Although numerous publications in the past several years have described the pharmacokinetics of vancomycin in various patient populations, disparity still exists regarding the most appropriate methods of monitoring, including therapeutic range, timing of peak determinations, and methods for adjusting doses.
Vancomycin is primarily effective against gram-positive cocci. Staphylococcus aureus and Staphylococcus epidermidis, including both methicillin-susceptible (MSSA & MSSE) or resistant-species (MRSA & MRSE), are usually sensitive to vancomycin with minimum inhibiting concentrations (MIC) less than 1.5 mcg/ml. Most strains of streptococcus are sensitive to vancomycin. Vancomycin is considered bactericidal (MBC/MIC < 4) except with enterococci and some tolerant (MBC/MIC > 32) staphylococci. When staphylococcal tolerance has been demonstrated, most clinicians add a second antibiotic such as an aminoglycoside to the regimen. Enterococcal infections should be treated with vancomycin combined with gentamicin. Vancomycin is also effective against the anaerobes, diphtheroids and clostridium species, including C. difficile.
Significant controversy has arisen in recent years regarding the efficiency by which vancomycin kills gram positive bacteria and the potential misuse of the drug. Several studies have shown that with both staphylococci and enterococci vancomycin does not kill the bacteria as quickly or sterilize the blood as rapidly as nafcillin or ampicillin. For this reason many authors suggest that unless the patient has an allergy to beta-lactams or has a methicillin resistant staphylococcal infection, the patient might be better served using a beta-lactam agent over vancomycin.
Concern over the ever increasing problems with vancomycin resistant enterococci (VRE) prompted the Center for Disease Control to issue a statement suggesting appropriate prescribing criteria for vancomycin.1 Situations in which the use of vancomycin should be discouraged:
Ototoxicity is an infrequent event occurring in fewer than 2% of patients receiving vancomycin. It is unclear whether elevated trough or peak levels are responsible for ototoxicity. Data in the literature suggest that trough levels of 13 to 32mcg/ml and peak levels of 21 to 62mg/ml are associated with this adverse effect. Such a wide range makes determination of the precise correlation of vancomycin serum levels with ototoxicity difficult.
A histamine-mediated reaction, often called 'red man syndrome', involves a rash over the upper body, possibly accompanied by hypotension. The syndrome is thought to be related to peak serum concentration. Healy (1990) reported that none of 11 volunteers receiving vancomycin 500mg every 6 hours demonstrated evidence of this reaction, while 9 of the same 11 showed symptoms consistent with 'red man syndrome' when receiving vancomycin 1000mg every 12 hours.
Vancomycin nephrotoxicity appears to be concentration-related, with an increased risk at trough concentrations greater than 15 mcg/ml.
Higher vancomycin doses carry a substantial risk for nephrotoxicity. Recent studies have shown that higher-dose vancomycin regimens are associated with a higher likelihood of vancomycin-related nephrotoxicity. A significant difference in nephrotoxicity between patients receiving >= 4 g vancomycin/day vs those receiving < 4 g vancomycin/day was noted: 34.6% vs 10.9%.19
Critically ill patients, patients receiving concomitant nephrotoxic agents, and patients with already compromised renal function are particularly at risk for vancomycin-induced nephrotoxicity. This risk is incremental with higher trough levels and longer duration of vancomycin use.20
Patients with multiple risk factors are particularly at risk for vancomycin induced nephrotoxicity.
The ideal dosing regimen for vancomycin maximizes the amount of drug received. Therefore, the 24h-AUC/MIC ratio is the parameter that correlates with efficacy. For vancomycin, a 24h-AUC/MIC ratio of at least 125 is necessary (some researchers recommend a ratio of 400 or more for problem bugs).
|< 125||4 (50%)||4|
|> 125||71 (97%)||2|
When given by IV infusion over 60 minutes, vancomycin follows a 2-compartment pharmacokinetic model; alpha (distribution) and ß (elimination). The alpha (distribution) phase is relatively long, averaging two hours. This has important implications for peak serum level sampling. If the peak level is drawn during the distribution phase, it cannot be used for analysis of the one compartment model.
Volume of distribution
Compared with aminoglycosides, the variability in the distribution volume of vancomycin is extreme. Published inter-patient variability has been reported as 0.26 to 1.30 L/kg, 0.21 to 1.51 L/kg, 0.2 to 1.3 L/kg, and 0.37 to 1.40 L/kg in a series of studies. The average Vd also varies widely in the literature, with early reports suggesting a value of 0.9 L/kg and more recent studies indicating a smaller Vd of 0.5 L/kg. There does not appear to be any readily identifiable clinical characteristic to explain this variability. Unlike the aminoglycosides where one can often predict a larger or smaller than average Vd based on fluid status, variability in vancomycin Vd appears to be completely unpredictable. Obese patients present another conundrum, some clinicians recommend LBW, others prefer TBW.
