Your eGFR might be a little inaccurate

Everyone with PKD gets their GFR (Glomerular Filtration Rate, estimated or otherwise calculated) tested every so often, depending on the condition of the kidneys. Or at least, you should be.  As I was curious, today I was researching just how they estimate your GFR and discovered that there’s some controversy from some doctors as to use and accuracy of some methods, and there are some caveats about its use.

Creatinine clearance (Ccr) measured from a 24-hour urine sample has traditionally been used to calculate your creatinine clearance, from which GFR could be estimated. That’s less popular these days because it can be incovenient, messy, and hard to get every last drop. So a lot of the time now your Ccr itself is estimated from your serum creatinine result, and then eGFR; or a formula  is used which uses serum creatinine instead of an estimated Ccr.

Most pathology labs in North America, as I understand to be the case, use the Cockroft-Gault equation to estimate your creatinine clearance. This equation takes into account your age, sex and serum creatinine. In Australia and the UK, the MDRD (Modification of Diet in Renal Disease) equation is overwhelmingly used to estimate GFR, and takes into account your sex, age, and serum creatinine. There are a few other equations in use, but those are the most common.

However, like many pathology tests, they have their limitations. Here are some reasons why it could be a good idea to discuss your results more fully with your doctor:

• MDRD equation is based on a standardised body surface area of 1.73m squared. It’s thought to be accurate enough for people whose BSA is from 1.5-1.9m squared. It also takes into account whether you are African-American or not.
• If you are very underweight, very overweight, have muscle wasting from disability or illness or malnutrition, have high muscle mass (eg, bodybuilders), are an amputee, or have other “non-standard” body composition, are elderly or a child, or are not of European or African-American ancestry, are vegans or vegetarians, or take creatine supplements –  then the eGFR using the MDRD equation may not reflect your actual kidney function.
• The Cockroft-Gault equation may be more accurate for adults in the above categories, possibly with an estimated lean mass – that is, your current body minus fat tissue. This may or may not be “ideal” body weight, depending on how much muscle tissue you have.
• The MDRD and Cockroft-Gault equations can also be inaccurate in those with reasonably good kidney function (Stage 1 or 2). The Mayo Quadratic equation is argued to be best in those people.
• The Salazar-Corcoran equation may be more accurate for fat people.

I should also note that doing the equations yourself via online calculators, etc, is just a guide, as pathology labs can take into account known margins of error in their equipment, which varies from lab to lab. This page on renally excreted drug dosing has an overview of some of the above issues. Remember, always check out sources and validity of any information you find, and discuss it with your doctor. Hopefully you have a doctor who is interested in patient collaboration!

Here are some articles and comments discussing the use of eGFR and methods, most of which have references for even more reading:

• The wikipedia article on renal function [*] has descriptions of some of the equations and links to articles on eGFR
• Chronic kidney disease and automatic reporting of estimated glomerular filtration rate: a position statement – Medical Journal of Australia 2005; 183 (3): 138-141
• About eGFR – The Renal Association (UK)
• Modified Glomerular Filtration Rate Estimating Equation for Chinese Patients with Chronic Kidney Disease – Journal of the American Society of Nephrology 17: 2937-2944, 2006
• Influence of body weight on the performance of glomerular filtration rate
  estimators in subjects with type 2 diabetes.
• Influence of body weight on the performance of glomerular filtration rate estimators in subjects with type 2 diabetes (PDF) – Diabetes Care, 2007
• Estimating and Reporting GFR and When not to use the MDRD – National Kidney Disease Education Program

[*] While Wikipedia shouldn’t be used as a primary source, much of the time it provides a good overview of a topic and links to other non-Wikipedia sources. I put a link to the renal function article as it had quite a few eGFR equations included.

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