What do you do for Vancomycin dosing when it comes to pts with lower renal function (CrCL~30-45)? Do you start dosing at intervals of Q24h or longer and check for the trough before the 3rd dose?
In the hope of preventing resistance, does anyone out there take more random levels to keep patients in a therapeutic range instead of waiting 3 days to find out they've been under-dosed? Instead of the typical loading dose, this method is using a shorter initial dosing interval as the 'loading dose' and achieves therapeutic dosing earlier.
This consult typically looks like the following:
Patient receives 1gram of Vanc in the ER for pneumonia. Patient's CrCL is ~35. A random Vanc serum level is scheduled for 12 hours later. That level typically comes back therapeutic (goal of 15-20). At this time, another 1 gram dose is given and a random serum level is ordered in 24 hours (accounts for accumulation and is basically the 'trough before the 3rd dose'). Once that level comes back therapeutic, it's fairly certain Q24h is the winner. It's one additional serum level but the patient doesn't drop below therapeutic range and you're not going 3 days of potentially being sub or supratherapeutic.
Well, I typed a long reply and was timed out in the process, so I'll try this again.
Vancomycin has been very tricky lately. I believe there to be an interaction between pip/tazo and vancomycin, but I can't prove it yet.
What I've found that works well for me is as follows.
Use the SCr you have available. Calculate a Ke using the population kinetics equation as an estimate. Up until a couple of years ago, we were targeting trough levels of 10-15 mg/L. You can get a rough estimate of your dosing interval by multiplying your half life by 2. Take the total body weight and multiply by 15 and 20 mg/L. Pick a convenient dose in between these two values, use your dosing interval as calculated, and that should get you between 10-15 mg/L.
However, with increasing MIC's, we are indeed having to target higher troughs of 15-20. Theoretically, multiplying the half life by 1.5 should get you closer to a dosing interval which will increase the trough to 15-20 mg/L. What I've started to do is a variant of this process that seems to produce reliable results.
Calculate your Ke estimate as before as well as your half life. Multiply the half life by 2 again and back up to the next more often dosing interval. Pick a convenient dose between 15-20 mg/kg and start the therapy.
For example, if you calculate a half life of 13 hours for a 70 kg pt, this would lead you to a dosing interval of roughly 26 hours, rounded to 24 hours to target a serum trough concentration of 10-15 mg/L. Instead of giving this q24hr, back up to q18hr. Multiplying the total body weight by 15 and 20 mg/L would lead you to a dose between 1050 and 1400 mg. This leaves you 3 realistic choices-- 1 Gm q18hr, 1250 mg q18hr, and 1500 mg q24hr. If you're concerned about hard to penetrate areas (lungs, distal cellulitis in a DM pt, etc.) 1250 q18 would be a reasonable choice. If you've got a "simple" bacteremia 1 Gm q18hr would be a reasonable choice. If you're concerned about CNS penetration you might consider 1500 or even 2000 mg q24hr to start.
As for the situation you described, a random level after one dose doesn't really tell me much. Most patients I run across reach the floor after presenting to the E.R. in such a state as you describe were dehydrated and after receiving fluids have serum creatinine levels lower than upon presentation. I think your best bets at that point are to pay close attention to I/O's and BUN as well as SCr. Positive fluid balances can dilute BUN and SCr and make the patient's renal function appear normal at first glance. Continual dosing at q24hrs may be inappropriate at that point as the patient may not be eliminating the drug as well as the numbers indicate.
Obtaining a trough level before the third dose as you describe must be taken with a grain of salt as well. Before the third dose, we're potentially only at approximately 87% of steady state, depending on the dosing interval's relationship to the half life. It's reasonable to expect a 12.5% increase (more or less) at steady state over the level obtained before the third dose and that does not account for any subsequent changes in renal function during the time it takes to reach steady state.
For example, if the half life is 12 hours and the dosing interval is 24 hours you've given 2 doses and had 5 half lives before the third dose. But, if the half life is 12 hours and the dosing interval is 18 hours you've given 2 doses and had 3 half lives. So, instead of that level being roughly 96% of steady state trough concentration, the obtained level is only 87.5% of steady state trough concentration.
Of course, the minutae of this is immaterial when the patient is in front of you. When in doubt, I'd rather be high than low (within reason) and I'd rather check a level than wildly guess.
Just my humble opinion and I'm certain others will disagree and have other methods that work well for them and produce equivalent (if not better) results.
John
..so you better get my trough!
