We had a patient with an active infection and bilateral below the knee amputations. The Creatinine was obviously going to over estimate kidney function due to the low muscle mass and I wasn’t prepared to trust the cystatin C in the presence of active inflammation. What to do? Can we McGyver the GFR by looking at the fall in vanco levels over time? Yes, of course we can.
Here’s how it works. Vancomycin is primarily eliminated by glomerular filtration, so its clearance approximates the GFR.
1. Get two vanco levels
You need two vancomycin concentrations drawn after the distribution phase (ideally 1–2 hours post-dose and a trough) and without an additional dose in between.
2. Calculate the Elimination Rate Constant (ke)
This gives you the rate at which the drug is disappearing from the plasma.
3. Estimate vancomycin Clearance
Vancomycin’s volume of distribution (Vd) is about 0.7 L/kg.
4. Convert to GFR (mL/min)
Since vancomycin is almost entirely renally cleared, its clearance approximates GFR.
The vanco clearance calculated above is in liters per hour, so to get conventional GFR units, multiply by 1000 and divide by 60
5. Caveats
- This only works if renal function is stable (no AKI or wild fluid shifts)
- Must use post-distribution levels
- Non-renal clearance of vancomycin is minimal but not zero
- Vd can be wildly off in critical illness, obesity, or fluid overload
Bottom Line
You can use vancomycin estimate GFR. It’s not perfect, but in the right context it’s a clever way to triangulate kidney function when the usual suspects lie.