Same Lab, Same Creatinine levels - Different Egfr's

[RickHow]

Same Lab - Same Creatinine levels - Different egfr's

Due to other health problems in addition to "ckd" my blood is tested every 3 weeks. I go to the same lab each time for testing. I keep track of my results over time. I noticed something of interest that I point out to others who track their Creatinine and Egfr. As we know Egfr is a calculation based up

Creatinine level, age, sex, race. Well here are some interesting results recently for me, all at the same constant age, sex, race. Notice that the same Creatinine level, is reported as a DIFFERENT EGFR level. Let's say that again for clarity. All the inputs to the Egfr formula are the same (creatinine, age, sex, race) yet the resulting eGFR is Different.

Creatinine 1.7, Egfr 40

Creatinine 1.6 Egfr 43

Creatinine 1.8 Egfr 38

Creatinine 1.8, Egfr 40!!!

Creatinine 1.7 Egfr 40 !!!!

Creatinine 1.75 Egfr 38

Creatinine 1.8 Egfr 40. !!!!

So I looked online. Several online calculators from reliable websites such as National Kidney Organization, Quest Diagnostics. They report;

1.7 Creatinine , EGFR 40

1.75 Creatinine, Egfr 73

1.8 Creatinine, Egfr 37

Same Creatinine levels produce different eGFR results, even at the same lab (i.e. 1.8 is 38gfr sometimes, 1.8 is 40 egfr sometimes, other labs 1.8 egfr is 37.

[orangecity41]

Did you mean eGFR 37 for 1.75 Creatinine and not 73. Was the age different in their calculations? Did they sometimes use age with a decimal? That is odd.

[RickHow in reply to orangecity41]

typing error on my part. for 1.75 it was 39. I agree with you it is so odd. I mentioned it to my doctor a few months ago when I first noticed it. I even gave him printed copies of results I got online using online egfr calculators (there are so many available). He welcomed the copies and said he would take it to the technicians in the blood lab. But never heard a word back. The confusing part for me is, which is wrong? The creatine level or their formula for calculating EGFR. All the numbers I gave were for the same age, although you are correct they may be using a decimal. BUT only one time was it reported with a decimal (1.75) all the rest were round numbers.

[userotc]

Whilst concerning, I presume it wont significantly affect the interpretation of trends in kidney function via serum eGFR and creatinine over time though it may explain some cyclical variations.

[kwikc]

What I see here are pretty consistent results within a certain range except that one which was 73 for a creat. level of 1.75 which strikes me as a gross error or typo.

Also, there are different formulas out there which give different results with the same input variables, some better, some worse at calculating values of eGFR between 50-60. You can’t mix them up and compare one with another. You also don’t exactly know what accuracy was used with the input variables. For example, was the creat. value used in the formula out to two or three decimal places or just one, and was your age rounded or used with fractions of a year?

There is also a way of calculating using Cystatin C as an input variable in addition to creat. This gives maybe more accurate results. You didn’t mention that. So except for that one anomalous value, your results are all consistent. You have CKD stage 3b. Not good, not catastrophic. Ain’t much you can do about it either.

I’m the same. Best advice: Stay hydrated, avoid drugs of all kinds as much as possible.

[RickHow in reply to kwikc]

Hi. Thanks for your input. Yes the one absurdly high value was a typo. I understand all the variables you mentioned. If I was getting these varied values at different labs, or over longer periods of time between testing, it MIGHT be more understandable. But these values are all from the same lab, every 3 weeks. And of course as you mention it can be variable by the person(s) doing the input rounding input numbers, not carrying out the same number of decimals, etc. So I did a little test of my own. I went to the online calculators of 2 RELIABLE websites. The National Kidney Organization and Quest. I used a Creatinine level of 1.8. I entered data and varied my age. I am 71. So I ran the formula using 71, 71.5, 72, 72.5. The resulting egfr did NOT change based upon the 3 ages. All the ages produced an egfr of 37. Yet "my lab" produced egfr of 40.I then did the same test and used creatinine levels of 1.7, 1.75, 1.77, 1.79. The online calculators produced an egfr of 1.7=39, 1.75=38, 1.79=37. NONE of the variable produced 40.

None of this is alarming in any way, but certainly is unusal.

[kwikc in reply to RickHow]

Well, you still haven’t said whether the equations used by all the organizations mentioned were the same each time. The formulas are obviously much more sensitive to creatinine changes than to age changes. Plus the eGFR is usually given as a rounded whole number which is approximate.

If you want the most accurate value, get the lab to measure GFR directly. This test is much more expensive though. There are several other tests that you could do to assess kidney function besides this one including ultrasound.

[RickHow in reply to kwikc]

I have no way of knowing what equations were used by the lab. I only know ALL the blood work was done by the SAME lab, and yet produced different results for the SAME Creatinine levels.

[Marvin8]

Calculations can be determined on your own, but how the creatinine levels are measured may be the more critical factor. If one lab's methodology for measuring creatinine levels differs from another, there's no way you'll be able to use the formulas across all labs.

[RickHow in reply to Marvin8]

Interesting. Didn't know that.

[Marvin8 in reply to RickHow]

"Most" of the labs standardize to an isotope dilution mass spectrometry (IDMS) reference measurement procedure, but the precise methodology can vary as can analyzers. Certain analyzers, especially ones that haven't been standardized must use a correction factor that takes their specific analyzer into account. Labs make mistakes. My creatinine levels always differ between Labcorp and Quest. It's just one of the reasons why I have the doctor test my Cystatin-C as well....in order to use it in the combined creatinine/cystatin-c formula.

[RickHow in reply to Marvin8]

thanks, good informations.

[kwikc]

Hi, again,I think the only way to get your questions answered is to call the lab and speak to the manager. He could tell you why you have the variances that you experienced with the results. I’ll stop here. Good luck!