Instead of a formal literature review, I just wanted to post a quick clinical comment on something that came up with a patient I saw on consultation in the hospital about 2 weeks ago. This 91 y/o female nursing home resident was admitted with a diagnosis of “cellulitis” of the right foot (of course much of this “cellulitis” resolved with elevation of the leg making it more PAD, than infection, but that is a topic for another time). As is common practice by some services in our hospital she was started on piperacillin/tazobactam 3.375g q6h and vancomycin 1g q12h.
When I saw the patient the next morning I noticed that her serum creatinine was 0.9. I was with a resident and I mentioned to him “Now, this is a problem”. His response was “How come? It is totally within normal limits?” You see where I am going with this…you CANNOT depend on the serum creatinine to determine renal function especially in an elderly patient. Although, I know there are newer, more sophisticated techniques that have been developed, the determination of creatinine clearance using the Cockcroft and Gault equation is an easy and handy way to determine the patient’s renal function for the purpose of antibiotic dosing. I know it is not easy to remember the equation. Most practitioners I ask can give me the “(140 – age)” part but that is about it. Just to refresh memories:
(140-age) x Wt(kg) / 72 x serum Creat all (x 0.85) for a female
I know it is not the easiest thing to remember. The good news is that…don’t worry, there’s an app for that! It doesn’t matter if you are on iPhone or Droid you can download a free application, usually bundled with multiple other medical calculators, which makes it easy to just plug in the appropriate numbers. If you don’t happen to have a smartphone handy, just go to a computer at the nursing station and Google “Cockcroft gault equation” and you will end up with about 31,000 results. Don’t bother fishing through, I like the very first one.
Back to this patient. She only weighed 110 lbs. Upon plugging in the numbers we found her Creatinine clearance to be about 30. This means she was receiving too much antibiotic. I changed her pip/tazo to the renal dose of 2.25g g6h, held the vancomycin and ordered it to be restarted after 24 hrs at 750mg q24h. (In a case like this, linezolid which does not have to be adjusted for renal insufficiency would have been an excellent alternative).
The bottom line of this brief anecdote is to just remind you that, when dosing antibiotics cleared through the kidney, you should be determining creat clearance and not just depending on the serum creatinine. You may be surprised at the result.