QUESTION: dr falkinburg, i have diabetes and hypertension.i read gfr in men over 70 is   not valid because of loss of muscle mass.i am 79 told my doctor i dont want gfr because at my age it is inaccurate.of coursev he ran it my serum creatinine was 1.4 and gfr 49.he w wantsts to run another gfr in 3 months.he is clueless.research suggests gfr of 45-59 in men over 75 is normal aging of kidney.dr richard glasscock,one of top nephrology in america sites studies that say.number one use cystatin c not serum creatinine gfr.cystatin more reliable then gfr because of loss of muscle mass.mayo clinic states men over 75 should not take gfr.also dr glasscock sites research yhat states in men over 75 proteinura is a more important predicitor of heart discease then gfr.i have run a microalbumin test last week.noprotein.i feel i can tell my clueless doctor  since i have no protein in my urine just test me once a year for cystatin c gfr testing in my case not warrantes?

ANSWER: Good evening, Howard.

Things are always more complicated than they seem.

The GFR stands for the "glomerular filtration rate" and represents the ability of your kidneys to filter poisons and toxins from your blood and excrete them into the urine. This is your kidney function.

The Creatinine clearance is a clinical estimation of your GFR.  Creatinine is a substance that is produced by the muscles of your body and is excreted by the kidneys into the urine. Because the kidney neither reabsorbs nor secretes creatinine, it clearance by the kidney can be used as an ESTIMATION of kidney function. The formula for calculating the creatinine clearance is:

Crcl = UV/P where:

Crcl = Creatinine clearance
U = Urinary creatinine concentration
V = 24 hour uninary volume in cubic centimeters
P = Plasma (blood) Creatinine

If you know algebra, you can see that a smaller muscle mass will result in less creatinine being produced, but will also result in a smaller amount of creatinine being excreted by the kidney. Therefore, the U/P Creatinine ratio will be unchanged and the formula will be accurate.  However, the plasma creatinine will be lower if your muscle mass is reduced (less creatinine will be produced by less muscle). So, the creatinine measured from your blood could be within the normal range and you could still have kidney disease. This is, in fact, one reason that we do a creatinine see if that normal creatinine is really normal.  Contrariwise, we will occasionally perform a Cr clearance on a young weight lifter. His creatinine will occasionally be slightly elevated due to the production of increased amounts of creatinine and we will want to be sure that his kidneys are normal. An increased production of creatinine will be associated with an increases excretion and the U/P creatinine ratio will be normal.

So, it is the Creatinine level in the blood (plasma) that will be effected by the changes in the muscle mass, not the Clearance itself.

There is a formula that calculates an estimated creatinine clearance that labs provide to clinicians.  This formula in NOT accurate if the kidney function is normal. A reliable creatinine clearance requires a 24 hour urine collection and is a pain to do.

I'm a little concerned that your Blood creatinine is not really completely normal.  A value of 1.4 mg% is more like a value that I would expect to see in a weight lifter not a 79 year old with a small muscle mass.  I would expect something like 0.5 - 0.7 mg%. Therefore, I would recommend to you that you have a creatinine clearance performed. I agree with you that this is not the picture of diabetic kidney disease.  Your micro-albumin and protein excretion are normal and this is inconsistent with diabetic kidney disease. In the setting of 79 years and high blood pressure you likely have a condition called nephrosclerosis which is essentially hardening of the arteries of the kidney and the treatment is good control of your blood pressure and possibly a "statin" for cholesterol. I would also recommend a renal ultrasound be done to be sure that there are no obstructive processes going on the might effect your kidney function.

Parenthetically, the GFR has nothing to do with heart disease, although, the two do tend to run together.

There are many substances that can be used to estimate the GFR and I'm sure that cystatin C is one of them.  The problem is that the laboratories only do the creatinine and in most clinical instances is OK.

By the way, Dick Glassock is a good friend of mine.  He trained me many moons ago.

Your question is very complicated, so is you wish to discuss this further, feel free to follow up.

Dr Falkinburg

---------- FOLLOW-UP ----------

QUESTION: dr falkinburg,your medical background is superior to mine.not to get in a pissing contest but an article in annals of internal medicine 10/2/2012 done by elke schaeffner and numerous other doctors did a study of 610 people over waS CALLED THE BERLIN INITIATIVE study.their research shows beyond a doubt that  current methods to estimate gfr are not validated and put elderly patipatients by mistake in wrong kidney class.they state only 2 tests for people over 70 one is b1s2or cystatin c.your answer to my question was based only on current gfr which is not reccomended for people over 70 bbecause loss of muscle mass affects accuracy of current proteinura in urine is better indicator of cardiac problems then also ignore research that states gfr 45-59 in men over 70 witn no proteinura  probably have normal aged kidneys. yetthe doctors continue to use outdated inaccurate gfr.they put elderly in sstage 3 based on gfr that is wrong and give them medicines not needed.a report in nevethelands state people in sstage 1 and stage 2 kidney class with proteinura have more kidney problems  then people in stage 3 w conclusion is doctors are not up to date my case can be aging kidneys and nothing colesteral is 125 and bad hdl is 50.these doctors that use gfr and scare the shit out of people are same doctors that insist on giving men over 75 needless psa i will get no ultrasound but  get cystatin c yearly along with microallbumin.sorry for long letter but needed you to hear my side of the western doctors stupid handling of kidney the way your buddy dr glasscock published article that gfr between 45 to 59 woth no proteinura usually is not kidney desease but aging to my kidneys being stiff they still do not let any protein in my urine

Good evening, Howard.

I have read the study you mention and I essentially agree with the points that they made.  However, those more accurate tests, such as cystatin C and others, are not routinely available in the clinical labs that primary care doctors use to help treat their patients. The creatinine, with its limitations, is the presently the best available test.  

I think you would be well served to have a consultation with a nephrologist who can review all of your records with you and help decide what would be the best tests to assess your kidney function.  Perhaps there is a place close to you where they are working with cutting edge investigational tests such cystatin C that could confirm of refute your GFR or, at least, give you an accurate assessment of your GFR.  

By the way, a GFR or Crcl of 49 in a 79 year old patient is fairly average and does not indicate terrible kidney failure. However, these aged kidneys are not normal when compared to younger patients. They don't work as well.  Nothing else works as well in the aged either. I was and am fully aware of that.

There are statistical concepts of "accuracy" and "precision" that are important here, but too complicated for me to explain in detail.  The bottom line is that a creatinine or creatinine clearance may not be as accurate as another test but it is still useful in following changes in kidney function over time, as long as there has been no significant change in the size of the muscle mass. Any error cancels itself.

Understand, however, that proteinuria has NOTHING TO DO with cardiac disease and the Berlin Initiative did not suggest that.  Understand, also, that  one can have very serious kidney disease, that will destroy the kidney, and have very little PROTEINURIA. And, you may well have it! You need to be studied to be sure that any apparent abnormalities are, in fact, due to only age and not to some other problem that is going to raise its ugly head and bite you in the ass when you least expect it.

Good luck, my friend and don't get too upset.  Everyone is trying as best as they can.

Dr Falk


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Newell R. Falkinburg, M.D., FACP


I am a board certified nephrologist and emeritus professor of medicine at a major medical school and past Director of Nephrology & Hypertension at a university affiliated hospital. I have expertise in all areas of clinical nephrology, dialysis, transplantation and plasmapheresis.


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