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GFR went way up right after lab changed both their auto analyzer and their test calibration method
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diitto posted:
I have only a single, functioning kidney (left) due to renal artery stenosis of my right renal artery. From 2003 when the stenosis first surfaced (with very high blood pressure) until 2008, over 22 Serum Creatinine (SCr) tests (and associated eGFR calculation), all of these test done at the same lab, my eGFR was 58 +/- 10 ml/min... Then, I recently discovered, this same lab made two big changes. First, they bought and installed a new auto-analyzer from a new company because the old system had reached end of life... Second, they changed the calibration method for how they calibrated their machine to measure (SCr) to the new IDMS-traceable standard (IDMS is Isotope Dilution Mass Spectrometry) that is slowly being adopted all over the world (at least so I read). They claimed that SCr would go down a small amount and they even came out with an adjusted eGFR equation (MDRD4-revised they called it) that was precisely 94% of what the old equation (original MDRD4) would output... But immediately after all this happened, the change of machines and the change of SCr calibration method, my SCr plummeted and my eGFR subsequently soared to a high of 86 ml/min. Since then it has trickled back over several tests to 59 ml/min, where it was for the five years prior to all this change at my testing lab... My nephrologist NEVER gave me a reasonable explanation for how my eGFR could have ever legitimately increased to what would be a pretty much normal GFR for a 56 year old man (how old I was when I got the 86 ml/min) with two good kidneys... She just said, "well, things have just gotten better"... But I was told by her and everything I've read that kidney function like that does not return once it's lost... So your thoughts on an 86 ml/min reading back in 2008 (I also had another one in that same time frame of 81 ml/min, again right after the testing lab changed both their machine and their cal method), where I had lived for years before and am again now at about 60 ml/min??? Would you expect a real bump upward of almost 50% in GFR or was this more likely errors and learning curve for the lab with new equipment and new calibration methods???? thanks... bob...
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john-skpt responded:
I really have no good guesses either.

What I suspect is this: always remembering that the eGFR measurements are /- 10% (and this 'acceptable lab variance' tends to apply to nearly all automated blood and urine tests), I think that the technicians were getting accustomed to the new calibration protocols and so the early tests showed false elevations in eGFR and similar decreases in SCr.

After a time, we see what we really would expect to see in stable cases: there is not a significant change outside the 10% parameter. The revision in the MDRD standard might have contributed to this shift too, but I suspect that most of the odd and temporary shift was due to learning, calibrating, and 'breaking in' the new machines.
 
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diitto replied to john-skpt's response:
Thanks for the response, John... On a related question, what is your understanding of what a "normal" SCr (or related GFR) might be for a person with only one functional kidney... I have read and heard so much conflicting information on that topic... On one side, I've heard that due to hypertrophy and renal reserve properties, etc., that blood tests might be expected to be the same whether you have two kidneys or just one... On the other side, I've seen one quote all over the internet that seems to have originated from the NIH that says, "People have a reduced GFR if they have only one kidney." Another quote I have seen repeated on many reputable websites says, "A person with only one kidney may have a normal level (SCr) of about 1.8 or 1.9"... Clearly that's a value that is higher than what would be considered normal for a person with two healthy kidneys... So what's your view on what are reasonable values for a person with a single functional kidney??? thanks... bob...
 
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john-skpt replied to diitto's response:
Traditionally, even though labs tend to list only one range for anyone and everyone, I think that docs make instant mental calculations to adjust the range for each patient.

If you dig into the details, the range should be generally higher for men than for women even if you account for differences in body mass, men tend to have a higher muscle/fat ratio and more muscle tissue goes with a higher serum creatinine. Same for age: it is expected that someone who is 60 will have less function than someone who is 20. But the labs don't recalcualte the numbers for anybody.

I know that my transplant lab uses a wider range than any other doc, generally with a high end at 1.6 mg/dL. And over time they tweak each patient's individual range once they have let things stabilize with the one transplanted kidney and a certain mix of medications. They are able to do it this way because of a limited number of patients, and a lot of data over a long time that they can use. But it is not practical to do in most cases or with large numbers of patients who have vastly different conditions and circumstances, there are just too many variables to account for.


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