Abbott AxSYM Vancomycin II assay: multicenter evaluation and interference studies

New liquid chromatography-tandem mass spectrometry method for routine TDM of vancomycin in patients with both normal and impaired renal functions and comparison with results of polarization fluoroimmunoassay in light of varying creatinine concentrations

A new LC-MS/MS method with simple sample extraction and a relatively short period of vancomycin analysis for routine therapeutic drug monitoring was developed and validated. 50μL serum was precipitated using 20μL 33% trichloroacetic acid and 0.5mol/L NH₄OH was added to increase pH before analysis. A RP BEH C18, 1.7μm, 2.1×50mm column maintained at 30°C and tobramycin as internal standard were used. Mass detection was performed in positive electrospray mode. The results obtained with LC-MS/MS method were correlated with an FPIA assay (Abbott AxSYM) using mouse monoclonal antibody. Subjects were divided into three groups according to creatinine levels (53.5±19.1, 150.2±48.4, 471.7±124.7μmol/L) and Passing-Bablok regression analysis and Bland-Altman analysis were used to compare vancomycin concentrations. The results of subjects with both normal and higher creatinine levels correlated very well and the linear regression model equations were near ideal (LC-MS_VAN=0.947×Abbott_VAN+0.192 and LC-MS_VAN=0.973×Abbott_VAN-0.411 respectively). Dialyzed patients with the highest creatinine levels showed about 14% greater vancomycin concentration with the FPIA assay (LC-MS_VAN=0.866×Abbott_VAN+2.127). This overestimation probably due to the presence of the metabolite CDP ought not to be of clinical relevance owing to the wide range of recommended vancomycin concentration.

Measurement and Calculation of the Extracorporeal Elimination of Vancomycin During Continuous Venovenous Hemodiafiltration and Continuous Venovenous Hemofiltration in Critically Ill Patients

Six surgical intensive care patients with continuous renal replacement therapy and therapy with vancomycin entered the prospective clinical study. The first day the patients were treated with continuous venovenous hemodiafiltration (CVVHDF) and the second day with continuous venovenous hemofiltration (CVVH). Three patients received 500 mg and three patients received 1000 mg of vancomycin every 12 hours. Monoclonal fluorescence polarization immunoassay (AxSYM) of vancomycin levels was performed from serum and dialysate/ultrafiltrate (during CVVHDF) or ultrafiltrate (during CVVH). Blood flow was 90 ml/hr, substitution 1 L/hr predilution, dialysate flow 1 L/hr (CVVHDF). The extracorporeal elimination of vancomycin during CVVHDF and CVVH is nearly linear but shows wide interindividual variation. The extracorporeal clearance of vancomycin was 24.2 ± 3.1 ml/min during CVVHDF (total clearance 60.4 ±18.1 ml/min) and 14.5 ± 2.4 ml/min during CVVH (total clearance 50.2 ± 14.9 ml/min). Intraindividual comparison revealed a significantly higher elimination of vancomycin by CVVHDF (p < 0.028). Peak serum vancomycin levels in patients receiving vancomycin 1g/day were 24.7 ± 5.3 μg/ml (CVVH) and 23.1 ± 5.2 μg/ml (CVVHDF), and with 2 g/day were 33.5 ± 2.7 μg/ml (CVVH) and 27.3 ±4.1 μg/ml (CVVHDF). The daily excreted amount of vancomycin during CVVHDF (r2 = 0.950, p = 0.01) and CVVH (r2 = 0.947, p = 0.01) can be calculated from a vancomycin level in the ultrafiltrate/dialysate outlet (CVVHDF) or the ultrafiltrate (CVVH) 8 hours after dosing. The 8-hour concentration of vancomycin in the ultrafiltrate from CVVH (or ultrafiltrate/dialysate from CVVHDF) during continuous renal replacement therapy serves as a basis for predicting extracorporeal elimination within 24 hours for the individual patient. Since critically ill patients show wide interindividual and intraindividual differences in the volume of distribution, clearance, and elimination half-life of vancomycin during therapy, the estimation of serum levels remains a necessity.

Advances in antibiotic measurement

Antibiotics are most commonly prescribed drugs in clinical practice. Therapeutic drug monitoring of these medications is typically associated with a select group of antibiotics such as aminoglycosides and vancomycin. Outside this group, other antibiotics such as chloramphenicol and antituberculosis agents may also require monitoring. Due to their wide therapeutic index, other classes of antibiotics such as penicillins, cephalosporin, sulfonamides, quinolones, and macrolides do not generally require routine therapeutic drug monitoring. Determination of serum or plasma concentration of these drugs, however, may be beneficial in those patients with compromised renal function. As can be expected, immunoassays for routine monitoring of aminoglycosides and vancomycin have been developed and are widely commercially available. Tests for other antibiotics, due to their infrequent use and low clinical application, are generally limited to in-house developed methods.

