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Author Topic: GFR vs. Creatinine Clearance  (Read 18267 times)

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Offline brenda

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GFR vs. Creatinine Clearance
« on: October 05, 2011, 08:20:52 PM »
Can someone explain the difference between GFR and Creatinine Clearance?
My pre-donation eGFR was 67; my Creatinine Clearance (CrCl) was 110; my post-donation eGFR = 42, my CrCl = 60. They are based on the same serum creatinine samples.  Obviously these two tests are measuring different things, but what?

My concern is to figure out how my kidney is doing. My transplant center says, "Go with the higher number! You're fine!"
When I read the research, CrCl is not usually the measure reported - though it is easy to calculate from the same serum samples that the subjects gave to get an eGFR.  For this - and many other reasons - I am not reassured by the Georgetown medical staff.  Anybody got a better explanation?

Thank you,
Brenda
Donated 5/17/2011
at Georgetown University Hospital

Offline Clark

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Re: GFR vs. Creatinine Clearance
« Reply #1 on: October 06, 2011, 08:22:17 AM »
Dear Brenda,

  Please consult with your primary care physician, and if warranted, seek a referral to an independent nephrologist.  Did you consult with an independent donor advocate before you donated?  Asking that individual for a clearer explanation may or may not get you one, but it should alert the transplant center that there's an as yet unfulfilled need for improvement.  Best wishes.  Do let us know what your learn and if you get better answers from the transplant center.
Unrelated kidney donor, 48 gallon blood & platelet donor
Past Rep to the OPTN/UNOS Boards of Directors & Executive, Kidney Transplantation, & Ad Hoc Public Solicitation of Organ Donors Committees

Offline Clark

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Re: GFR vs. Creatinine Clearance
« Reply #2 on: October 06, 2011, 08:26:46 AM »
Also, you may want to print Bill Freeman's excellent thread on this topic and take it with you: http://livingdonorsonline.org/ldosmf/index.php?topic=141.0
Unrelated kidney donor, 48 gallon blood & platelet donor
Past Rep to the OPTN/UNOS Boards of Directors & Executive, Kidney Transplantation, & Ad Hoc Public Solicitation of Organ Donors Committees

Offline Donna Luebke

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Re: GFR vs. Creatinine Clearance
« Reply #3 on: October 08, 2011, 08:24:21 PM »
Most electronic medical records use the MDRD-GFR which is one calculation and per your note, this number is low.  While your creatine clearance is 60, this is not the number used in the MDRD-GFR which says if is >60 you are OK.  Ask what is the transplant center lab's cutoff for creatinine clearance.  Get copies of all test and medical record notes the transplant center has and take this to a nephrologist.  If is a surgeon telling you that you are OK, remember that surgeons are not medical experts in kidney function or kidney disease.  Their training is not focused on medical care of patients. 

Donna
Kidney donor, 1994    Independent donor advocate
MSN,  Adult Nurse Practitioner
2003-2006:  OPTN/UNOS Board of Directors, Ad Hoc Living Donor Committee, Ad Hoc Public Solicitation of Organs Committee, OPTN Working Group 2 on Living Donation
2006-2012:  Lifebanc Board of Directors

Offline WilliamLFreeman

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Re: GFR vs. Creatinine Clearance
« Reply #4 on: October 17, 2011, 12:24:34 PM »
Brenda,

You already received good advice in the prior responses.  Let me add a bit.

It looks like you thought the answer "Go with the higher number! You're fine!" was not satisfactory.  If I had received it, I would have thought it unsatisfactory, too.  Why go with the "higher" number?  Any "higher" number?  Why not "lower"?  Etc.  Sometimes a simple, quick, re-assurance produces more worry, because it is too simple and too quick.   :(

"GFR" and "creatinine clearance" and "e-GFR" [= estimated-GFR] refer to the same thing -- Glomerular Filtration Rate [= "GFR"].  But each gets there in different ways.  So, let's start at the beginning.  [This will be a long post.]

