Lesson #4: Vancomycin Dosing
After reviewing the important terminology, background, and equations, it is finally time to grab your calculator.
You will be going through step by step on considerations when dosing vancomycin and will get the opportunity to practice and test your comprehension of the equations.
So, if you are ready, let’s get started!
Step 1: Assess the Patient
Before you start on your calculations, it is important to assess whether it is appropriate to give vancomycin by checking the following:
- Allergies
- Indication for therapy
- Current antibiotic therapy
- Microbiology results if available
- Serum creatinine (Scr)
Allergies
Determine if the patient has had anaphylactic reactions to vancomycin in the past. Oftentimes, vancomycin-infusion reactions (previously known as red man syndrome) are listed as an allergy though it is not a true allergic reaction but a rate-dependent infusion reaction that can be avoided by extending the infusion time.
Indication for therapy
Evaluate whether vancomycin is appropriate to give. This can be difficult as vancomycin is oftentimes ordered empirically while awaiting cultures to result. The CDC has established guidelines on situations where vancomycin is appropriate to give:
- Serious infections caused by beta-lactam-resistant gram-positive organisms
- Infections caused by gram-positive organisms in patients with serious allergies to beta-lactam antimicrobials
- Antibiotic-associated colitis
- Prophylaxis for endocarditis
- Prophylaxis for major surgical procedures at institutions with high rates of MRSA or MRSE
Current antibiotic therapy
Review the patient’s antibiotic regimen and assess if it is appropriate.
- If we are starting vancomycin, is the patient currently on another antibiotic that covers the exact same organisms? If so, is double coverage needed?
- Is vancomycin an appropriate antibiotic to give based on the infectious disease we are treating? (e.g. we would not want to give IV vancomycin if we are treating Clostridioides difficile, we would switch it to oral vancomycin which does not require us to calculate a dosing regimen)
- Will vancomycin cover the suspected organisms for the infectious disease we are treating? (e.g. vancomycin does not cover pseudomonas so we would want to pick another antibiotic)
Microbiology results if available
Review the patient’s cultures (e.g. blood cultures, urine cultures, sputum cultures) and assess for opportunities to narrow therapy or change therapy based on antibiotic resistance.
Serum creatinine (Scr)
Assess the patient’s renal function by collecting important laboratory values such as serum creatinine.
Back to Dr. Vanko
During this time, Dr. Vanko developed fevers and a high white count. Blood cultures were drawn, and preliminary results show staphylococcus in the blood. No sensitivities have returned yet.
In the meantime, it was decided that he should be put on broad-spectrum antibiotics, vancomycin and cefepime. I like to call this very popular combination, vancopime. 
Dr. Vanko
Age: 65 years old
Height: 176 cm
Actual body weight: 72 kg
Allergies: atorvastatin, peanuts
Temp: 100.4° F
WBC: 24
Scr: 1.2
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Step 2: Calculate the Creatinine Clearance
The next step is to calculate an estimated creatinine clearance (CrCl) using the Cockcroft-Gault formula
CrCl (female) = CrCl (male) x 0.85
Clinical Note
This step may seem deceptively easy, but a lot of people tend to over or underestimate a patient’s CrCl by inputting incorrect weights and Scr into the equation. The CrCl is the backbone for all further pharmacokinetic equations, so it is important to estimate it correctly! Let us take a moment and discuss it through.
- Ideal body weight (IBW) (men) = 50 kg + 2.3 kg for each inch over 5 feet
- Ideal body weight (IBW) (women) = 45.5 kg + 2.3 kg x for each inch over 5 feet
Weight
When determining the weight used for the CrCl equation, ideal body weight is recommended unless:
Scenario | Recommended weight to use: |
Actual body weight <IBW | Actual body weight |
Actual Body Weight >30% over IBW | Adjusted body weight |
Dr. Vanko, 65 year old Male
Height: 5’9” (69.3 inches)
Actual body weight: 72 kg
Allergies: atorvastatin, peanuts
Temp: 100.4° F
WBC: 24
Scr: 1.2
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Serum Creatinine
Clinical judgment is needed when deciding what Scr to use. Why can’t we use the patient’s Scr that we obtained from their lab work, you ask? Well, it turns out that Scr is not the best marker for renal status in patients who have low muscle mass (e.g., bed-bound, elderly). Because of this, we will have to ‘tweak’ our Scr value in certain circumstances. Mastering this will require some experience and practice on your end, but be sure to refer to your institution’s policy.
