Crystalline degradation product cross-reactivity with vancomycin fluorescence polarization immunoassays

Vancomycin and creatinine determination in dried blood spots: Analytical validation and clinical assessment

This study aims to develop a liquid chromatography tandem-mass spectrometry (LC-MS/MS) method for vancomycin and creatinine measurement in dried blood spots (DBS) and to evaluate its clinical application. The analytes were extracted from DBS and analyzed by LC-MS/MS. Vancomycin and creatinine DBS and plasma concentrations were compared in 54 and 35 samples, respectively, from 29 patients. Accuracy was 94.4-102.6%, intra-assay precision was 2.1-5.6%, and inter-assay precision was 3.5-7.0%. Patients vancomycin plasma to DBS concentration ratios were highly variable (1.148-5.022), differently from creatinine (0.800-1.283). The assay has adequate analytical performance. Plasma concentrations can be satisfactorily predicted from DBS measurements for creatinine, but not for vancomycin, which limits its clinical application.

Factors impacting unbound vancomycin concentrations in different patient populations

The unbound drug hypothesis states that only unbound drug concentrations are active and available for clearance, and highly variable results regarding unbound vancomycin fractions have been reported in the literature. We have determined the unbound vancomycin fractions in four different patient groups by a liquid chromatography tandem mass spectrometry (LC-MS/MS) method and identified factors that modulate vancomycin binding. We have further developed and validated a prediction model to estimate unbound vancomycin concentrations. Vancomycin (unbound and total) concentrations were measured in 90 patients in four different hospital wards (hematology [n = 33 samples], intensive care unit [ICU] [n = 51], orthopedics [n = 44], and pediatrics [age range, 6 months to 14 years; n = 18]) by a validated LC-MS/MS method. Multiple linear mixed model analysis was performed to identify patient variables that were predictive of unbound vancomycin fractions and concentrations. The variables included in the model were patient age, ward, number of coadministered drugs with high protein binding, kidney function (estimated glomerular filtration rate [determined by Chronic Kidney Disease Epidemiology Collaboration formula]), alpha-1-acid glycoprotein, albumin, total bilirubin, IgA, IgM, urea, and total vancomycin concentrations. In the pediatric cohort, the median unbound vancomycin fraction was 81.3% (range, 61.9 to 95.9%), which was significantly higher (P < 0.01) than the unbound fraction found in the three adult patient cohorts (hematology, 60.6% [48.7 to 90.6%]; ICU, 61.7% [47.0 to 87.6%]; orthopedics, 56.4% [45.9 to 78.0%]). The strongest significant predictor of the unbound vancomycin concentration was the total drug concentration, completed by albumin in the pediatric cohort and albumin and IgA in the adult cohorts. Validation of our model was performed with data from 13 adult patients. A mean difference of 0.3 mg/liter (95% confidence interval [CI], -1.3 to 0.7 mg/liter; R(2) = 0.99 [95% CI, 0.95 to 0.99]) between measured and calculated unbound vancomycin concentrations demonstrated that the predictive performance of our model was favorable. Unbound vancomycin fractions vary significantly between pediatric and adult patients. We developed a formula to estimate the unbound fraction derived from total vancomycin, albumin, and IgA concentrations in adult patients.

Vancomycin assay performance in patients with end-stage renal disease receiving hemodialysis

STUDY OBJECTIVE

To compare the performance of polyclonal fluorescence polarization immunoassay (pFPIA) with that of enzyme-multiplied immunoassay technique (EMIT) in patients receiving vancomycin and hemodialysis.

SETTING

Outpatient hemodialysis center.

PATIENTS

Seven subjects with end-stage renal disease treated with hemodialysis 3 times/week with a cellulose triacetate hemodialyzer.

INTERVENTION

Subjects received vancomycin 1000 mg intradialytically during the first study session and 750 mg every other hemodialysis session thereafter for 4 weeks.

MEASUREMENTS AND MAIN RESULTS

Blood samples were obtained throughout the study, and vancomycin serum concentrations were determined by pFPIA and EMIT. The mean +/- SD difference (estimate of bias) between assays was -1.10 +/- 1.35 mg/L. The limits of agreement (mean difference +/- 1.96 x SD) between them were -3.80-1.60 mg/L.

CONCLUSION

Our data suggest that the manufacturer's changes in the vancomycin pFPIA eliminated overestimation of drug concentrations in patients undergoing high-permeability hemodialysis.