eGFR Frequently Asked Questions

What is the eGFR?

The GFR (glomerular filtration rate) is the volume of fluid filtered from the renal glomerular capillaries into Bowman's capsule per unit time and it is the most accurate indicator of glomerular function. The “eGFR” is an estimated value derived from a measured serum creatinine and population means for age and gender.

Can the GFR be measured directly?

Yes. The GFR can be measured directly. Radioisotope methods are very accurate but expensive, time-consuming, and invasive. Newer techniques such as serum cystatin appear promising but are significantly more expensive and not as widely available as serum creatinine. The time-honoured method of creatinine clearance requires the extra expense of a timed urine sample that is difficult to collect and analyze accurately.

Why not just use the s-creatinine or s-urea?

The relationship between s-creatinine and GFR is not linear. Thus, large changes in renal function are reflected by small changes in s-creatinine. In fact, s-creatinine will detect changes in renal function only after the GFR has declined by 50%. In addition, because s-creatinine is influenced by age and muscle mass, a set of at least 10 separate reference intervals would be required to accurately define normal adult function.

S-urea is influenced significantly by hydration and protein intake and is a less accurate indicator of glomerular function than s-creatinine.

Why report the eGFR?

The eGFR is the simplest, most accurate and inexpensive test for kidney function that is available today and is the common “starting point” for the Clinical Practice Guidelines that deal with kidney disease.

When should an eGFR be requested?

The eGFR should be requested every 1-2 years in patients at risk for chronic kidney disease. This includes: patients with diabetes, hypertension, cardiovascular disease, a family history of kidney disease, those belonging to certain high-risk ethnic groups (First Nations and Pacific Islanders), as well as patients with known kidney disease, demonstrated reductions in eGFR, or abnormal urinalysis.

What is a normal eGFR?

A “normal” value is ³ 60 mL/min in the absence of systemic disease or other evidence of kidney disease.

What is an abnormal eGFR and what do I do with a patient who has one?

A value of < 60 mL/min should be confirmed by repeat testing and consideration given to the factors that may cause false eGFR results (see lists below). If the result remains low for over 3-months then chronic kidney disease is present. The patient should by classified using the staging system of the National Kidney Foundation.

Is the eGFR the same in different laboratories?

Because different analytical equipment manufacturers calibrate s-creatinine assays differently, the results from different labs will be slightly different. To address this problem, a standardization system has been developed. Today, even though s-creatinine values may differ between laboratories, their reported eGFR values are equivalent.

Are there factors that can cause false eGFR results?

Yes; there are some general and specific limitations, as follows:

General Limitations (calculation not necessarily valid)

  • Children under the age of 18 years (alternative calculation required)
  • Racial groups (not yet defined)
  • Rapidly changing renal function
  • Rapidly changing body fluid distribution
  • Pregnancy
  • eGFR < 30 mL/min

Increase in serum creatinine (decreased eGFR)

  • Kidney disease (reduced glomerular function)
  • Reduce renal perfusion
  • Large muscle mass (do not confuse with fat)
  • Abnormally rapid muscle breakdown
  • High protein intake (cooked meat may transiently increase creatinine production; there may be a transient increase in actual GFR)
  • Marked extra-cellular fluid decrease
  • Very poor glucose control
  • Drugs causing increased creatinine release from muscle (active vitamin D metabolites, corticosteroids, fenofibrate)
  • Drugs causing decreased proximal tubular secretion of creatinine (cimetidine, phenyl acetamide, pyrimethamine, salicylates, trimethoprim
  • Drugs having an effect that is not understood (nonsteroidal anti-inflammatories, pyrimethamine)
Analytical
  • Chromogens causing a positive analytical interference (ascorbic acid, bilirubin,ketoacids (picric acid method), nonspecific chromogens (some methods), sarcosine
  • Drug causing a positive analytical interference (dopamine)
  • Drugs causing a positive analytical interference that varies with different assay conditions (cephalosporins,flucytosine)

Decrease in serum creatinine (increased eGFR)

  • Kidney disease (tubular secretion may increase in some disorders, muscle mass may decline in CKD, a significant amount of creatinine can be degraded by bacterial overgrowth in the small bowel in severe CKD)
  • Reduced muscle mass (small people, amputation, malnourishment, paraplegia, quadriplegia)
  • Low protein intake
  • Massive extra-cellular fluid increase

No effect (though effect previously reported)

  • fluoroquinolones

 
 
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