetarian diet, ingest little protein, or who take creatine dietary supplements. Likewise, the eGFR equations are not valid for those who are 75 year of age or older because muscle mass normally decreases with age. For these individuals an eGFR based upon serum cystatin C may be more useful.

The eGFR test may also be affected by drugs, such as gentamicin, cimetidine, trimethoprim and cisplatin which can increase plasma creatinine concentration , and by any condition that decreases blood flow to the kidneys.

The most commonly used equation in the UK for calculating eGFR, is called the MDRD (Modification of Diet in Renal Disease study) equation. It requires a person’s serum creatinine, age, and assigned values based upon gender and race. However, the CKD-EPI equation based upon serum creatinine, age, gender and ethnicity is recommended by NICE CG182.

When an improved assessment of eGFR is needed a serum cystatin C test can be used. This test is not currently available in all UK laboratories.

Direct measurement of the GFR is complicated and requires experienced personnel. The most commonly used method for determining the GFR is the creatinine clearance method. However, this is liable to errors of urine collection as well as problems with testing methods. The best method for determining the GFR is a procedure called an ​‘inulin clearance’. It is not routinely used, however, because it is expensive and awkward. It involves introducing a fluid containing the marker molecule inulin (NOT insulin) into your veins (IV – intravenous infusion) and then collecting timed urines over a period of hours. The urine volumes are noted and the inulin in each sample is measured to allow determination of the GFR. Other methods of determining GFR use non-radioactive and radioactive markers, and have similar drawbacks. The UK now uses serum creatinine concentration to calculate the eGFR.

As mentioned above, eGFR is routinely calculated in the UK with all serum creatinine requests. However, if for some reason your serum creatinine only is known to you, there is an online eGFR calculator on the Renal Association website. However an eGFR calculated by a local laboratory may differ because it is likely to be more accurate than the website calculator which cannot take into account local variation in creatinine measurements.