Standard vancomycin dosing will not work for every patient. Let me walk you through 3 often overlooked patient characteristics that suggests the need for closer scrutiny and off-protocol dosing.
This isn’t a post about how to perform detailed vancomycin calculations. It’s about learning how anticipate and be responsive to the body mechanics of patients to prevent both surpratherpaeutic and subtherapeutic dosing of vancomycin and maximize patient outcomes.
The Knowledge Gap
Despite the availability of algorithms, protocols and calculators, vancomycin dosing still remains a challenge for many. A knowledge gap exist when there is a dissociation between the components of a calculation and the patient variables they connect to.
Let’s close the gap. These are 3 red flags that should make you pause and reconsider the use of standard vancomycin dosing protocols.
- paraplegia/quadriplegia
- “normal” renal function
- ascites
I have seen dosing errors occur in each of these situations multiple times in my career. Let me help you to never make these mistakes.
Benefits of Vancomycin Dosing Protocols
Vancomycin protocols are developed with good intentions. They are helpful.
In general protocols allow for standardization of care which is useful when different providers see the patient from day to day. There is an inherent understanding of what as done and why when a protocol is followed.
Vancomycin dosing protocols are usually well vetted and based on evidenced based guidelines which can translate to better patient outcomes.
Protocols are a great starting point and every institution should have one for vancomycin dosing.
It lays out an approach for vancomycin dosing that will likely be successful for the majority of patients.
However, no protocol replaces clinical judgement. No protocol can account of every clinical scenario. Which is why we must also know when to stray away from vancomycin dosing protocols.
The Problem with Vancomycin Protocols
All protocols make assumptions. They have to do this if they are to be applied broadly. They often make linear correlations between patient characteristics and how vancomycin will be processed by the body.
Most vancomycin protocols rely heavily on 2 patients factors: their weight and renal function. Weight is used to estimate the dose and renal function to determine the frequency of dosing.
Within the boundaries of weight and renal function set by the protocol there are many patient variables that influence how the body will process vancomycin.
Variables of Weight
A patient’s weight is a function of muscle, fat and fluid. Variations the relative amounts of these will affect how vancomycin will be processed. 2 patients weighing 72kg may have very different body compositions and therefore process vancomycin differently.
Variables of Renal Function
Renal function is estimated by creatinine clearance (CrCl). CrCl is calculated using Scr, weight and age. 2 patients with a CrCl of 45ml/min can vary significantly in each of these variables but have an overall similar calculated value. They will process vancomycin differently because of these variabilities.
It would be very difficult for any protocol for account for all these patient variables. The first step is being aware of the limitations of vancomycin dosing protocols. Then we need to actively seek out what would make our patients ineligible for protocol dosing. We should be looking to rule out protocol use. This starts with a thorough chart review.
1. Quadriplegia & Paraplegia
I’ve seen this error multiple times in my career.
Always read the patient’s history prior to dosing vancomycin. If quadriplegia, paraplegia or really any spinal cord injury is documented, veer off the protocol. Here’s why.
Paraplegia refers to paralysis of the legs, quadriplegia is paralysis of both arms and legs.
Patients with spinal cord injury, acute or chronic, have a substantially higher risk of renal failure compared to the general population.
Renal deterioration can occur at any time after injury. Routine assessment of renal function is required.
Estimating Renal Function in Paralyzed Patients
The issue is further complicated by the use of creatinine clearance as an estimate of renal function in this population. Most vancomycin dosing protocos rely on this calculation to determine the frequency of dosing.
Creatinine is a waste product of creatine degradation.
Our bodies derive creatine from protein metabolism and uses it to store energy in muscle as creatine-phosphate.
Once creatinine-P has donated it phosphate stores to make energy in the muscle, it once again becomes creatine. Creatine will naturally degrade overtime to become creatinine which is released from muscle then cleared from the body by the kidneys.
In a non paralyzed patients serum creatinine works are a measure of renal function because the muscles are constantly releasing it and the kidneys then clear it.
In paralyzed patients serum creatinine is low not because the kidneys are clearing well but because it is not being produced. A calculation cannot discern this difference, we must be able to recognize the risk and adjust our calculations and dosing.
These patients have very restricted mobility so they lose muscle mass quickly. Diminishing muscle mass results in low levels of serum creatinine, consequent overestimation of renal function by creatinine clearance calculations and masking of any renal dysfunction.
