Estimated glomerular filtration rate
The estimated glomerular filtration rate (eGFR) is widely used as a surrogate marker of renal function and is mathematically-derived from the patient's serum creatinine, using their age, sex and ethnicity.
Calculation
The eGFR is calculated using a four variables Modification of Diet in Renal Disease (4-v MDRD) formula:
eGFR (mL/min/1.73m) = 186 (serum creatinine in µmol/L × 0.011312) × (age) × (0.742 if female) × (1.212 if African/American Black)
Prior to the introduction of this formula, eGFR derivation required an accurate body weight, which was a significant impediment to its easy adoption into routine clinical practice.
Glomerular filtration rate
The actual glomerular filtration rate (GFR), as opposed to the estimated GFR, is of course more accurate but is much more laborious to measure, with methods including:
- Cr-51 labeled ethylenediaminetetraacetic acid (EDTA)
- Tc-99m labeled diethylenetriaminepenta-acetic acid (DTPA)
- I-125-iothalamate
- inulin clearance
- inulin, a polysaccharide, is completely filtered at the glomerulus with no tubular secretion or reabsorption
- creatinine clearance
Therefore in routine clinical practice the eGFR is the usual test of renal function.
Limitations
The published literature advises caution in the use of the MDRD calculation in:
- acute renal failure
- any condition where body weight may markedly differ from that theoretically expected from age and sex:
- pregnancy
- anasarca
- sarcopenia
- amputees
- paraplegics
- morbid obesity
- malnutrition
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