Study on pharmacokinetics and tissue distribution of norvancomycin in rats by CE with electrochemical detection

In this paper, we developed a sensitive and simple method to study the pharmacokinetics and tissue distribution of norvancomycin (NVCM) in experimental animals by using CE with electrochemical detection. Pharmacokinetics investigation was performed by the collection of blood samples at timed intervals following administration of NVCM. Pharmacokinetic parameters were calculated by the 3P87 pharmacokinetic program. The elimination half-life of NVCM was 42.4742 min with a clearance rate of 0.0233 mL x kg(-1) x min(-1). Additionally, drug distribution was studied by measuring the NVCM levels in kidney, lung, stomach, intestine, spleen, heart, liver, and cerebrum. Electrophoresis conditions such as buffer solution, working potential, separation voltage, and sampling time were also discussed. The linear range was from 0.8 to 540 microg/mL with a correlation coefficient of 0.9991. The detection limit was 0.3 microg/mL. This method was for the first time applied to study the pharmacokinetics and tissue distributions of NVCM in experimental animals.

Determination of vancomycin in human serum by micellar electrokinetic capillary chromatography with direct sample injection

BACKGROUND

Vancomycin (VCM) has a bacteriostatic effect on gram-positive bacteria such as the methicillin-resistant Staphylococcus aureus.

METHODS

A new assay for measuring vancomycin concentration by micellar electrokinetic capillary chromatography using direct serum injection was developed. A borate buffer (pH 10.0) containing 100 mmol/l sodium dodecyl sulfate was used as an electrophoresis buffer, and the detection was at 210 nm. The migration time of vancomycin was approximately 7 min.

RESULTS

The linearity was from 0 to 100 microg/ml, with the limit of detection of 1.0 microg/ml (S/N=3). The within-run CV was 3.99-5.53%, and the recovery rate was 91-103% for a concentration range of 6.5-45.5 microg/ml. The between-day CV was 6.76% at 22.2 microg/ml. There was no interference from 32 other antibiotics. The correlation coefficient between the assay and fluorescence polarization immunoassay and direct injection HPLC was 0.982 and 0.985, respectively. The assay required no sample preparation of serum and used only microquantities of an electrophoresis buffer and samples.

CONCLUSIONS

This assay is cost-effective and suitable for routine clinical use.

Fluorescence polarization immunoassay: can it result in an overestimation of vancomycin in patients not suffering from renal failure?

It has been reported in scientific data that fluorescence polarization immunoassay (FPIA) results in overestimation of vancomycin in patients with renal failure. This overestimation is caused by interference of the degradation product, CDP-1, in this assay. Increases in vancomycin levels have also been reported in patients not suffering from renal failure (nonrenal failure patients) who are receiving vancomycin therapy for approximately 10 days or more. The authors tested whether this increase in vancomycin in nonrenal failure patients is a result of CDP-1 interfering with FPIA or a change in the pharmacokinetics of the drug. Serum vancomycin peak and trough samples were obtained from 10 adult (mean age +/- SD: 55.9 years +/- 17.5) nonrenal failure patients (mean ClCr +/- SD: 76.2 mL/min +/- 29.20) receiving vancomycin therapy for at least 10 days. These peaks and troughs were obtained at steady state and again at approximately 10 days of therapy. All serum samples were analyzed initially by fluorescence polarization immunoassay (FPIA, TDx) (Abbot Diagnostics; Irving, TX) and again by enzyme multiplied immunoassay (EMIT Vancomycin Assay) (Dade Behring; San Jose, CA). Statistical analysis (Wilcoxon signed-rank test) determined that there was no difference between the values obtained from the two assays. This demonstrates that the increase in vancomycin levels is not caused by the accumulation of CDP-1 and may be the result of a change in the pharmacokinetics of the drug.

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.

Accuracy of measured vancomycin serum concentrations in patients with end-stage renal disease

OBJECTIVE

To review information related to the accuracy of vancomycin serum drug concentrations in patients with end-stage renal disease, focusing on available assays and mechanisms of cross-reactivity.

DATA SOURCES

Primary and review articles identified from a MEDLINE search (January 1980-June 1999) and through secondary sources.

STUDY SELECTION AND DATA EXTRACTION

All articles identified were evaluated, and all relevant information was included in this review.

DATA SYNTHESIS

Falsely elevated vancomycin serum concentrations may occur in patients with renal dysfunction. The underlying mechanism is due to the formation and accumulation of a pseudo-metabolite, the vancomycin crystalline degradation product (CDP). Vancomycin is converted to CDP when exposed to heat, including normal body temperature. Because the molecular structures of CDP and vancomycin are similar, both molecules are detected by polyclonal immunoassay systems used in clinical laboratories. This cross-reactivity leads to falsely elevated serum vancomycin concentrations in excess of 50-70%. Such large assay inaccuracies may result in improper dosage adjustments and therapeutic failures. A monoclonal immunoassay system has been developed that does not significantly cross-react with CDP.

CONCLUSIONS

To appropriately interpret laboratory results, it is essential for clinicians to be aware of the vancomycin-CDP cross-reactivity problem and to be familiar with the specific assay used to measure vancomycin concentrations in patients with renal dysfunction.