Impaired renal function leads to reduced drug clearance and vancomycin accumulation. I have seen this happen multiple times in my career when creatinine clearance is taken at face value in paralyzed patients.
Vancomycin Dosing Interval in Paralyzed Patients
When dosing vancomycin in quadriplegic and paraplegics, extend the dosing interval beyond what the calculated creatine clearance suggests. They will require an extended window for drug clearance. Most times, it appears they can tolerate Q8H dosing. I assure you, they cannot.
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2. “Normal” Renal Function
Never dose on a single value, always dose on a trend.
The trend we are most concerned about is serum creatinine. This is the lab value that will be used in the estimation of renal function. Most practitioners understand the importance of stable renal function but can underestimate what stability really means.
What is normal serum creatinine?
Normal serum creatinine in the general population ranges anywhere from 0.6-1.2mg/dL. Normal serum creatinine for YOUR patient is dependent on their baseline. Though a value between 0.6-1.2 mg/dL is considered normal, a value of 1mg/dL can signal renal impairment for your patient. Here’s how.
Acute kidney injury is defined as an increase in serum creatinine greater than 0.3mg/dL within 48 hours. This means that a patient whose serum creatinine is normally 0.7mg/dL would have renal impairment if they presented with a serum creatinine of 1.1mg/dl even though they would still fall within the “normal” range.
Sample Patient
There is a significant difference in the rate of filtration even though both are “normal” values. If we simply assume that if serum creatinine at presentation is “normal” because it falls within the normal range we would miss the opportunity to proactively guard against drug accumulation.
Most patients who are being treated for less than 5 days with vancomycin may not require a level but a patient like this may require assessment of levels and more frequent monitoring of renal function.
Vancomycin Dosing with “Normal” Renal Function
Whenever possible, always compare the serum creatinine at the time of dosing with prior labs even when values fall within the “normal” range.
3. Ascites
A patient’s weight should be assessed from a perspective of body composition: fat versus muscle versus fluid weight.
Again, always read the patient’s full history and chief complaint before dosing vancomycin.
If ascites is mentioned, raise the red flag.
Ascites occurs when fluid accumulates in the abdomen usually secondary to cirrhosis, cancers, alcoholism, viral infections among other things.
Vancomycin is hydrophilic (water loving) but it has a large volume of distribution (~0.7L/kg). This means that it distributes widely throughout different tissues in the body which is why we can effectively use it to treat skin infections, brain infections, bone infections, bloodstream infections and ascitic infections.
A patient who is volume overloaded due to ascites can have a significant amount of their reported weight influenced by fluid accumulation in the abdomen.
When a patient presents with large volume ascites, part of the treatment plan is to alleviate this fluid congestion. High volume paracentesis may be performed removing usually more than 5L via a catheter.
High Volume Paracentesis
A patient with severe ascites requiring large volume paracentesis can have anywhere from a 10-44 lb (4.5-20kg) difference in weight after fluid has been removed from the abdomen.
Vancomycin is dosed at 10-15mg/kg. That is a potential 45mg-300mg difference in the dose used. This accumulation can be significant with repeated dosing.
Vancomycin dosing in patients with cirrhosis is further complicated by the reduced production of creatinine.
This can result in an overestimation of renal function via creatinine clearance equations as we discussed above.
Because vancomycin is able to penetrate ascitic fluid when that large volume has been removed it may reduce the volume of distribution of vancomycin further increasing the risk of accumulation.
If a patient continues to receive the dose calculated with their pre-paracentesis weight there are a multiple factors that places them at risk for supratherapeutic dosing.
Vancomycin Dosing with Ascites
If severe ascites is mentioned in a patient’s chart it should raise an alert. Investigate the degree of ascitic accumulation and monitor for any recommendations for paracentesis.
Vancomycin dosing protocols are a valuable resource that can be accurately applied to a vast majority of patients. Because of this the best approach is to use them with skepticism. We are trying rule out our patient’s eligibility for protocol dosing via a thorough chart review.
As you read any mention of paraplegia, quadriplegia, ascites, paracentesis should trigger an internal alert. Strict protocol dosing is probably not the best approach for these patients. Assessing renal function relative to the individual patient’s baseline rather than population parameters of “normal” is also best practice.
If you’ve found this unit helpful I would love to hear from you. Leave a question or comment below.
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The information on this website is intended to be used solely for educational and informational purposes. While the content may be about specific medical and health care issues, it is not a substitute for or replacement of personalized medical advice and is not intended to be used as the sole basis for making individualized medical or health-related decisions.
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