Evaluation of an enzyme immunoassay for serum gentamicin

Prospective Analysis of Various Dosing Methods for Aminoglycosides and Vancomycin Therapy in Chronic Spinal Cord Injury Patients

BACKGROUND/AIM

Several methods have been proposed for the better estimation of creatinine clearance (CLCR) in spinal cord injury (SCI); however, there is no standard method to date. In 2011, Lee-Dang derived an equation to predict CLCR more accurately, although it was limited by problems inherent in any retrospective study of previously collected data. Thus, a prospective investigation was conducted to evaluate the feasibility of clinical implementation and accuracy of the Lee-Dang method against 5 other methods while controlling sources of bias and confounding.

METHODS

A prospective investigation included 100 patients who met the inclusion criteria, given as follows: a diagnosis of SCI and administration of intravenous aminoglycosides (AG) or vancomycin with their dosing regimen determined by the Lee-Dang equation. The percentage of patients achieving therapeutic goal was determined, and the measured AG and vancomycin drug clearance (mCL(DRUG)) values were compared with the results that were estimated from 6 equations.

RESULTS

The Lee-Dang equation was found to be more accurate, with a smaller magnitude of variance from mCL(DRUG) and smaller residuals and bias compared with the alternatives. The Lee-Dang method resulted in >90% of patients achieving the steady state serum concentrations within the pre-defined goal for AG and vancomycin. All other predictive methods grossly overestimated CLCR and produced markedly lower frequency (<50%) of achieving the goal for both antimicrobials (p < 0.05).

CONCLUSION

The prospective data shows that the Lee-Dang method for predicting CLCR is more accurate relative to the alternative methods in the study population with SCI, thus recommend using the Lee-Dang method for pharmacokinetic dosing of AG and vancomycin in SCI.

A More Accurate Method to Estimate Aminoglycoside Clearance From Serum Creatinine in Patients With Spinal Cord Injury

Background: Current literature reports that the traditional methods overestimate renal function in spinal cord injury (SCI); however, there is no accepted standard method. Objective: This study evaluated 6 published methods against measured aminoglycoside (AG) drug clearance and determined the frequency with which each method would achieve target peak and trough AG concentrations within a specified range. Methods: A chart-based investigation was conducted at a hospital with a large SCI population, and a total of 35 patients met the inclusion criteria: a diagnosis of long-standing SCI, administration of AG via intravenous infusion, and at least one set of steady-state AG peak and trough concentrations. Pharmacokinetic analysis was performed to compare the measured AG clearance values against the values resulting from 6 methods of estimating the glomerular filtration rate (GFR). Patient-specific pharmacokinetic parameters were used to simulate steady-state peak and trough AG concentrations from doses derived from each method. Results: Compared with the other methods, the Lee–Dang method was found to be more accurate, with the smallest magnitude of variance from the measured AG clearance values. Five alternative methods significantly overestimated AG clearance, by approximately 70% to 160% ( P < .05). The Lee–Dang method underestimated AG clearance (by 10%), however not to a significant degree ( P = .079). Compared with the alternative methods, the Lee–Dang method resulted in a higher frequency of steady-state peak and trough AG concentrations within the target range specified. Conclusion: The Lee–Dang equation for predicting GFR was more accurate relative to the other methods in the study population of patients with long-term SCI.

Osseous concentrations of gentamicin after implantation of acrylic bone cement in sheep femora

17 sheep received femoral implantation of 'Cérafix Genta', containing gentamicin 0.6 or 1.2 g per dose either unilaterally (first 3 sheep) or bilaterally (remaining 14 sheep). Highest concentrations in bone were measured after 24 and 48 post-operative hours (respectively 36 and 72 mg/kg for Cérafix 0.6 g and 1.2 g). Gentamicin concentrations in bone marrow and femoral heads were in the same range. Blood and urine concentrations of gentamicin measured in sheep were comparable to those obtained in man. An extrapolation of the gentamicin behaviour from sheep to humans, might be possible: the sheep weight and femoral dimensions were comparable to those of humans and the method of implantation was similar in man and animal. For up to 18 months after operation, gentamicin levels in bone were always higher than critical concentration (4 mg/kg). For the early post-operative period, blood and urine levels always remained below gentamicin toxic concentrations.

