ESTIMATING GFR (GLOMERULAR FILTRATION RATE)
LDO has had several long postings on the significance to living kidneY donors (LKDs) of their estimated GFR (eGFR) calculated from serum creatinine. An article in this month's American Journal of Kidney Disease
(Mar 2011, volume 57, Supplement 2, page S9-S16) has important new information ( http://www.ajkd.org/article/S0272-6386(10)01594-5/abstract
). Briefly, the usual equation used to calculate eGFR is called "MDRD" (Modification of Diet in Renal Disease Study), and was developed from 1,628 people already with chronic kidney disease with the average GFR of everyone being 40 mL/min/1.73m2 (= the number of milliLiters of blood filtered per minute adjusted for size of the person). That equation is known to often underestimate real measured GFR at the upper range (from, say, 50 upward).
A new equation called "CKD-EPI" (Chronic Kidney Disease - Epidemiology Collaborative) was developed from 8,254 people, and validated as a separate data set of 3,896. Both groups had people both without and with chronic kidney disease, and had an average GFR of 68. The Kidney Early Evaluation Program Study then used both equations to calculate the eGFR for 116,321 people enrolled, followed them for an average of 3.7 years, determined who had died in that interval. Using those data, it determined the mortality rate for each group of people defined by their GFR as determined by the MDRD equation and separately by the CKD-EPI equation. RESULTS: The CKD-EPI equation was a better predictor of mortality rate than was the MDRD. In particular, people with eGFR 45-59 by MDRD but 60-89 by CKD-EPI, had a lower mortality rate than even the people with eGFR 60-89 by both equations. That is, CKD-EPI-defined groups predicted risk more accurately than MDRD's.
The National Kidney Foundation now recommends replacing MDRD with CKD-EPI to calculate eGFR, because CKD-EPI is more accurate. (Both equations require the laboratory to calibrate its serum creatinine measurements to the Cleveland Clinic Research Laboratory.)SIGNIFICANCE for us LKDs
If your eGFR results are worrisome, and if your lab or primary care provider (PCP) uses MDRD to calculate eGFR, I recommend that you ask the PCP to use CKD-EPI & re-calculate, and also to check if the lab calibrated its serum creatinine measurements to Cleveland Clinic Research Lab. If those results are still worrisome, ask to get a measured GFR (mGFR). The usual mGFR is done by a 24-hour urine collection plus blood test. (It, too, can be inaccurate. A more accurate test uses Iothalamate, but is expensive.)More importantly
, please realize what the classification of GFR of 30-60 by itself
really means to us LKDs. By itself, it means to LKDs: no-one knows
That group, "Chronic Kidney Disease Stage 3," is defined by the value of GFR alone; the definition does not require having chronic kidney disease or another disease known to be life-shortening and also adversely affecting GFR -- namely diabetes, high blood pressure, or heart disease -- in people with 2 kidneys. (CKD 4 and CKD 5 require having chronic kidney disease.) Most people with 2 kidneys and GFR 30-60 have one or more of those diseases, and thus are at high risk to die prematurely from them. That definition, however, was not developed in LKDs otherwise healthy
, that is, LKDs without chronic kidney disease (shown by abnormal results of urine or blood or other tests) or diabetes or high blood pressure (especially not well controlled) or heart disease -- but simply "GFR 30-60." That definition also did not account for person's age; because most people's GFR decreases slowly as they get older, even if they remain healthy.
No-one has studied a large group (2,000+ probably is needed) of LKDs of different ages without evidence of chronic kidney disease, diabetes, high blood pressure, or heart disease -- and followed them for several years to see how many died and what happened to their GFR, and compare those results to a similar large group of people with 2 kidneys. Until such a study (or similar) is done, no-one knows if mGFR of 40-60 is a risk factor at all for LKDs, or carries the same risk as it does for people with 2 kidneys with the same mGFR, or ...?
GFR has more value to the health of us LKDs for a different reason: it shows trends. If your/our GFR gets lower relatively rapidly, it may well indicate a problem that needs to be evaluated, diagnosed, and treated. Thus, the trend of GFR over time is more important than in which group your/our GFR is as defined by and for people with 2 kidneys.
Even more important, of course, is doing what so many have written so frequently on LDO:
* measure blood pressure and blood glucose at least yearly;
* follow a healthy lifestyle even more carefully than before donating (= low salt, low fat, diet; moderate exercise 30 min/day 5 days a week);
* treat early high blood pressure (systolic 130+ or diastolic 80+) with more life-style adjustment (more intense diet, exercise, etc.), and if needed medicines;
* treat diabetes with more life-style adjustment (more intense diet, exercise, etc.), and probably medicines.
I have the full article. If you want to read it, please e-mail me (see my Profile).