Scenario | Recommended Scr to round to: |
Scr <0.8 mg/dL | 0.8 mg/dL |
Age >65 years or quadriplegic/bed bound | 1 mg/dL |
Dr. Vanko, 65 year old Male
Height: 5’9” (69.3 inches)
Actual body weight: 72 kg
Allergies: atorvastatin, peanuts
Temp: 100.4° F
WBC: 24
Scr: 1.2
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Step 3: Determine Your Dosing Strategy
Now that you have calculated the patient’s CrCl, the next step is to determine your approach to dosing based on the patient’s renal function.
In a patient with good renal function, you want to continue with scheduled vancomycin dosing calculating a maintenance dose and interval.
In a patient with unstable renal function (AKI) or poor renal function (CKD), you would want to dose by levels or start hemodialysis dosing.
Vancomycin renal dosing strategies are discussed in another course.
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Step 4: Loading Dose
Loading doses are strongly recommended in severe infections (e.g., sepsis, bacteremia), obesity, or any time steady-state concentrations are difficult to achieve. Compared to the equation for maintenance dose, the loading dose equation is not dependent on renal status; therefore, administration of a loading dose can be considered without regard to the patient’s creatinine clearance.
There are two methods for calculating a loading dose.
Method #1: Use the weight based equation
LD = 20-25 mg/kg x actual body weight
Method #2: Use the pharmacokinetic equation
LD = desired peak x Vd.
For desired peak, use estimated peaks of 30-40 mcg/mL.
*2020 Guideline Update Alert*
ASHP released new recommendations in critically ill patients with suspected or documented serious MRSA infections, a loading dose of 20-35 mg/kg (max dose of 3,000 mg). The magnitude of the loading dose should be driven by the severity of the infection and the need to get to therapeutic concentrations rapidly.
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Step 5: Calculate the Dosing Interval
Now that you have the patient’s creatinine clearance, you can calculate the estimated elimination constant (Ke). We will need this to calculate an interval for the patient.
Ke = (0.00083 x CrCl + 00.0044)
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In practice, there are two methods for calculating the dosing interval
Method #1: Use the dosing interval equation
Estimated Dosing Interval = [ln (desired peak/desired trough) / Ke] + t’
This method allows you to estimate a more precise interval that correlates to your patient’s estimated creatinine clearance.
Dosing intervals should be rounded to clinically acceptable intervals of 8 hours, 12 hours, 18 hours, 24 hours, etc.
Method #2: Determine an interval off of estimated population creatinine clearance tables
Creatinine Clearance (mL/min) | Recommended Interval |
>90 | Q8hr |
50-90 | Q12hr |
15-49 | Q24hr |
<15 | Pulse dosing |
Reference: Lexicomp Inc.
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Step 6: Calculate the Maintenance Dose
In practice, there are two methods for calculating the maintenance dose
Method #1: weight-based calculation using the actual body weight
MD = 15 mg/kg x Actual Body Weight
Method #1 is often the most popular method
Method #2: using the maintenance dose pharmacokinetic equation
MD = LD x 1-e-KeT
Method #2 can be calculated by hand or using an institution-approved dosing calculator
Regardless of the method chosen, remember to round the dose to the nearest 250 mg increments.
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Step 7: Calculate the Estimated Peak and Trough
To determine the estimated peak and trough from the dose and interval you have chosen, the following equations are used:
Cpeak = (Dose/t’)(1-e-Ke(t’)) / Vd(Ke)(1-e-Ke(T))
Cmin = Cpeak (e-Ke(T-t’))
In practice, institution-specific calculators are used to assist with calculating a peak and trough.
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As you can see from this example, 750 mg every 12 hours will get us closer to the desired goal trough and peak.
Whohoo! You made it through the TOUGHEST section. Those calculations were just a warmup. If you’re looking to get additional practice. Check out this worksheet for additional vancomycin dosing problems!
Let’s review what you have learned about vancomycin dosing in this quick 5-minute quiz.
Lesson #4: Vancomycin Dosing Read More »