The "glomerulus" is the first part of the microscopic unit in the kidney that filters certain chemicals from the blood, such as urea, and passes them out in the urine.  I do not remember how many glomeruli [= plural of glomerulus] a typical person has in their 2 kidneys, but I assume the number is in the 100-thousands to millions.  The total process to produce urine is quite complicated, but we do not need to discuss it here.  The important point is that most chronic kidney disease, including chronic renal failure, is due to a decrease in the effectiveness of that first filtering step, by the glomeruli.

"GFR" is expressed in terms of volume filtered per unit time -- usually "ml/min" [= milliliters per minute {or cc -- cubic centimeters -- per minute}].  The "normal" youngish adult has a GFR of 100 to 130 ml/min.

A way to measure "real GFR" [my term] -- GFR that is totally accurate -- is cumbersome and expensive.  (It is done by injecting a harmless chemical, inulin, into the blood, and then measuring how much inulin is "cleared" or filtered in a given unit of time.)  Moreover, that method measures real GFR [= the filtration rate of all glomeruli added together] only over a short period of time; but real GFR varies somewhat during the course of a day -- asleep vs. awake, exercising vs. resting while sitting, etc.

A simpler method uses creatinine, a chemical that the body makes continually, and that is cleared or filtered by the glomeruli.  That measurement is "creatinine clearance [CrCl]."  It is not quite correct to say that CrCl "is easy to calculate from the same serum samples that the subjects gave to get an eGFR."  CrCl requires not only serum creatinine [Cr], but also the total urine produced over a time period (usually 24 hours).  The time interval is needed to get the "per unit time" of the CrCl.  The urine is needed to get "how much creatinine did the glomeruli filter from the blood into the urine during that time," that is, the Creatinine Cleared [= "filtered"].

To measure CrCl one needs:
  *  serum creatinine level [in milligrams per ml/Dl = milliliters per deciLiter];
  *  total urine volume produced over 24 hours [in milliliters];
  *  total urine creatinine in the total urine [in milligrams].
Note that 2 of the 3 measures needed come from the 24 hour urine.

The formula for CrCl is:  (urine creatinine X urine volume over 24 hours) divided by (serum creatinine X 24 [hrs] X 60 [min]).  The CrCl, then, is a "measured GFR" [mGFR].

Unfortunately, the 24 hr CrCl is not fully accurate, compared to the gold standard method using inulin.  ON AVERAGE, it overestimates "real GFR" by 10% - 20%.  That range of overestimating is an average, meaning that is some people the overestimation is more than 20%, and in others is less than 10%.

CrCl is inexpensive -- but, as we know, is cumbersome for the person collecting the urine.  An even less expensive, and not as cumbersome, method is "eGFR" -- estimating GFR from the serum creatinine.  Several different formulae exist, a recent one being "MDRD" that Donna referred to -- Modification of Diet in Renal Disease.  A more recent formula is "CKD-EPI" -- Chronic Kidney Disease - Epidemiology Collaborative."  See my post listed second from the top of Living Donors Forum p. 1, "ESTIMATING GFR (GLOMERULAR FILTRATION RATE)" for a description of both, and why CKD-EPI may be a closer estimation of real CFR than MDRD.  Both tend to underestimate real GFR, on average, although CKD-EPI less so than MDRD.  (Brenda, the Georgetown Lab probably used MDRD -- feel free to ask to find out.  Also, feel free to ask them to calculate eGFR by the CKD-EPI formula.)  Your eGFR probably was an underestimate of your real GFR.  The "e" of "eGFR" says it was an "estimate by a simply calculation of serum Cr alone."

So, what is YOUR "real GFR"?  You, Georgetown, nephrologists, cannot say for sure, without a more accurate test being done.  (An iothalamate test is more accurate than a 24-hour urine CrCl, but is quite expensive.)

The more important question is, "What will a more accurate GFR mean to you and your life?"  Please see the section "SIGNIFICANCE for us LKDs" in my prior post mentioned above.  Let me summarize that here.