Like the aminoglycosides, vancomycin is primarily cleared by glomerular filtration. Correlation of vancomycin clearance to creatinine clearance typically gives values for slope of between 0.5 and 0.8 and y-intercept (non-renal clearance) of up to 15 ml/min. All studies have demonstrated a strong correlation between vancomycin clearance and creatinine clearance, however, there is significant variability in the non-renal clearance component. This unpredictability is particularly evident in patients with impaired renal function who are more dependent on nonrenal clearance. Therefore, extra caution is required when estimating clearance in patients with markedly decreased renal function.
Given the variability of Vd and clearance seen with vancomycin, standard doses are likely to be associated with a significant degree of variability in serum concentrations.
Regardless of the pk model used to assess the serum concentration time profiles, the terminal elimination half-life of vancomycin is prolonged and the total body clearance is reduced in patients with impaired renal function:
The relatively unpredictable relationship between dose and resultant serum levels of vancomycin has prompted the development of a wide variety of dosing methods. For patients with creatinine clearances of 15ml/min or greater, the method of Lake and Peterson appears to be the least biased and most precise predictor of vancomycin dosage.9 In their evaluation, 71% of peak concentrations were within the range of 20 to 30 mcg/ml, and 81% of trough levels were within the range of 5 to 10 mcg/ml.10
Modification of vancomycin dosing is a major concern in the patient with renal insufficiency. For patients with creatinine clearances less than 15ml/min, the method of Matzke may be the best choice.8 Vancomycin is not removed appreciably by hemodialysis and thus is administered only every 7 to 10 days in dialysis patients. Clearance during peritoneal dialysis is more controversial. Originally thought not to be removed by peritoneal dialysis, recent reports have shown that vancomycin is cleared by this route. Dosing of vancomycin in peritoneal dialysis patients remains controversial.
For evaluation of serum level data, methods incorporating Bayesian principles appear to give the best overall predictive performance compared with traditional methods of vancomycin dosage adjustment. The Bayesian approach combines both population and patient-specific information (i.e., serum level data) in predicting dosage requirements.
The pharmacokinetic model most widely used by clinicians has been one-compartment. Because vancomycin exhibits a multi-compartment pharmacokinetic profile, the clinical application of the one-compartment model requires post-distribution serum samples which are often difficult to accurately obtain. Compared with the 1-compartment model, the 2-compartment model results in a significant improvement in both bias and precision in predicting vancomycin peak and trough concentrations.
Considerable controversy exists concerning peak levels. Less controversial are recommendations for trough levels. Because of the pharmacodynamics of vancomycin, trough levels must remain above the MIC for continual anti-bacterial activity.
The trough sample should be obtained just prior to the dose. The timing of peak levels continues to be an area of controversy. Most experts now agree that peak samples are most appropriately obtained 15 to 30 minutes after infusion rather than 1-2 hours after, because peaks drawn later substantially underestimate the true peak levels achieved immediately after infusion.
Drawing at exactly the right time is not as important as having the lab note the exact times that the samples were drawn. Also, have the nurse note the exact times that the sample infusion was started and when it ended. Please be aware of the common policy of nursing personnel to record a dose as having been given exactly as ordered if it is given within 30 minutes of the recorded time. This could lead to significant errors in analysis, therefore it is important to record the exact times.
This issue cannot be stressed enough. Inaccurate recording of drug administration times and lab draw times are the greatest source of calculation error, having a greater effect than pharmacy preparation error or lab assay error.
In general the Bayesian approach to the determination of individual drug-dosage requirements performs better than other approaches. However, outlying patients in a population (ie, those patients whose pharmacokinetic parameters lie outside of the 95th percentile of the population) may be put at risk. As is always the case, the computerized algorithms outlined below can only assist in the decision-making process and should never become a substitute for rational thought or informed judgement.
Recent data have shown that prolonged treatment with vancomycin (>10 days) may result in a decline in the drugs clearance despite stable renal function. Given this risk of decreased elimination, close monitoring of serum levels is advisable even in patients with normal and stable renal function.18
Vancomycin pharmacokinetics are highly variable, it is a difficult drug to model empirically, look at the divergent methods in the literature. In short, vancomycin is not a drug to hang your "pk hat" on.
Before calculating an initial dose or analyzing serum level data, enter the target trough level and the standard length of infusion.
The program supports five different serum level analysis methods for the one-compartment model:
The Kinetics© program adds optional two compartment analysis which requires one or two serum levels. In general the more data input into the model, the more accurate the calculation.
The vancomycin models are not hard-coded into the program. The parameters are found in the drug model database and are fully user-editable. You can tailor each drug model to fit your patient population, or you can create your own models. See the Edit drug models section of the help file for further information.
CL = CrCl x 0.065
Vd = 0.7 L/kg
There is one important caveat when using a 1-compartment vancomycin model. Because of the long distribution phase of vancomycin, peak sampling time is an important consideration. If the one compartment model is used, the peak level must be drawn after the distribution phase, which is at least one hour after the end of the infusion.