It is important to bear in mind the following pitfalls and cautions when interpreting the eGFR

• It is only an estimate of kidney function and a significant error is possible. The eGFR is most likely to be inaccurate in people at extremes of body type e.g. patients with limb amputations, severely malnourished and morbidly obese individuals
• Confidence intervals: The 90% confidence intervals are quite wide, e.g. 90% of patients will have a true GFR within 30% of their estimated GFR and 98% have measured values within 50% of the estimated value. For an individual patient, however, serial values will be much more consistent than this just as creatinine values are – e.g. a 20% fall in a patient’s eGFR is certain to reflect an important change.
• Look at the trend in eGFR – Identifying trends in eGFR is often more informative than one-off readings. i.e. a progressive fall in eGFR across serial measurements is more concerning than stable readings which don’t change over time.
• Race: Some racial groups may fit the equations used to calculate the eGFR less well. This is mainly because the datasets used to derive these equations were predominantly white and black US citizens. There is usually a need to apply a correction factor for Black people which varies depending on the formula used: add 21% if MDRD equation and 16% if CKD-EPI is used. In the UK population both the MDRD and CKD-EPI equations seem to work quite well.
• Not so good near normal: The equations used for estimating eGFR tend to underestimate normal or near-normal function therefore slightly low values (i.e. around the 60 ml/min/1.73m2 CKD “cut-off” level) should not be over-interpreted. In this situation repeat testing, looking for a progressive decline in renal function over time, or measurement of cystatin C (a different biomarker of kidney function) may be useful. Additional indicators or renal disease should also be sought in these patients – e.g. urinalysis looking for blood and protein, positive family history of renal disease etc… Routine reporting of eGFR values >90 is not recommended and many labs are now reporting all values over 60 as >60. Note, however, that a significant (e.g. 20%) rise in creatinine while eGFR is >60 may still be important as it will usually reflect a real change in GFR.
• Values can differ between laboratories. Creatinine measurements can differ significantly between laboratories depending on the methods used to measure it. Furthermore, different laboratories may use a different formula to calculate the eGFR further complicating comparison of eGFR measurements obtained from different laboratories.
• Creatinine level must be stable: eGFR calculations assume that the level of creatinine in the blood is stable over days or longer i.e. steady-state; they are not valid if it is changing. It is therefore not valid in patients with acute kidney injury or in patients receiving dialysis, for example.
  Age: MDRD and CKD-EPI equations are not valid for individuals under 18 years of age.

Patient information

The level of creatinine in the blood is a useful guide to kidney function or the glomerular filtration rate (GFR). (The terms kidney function and GFR should largely be considered as interchangeable.) Formulas which combine the serum creatinine level with other information about a patient such as their age, sex and race, can provide a more accurate measure of kidney function which is termed the estimated GFR or eGFR. The eGFR is usually calculated and reported by the laboratory measuring the creatinine level. If not, it can be calculated using an online eGFR calculator

“Normal” GFR is approximately 100 but you will often see it reported as >90 (“greater than 90”) or >60 (“greater than 60”). It is for this reason that patients (and some doctors) sometimes quote the eGFR as a percentage of normal kidney function. Whilst not factually correct, it does help to make the numbers easier to understand.

Kidney damage is graded into 5 stages based on the eGFR – see stages of CKD. This staging system is useful when planning management and follow up.