SUMMARY of "SIGNIFICANCE for us LKDS"

  1]   No-one knows what range of values of eGFR or mGFR would represent "Stage 3" of Chronic Kidney Disease in people with one kidney due to having donated the other kidney!  ["Stage 3" in people with 2 kidneys is "less than 60 ml/min" -- specifically, GFR 30-59 ml/min with or without other evidence of kidney disease.  LKDs end up with their GFR being, on average, 65% [60% to 75%] of their pre-donation GFR.]  "No-one knows" because medicine, transplant medicine specifically, has not yet studied the question -- or not yet finished such a study, that would require 20 years of follow-up of thousands of LKDs, recording *EVERYONE's* cause of death, and total renal and cardiovascular disease status.  (For what is it worth, I expect the equivalent value for us LKDs would be something like "less than about 50 ml/min" -- but I do not KNOW that!  Take it or leave it.  ;) ]

  2]   Is there anything Georgetown, I, or anyone can do to reduce your worry?  I recommend the opposite:  all LKDs, including I, should continue to worry, no matter what our eGFR or mGFR is -- but worry productively.  Why do I recommend that all us LKDs be worried?  God, evolution, or God through evolution [your choice], gave us 2 kidneys for a reason.  The second kidney is NOT "superfluous."  It is a kidney in reserve, in case something bad happens to one kidney, or chronic kidney disease starts.  (If people with chronic kidney disease have 2 kidneys, the disease will progress more slowly.)  We LKDs gave away our "reserve kidney" to someone else.  We LKDs thus should worry productively, about what can we to take good care of our one kidney.

  3]   What should productive worry focus on?  Making sure our one kidney is happy and healthy, by adopting a lifestyle that follows what Donna and so many others have preached here 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 "pre- high blood pressure" (systolic 130+ or diastolic 80+) with more life-style adjustment (more intense diet, exercise, etc.), and if needed medicines -- to prevent actual high blood pressure;
   *   if overweight, prevent type 2 diabetes with more life-style adjustment (more intense diet, exercise, etc.);
   *   prevent heart attacks, strokes, and blockage of the larger arteries to the kidneys by keeping cholesterol levels where they should be by diet, and by medicines if needed;
   *   if high blood pressure or diabetes does/do develop, treat / manage it/them aggressively with diet, exercise, weight loss, and probably medicines.

  5]   "What, me worry?"  YES, just enough to make my one kidney happy by keeping it healthy.  :D


For those who want to see a good schematic drawing of a glomerulus [= an arteriole tuft enclosed in a bag with microscopic holes that let urea etc. out to the surrounding capsule that collects the urine], see http://en.wikipedia.org/wiki/Glomerulus

For those who want to read more detail about GFR, see http://en.wikipedia.org/wiki/Glomerular_filtration_rate

Bill
Bill - living kidney donor (non-directed, Seattle, Nov 24, 2008), & an [aging] physician  :-)

Offline Donna Luebke

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Re: GFR vs. Creatinine Clearance
« Reply #5 on: October 19, 2011, 05:37:38 AM »
Bill

Thanks for this excellent and detailed note.  Many of the lifestyle or health issues you note become more of a concern as we 'age' with one kidney.  There are aging effects that occur to our kidneys so one kidney means we 'worry' just a little more.  I donated in 1994 and yes, I do worry.  I check my blood pressure several times per week, get at least annual bloodwork, and address any risk factors for cardiovascular disease.  My primary physician calculates an estimated creatinine clearance just to be sure my trend is not going south.  I do not have any family risk factors either which has always been reassuring.  No diabetes, heart disease, stroke or hypertension. 

Donna

Donna
Kidney donor, 1994    Independent donor advocate
MSN,  Adult Nurse Practitioner
2003-2006:  OPTN/UNOS Board of Directors, Ad Hoc Living Donor Committee, Ad Hoc Public Solicitation of Organs Committee, OPTN Working Group 2 on Living Donation
2006-2012:  Lifebanc Board of Directors

Offline sherri

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Re: GFR vs. Creatinine Clearance
« Reply #6 on: June 23, 2013, 11:59:04 AM »
Bumping up this thread for Nancy
Sherri
Living Kidney Donor 11/12/07

Offline Fr Pat

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Re: GFR vs. Creatinine Clearance
« Reply #7 on: September 28, 2013, 06:51:04 PM »
Just bumping this up again as there has recently been a question on the topic.
    Fr. Pat

 

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