A few weeks ago I had to check the dose of pregabalin (Lyrica) in kidney failure and found myself in the drug label (this is what happens when you fact check ChatGPT) and saw that they included the Cockcroft and Gault equation…
It was like a blast from my earliest memories of internal medicine. The orignial eGFR formula, MDRD, was already being adopted when I arrived in Chicago for fellowship, but during residency, if you wanted to know the GFR, you would whip out the old CG equation and calculate the creatinine clearance. And if you were really good, you would recognize that the constant, 72, in the denominator would cancel the weight for patients with roughly a normal size, so the formula was just (140 – age) divided by serum Cr, something that could be approximated in your mind. This hack was even pointed out in the original paper:
This ability to mental math the GFR disappeared with the MDRD formula:
This was the early days of handheld PDAs in medicine and the MDRD equation was a great reason to carry one with a medical calculator (MedCalc doesn’t arrive until 2005).
For the last year I have been working a with a team of academic internists from West Virginia on an editorial about the difficulties of assessing GFR in hospitalized patients. and an examination of the absurdity that we have a a lot of approved drugs where dosing guidelines were explored and established using the obsolete Cockcroft and Gault equation. The editorial has faced an uphill battle and is currently being revised for submission to our fourth journal. This is a new experience for me since in the past if I wanted to get my opinion out there I wouldn’t go through a journal but just post it here. Blogs have ruined my patience for the editorial process.
Though CG is antiquated and poorly validated it still performs well enough for quick assessments and areas where precise knowledge of the GFR is not needed. For example, in an 80 year old male with Cr of 2:
- Cockcroft Gault provides a CrCl of 30 ml/min
- MDRD gives you an eGFR of 32 ml/min/ 1.73m^2
- CKD-EPI creatinine-no-race (2021), generates an eGFR of 33 ml/min/ 1.73m^2
It is hard for me to imagine a situation where there is a meaningful difference between 30, 32, and 33.
Flip the gender and it still performs well:
- CG: 25 ml/min
- MDRD: 24 ml/min/ 1.73m^2
- 2021 CKD-Epi: 25 ml/min/ 1.73m^2
One last thought on this topic: Cockcroft Gault, MDRD, and CKD-Epi all come from a time when we thought that GFR was the defining characteristic of kidney disease. It underlies the entire concept of CKD, where the specific etiology of kidney disease is less important than the specific GFR, with the unstated assumption that all patients with a similar GFR behave similarly and can be approached similarly. I do not think this theory has borne out and I think nephrology would benefit from moving beyond eGFR to a more nuanced vision of kidney disease.
Addendum, this post was inspired by this post on Bluesky
And I had ChatGPT make a picture for this post:
Picture:
