What are the limitations of a GFR test?

“GFR is only an estimate of one aspect of kidney function and a significant error is possible.”

What affects GFR apart from glomerular filtration?

The GFR varies according to:

Age – decreases with age. Many doctors believe GFR naturally falls by 1 ml/min per year from the age of 40 – and that is a normal part of ageing and not a disease. Hence someone who has a GFR of 100 ml/min at 40 years, may have a GFR of 59 ml/min at 81 years, and according to the CKD/GFR 5 stage classification, has Stage 3 CKD. They do not. They are normal
Gender – higher in men
Body size – higher in larger people
Creatinine blood test – as its based on the blood creatinine level, it carries many of the limitations of the creatinine test.

CKDEx has more information on the effects of age and gender.

So. What are the limitations of a GFR?

The eGFR is inaccurate:
– in people at extremes of body type – e.g. patients with limb amputations, severely malnourished and morbidly obese people
– at very low GFR levels – e.g. below 15 ml/min
Values differ between 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. in steady-state. It is therefore not valid in patients with acute kidney injury (AKI) or in patients receiving dialysis, for example.

Why GFR should not be equated with %kidney function

As stated above, glomerular filtration is only one (of seven) functions of the kidney. Therefore, it is wrong, as many doctors and nurses do, to express GFR as a % kidney function. The other six functions are described on CKDEx here.

Also. As average human GFR is 120 ml/min, and is not 100, GFR should not be expressed as a percentage for that (mathematical) reason.

Summary

We have described what are the limitations of a GFR test. It needs to be interpreted understanding its limitations. We hope it has been helpful.

Other resources

The UKKA has more information on GFR and its meaning here.
What is normal GFR?
What are the limitations of a blood creatinine test?
CKD: disease or normal ageing?

Last Reviewed on 16 April 2024