Comparative study of gentamicin release from normal and low viscosity acrylic bone cement

The pharmacokinetics of gentamicin were studied after total hip joint arthroplasties in 2 groups of 10 patients. The prosthesis was performed in the first group with 'Palacos R plus gentamicin' (normal viscosity), manufactured by Schering, and in the second group with 'Cerafix genta R' (low viscosity) manufactured by Ceraver-Osteal. Both cements included similar concentrations of gentamicin. Urine was collected at 12-hour intervals for 15 days after operation, and drainage fluids for 48, 72 or 108 hours. Blood samples were taken 3 and/or 5 hours after prosthesis implantation. In both cases, high concentrations of gentamicin were found in drainage fluids and urine during the early postoperative period. Mean gentamicin excretion curves were calculated by a computer-aided design program (SIAM) for the 2 cements. The release of gentamicin was biphasic in both cases, although the slow elimination phase appeared to be longer for 'Cerafix'. In the first postoperative period, the drug had a better bioavailability during the rapid elimination phase in the case of 'Palacos'. The calculated peak blood concentration was in the same range for both compounds. The conclusion is drawn that, in patients undergoing total hip joint arthroplasties, gentamicin concentrations reach local levels higher than the minimum inhibitory concentrations of most of the likely sensitive pathogens. However, in both cases, as blood concentrations appear to be low, patients will not be protected against systemic infections. Both cements have similar antibacterial properties but the mechanical properties of 'Cerafix' are the better of the two.

Release of gentamicin from acrylic bone cement

The pharmacokinetics of gentamicin were studied after total hip joint arthroplasties performed with "Palacos R plus gentamicin' in 10 patients. Urine was collected at 12-hour intervals for 15 days after operation, and drainage fluids for 48, 72 or 108 hours. Blood samples were taken 3 and/or 5 hours after prosthesis implantation. High concentrations of gentamicin were found in drainage fluids. Excretion curves in drainage fluids or urine were fitted by a computer-aided design program (SIAM) and the mean curves established. Elimination of gentamicin was biphasic in both cases. The rapid phases had a half-life of 2.97 hours in drainage fluids and 7.16 hours in urine. Half-lives of the slow phases were 13.5 and 47.12 hours, respectively. The mean percentage of total gentamicin released by the two routes was 5.78% of the quantity implanted. The calculated peak blood concentration was 0.12 mg/L. It is concluded that gentamicin concentrations locally reach levels higher than minimum inhibitory concentrations of most of the likely pathogens in patients undergoing total hip joint arthroplasties with "Palacos R plus gentamicin' bone cement. However, as blood concentrations appear to be low, patients may not be protected against systemic infections.

Effect of irrigation and tourniquet application on aminoglycoside antibiotic concentrations in bone

Preoperative antibiotics are an important measure taken to prevent infection in joint replacement surgery. The local availability of these antibiotics in the operative site is absolutely necessary to ensure adequate prophylaxis against contaminant bacteria. Because pulsatile-lavage rapid-recovery systems have become a routine technique of bone preparation in joint reconstruction, we chose to study the effect of these systems on local antibiotic concentrations. We further investigated the effect of irrigation with and without use of a limb tourniquet. For our study, we obtained 23 bone specimens in 16 patients undergoing joint reconstruction (14) or amputation (2). The patients were classified into one of four groups based on whether a tourniquet was applied during the procedure and whether the bone specimens were irrigated. In addition, matched blood samples were obtained to establish that therapeutic serum levels were achieved. Nine patients contributed 13 bone specimens, which underwent vigorous irrigation in vitro. None of these specimens had detectable levels of antibiotics, regardless of whether a tourniquet was used. Seven patients yielded 10 bone specimens, which were not irrigated. Five of these seven patients had detectable levels of antibiotics. In addition, the specimens from limbs without tourniquets had levels that averaged 0.51 microgram/ml while those with tourniquets averaged below 0.2 microgram/ml. Therefore, the use of vigorous irrigation in bone preparation has a significant deleterious effect on the local presence of previously administered systemic antibiotics. This effect is compounded if the operative site is isolated from continuous blood flow by use of a tourniquet. We therefore recommend that additional measures be taken to ensure that adequate antibiotic levels are present.(ABSTRACT TRUNCATED AT 250 WORDS)

Antibiotic monitoring in body fluids

Analytical procedures recently described for the quantitative determination of antibiotics in body fluids are reviewed. High-performance liquid chromatography (HPLC) and immunoassays appear as an alternative to current microbiological assays. HPLC has been applied to most antibiotics in clinical use and a major part of the review deals with this technique. Attention is given to sample pretreatment, characteristics of chromatography and detection, and limit of sensitivity. Non-isotopic immunoassays have been essentially applied to aminoglycosides and vancomycin and are also reviewed. Advantages and drawbacks of HPLC and immunoassays are presented.

Gentamicin disposition kinetics in humans with spinal cord injury

The disposition kinetics of gentamicin, an aminoglycoside antibiotic, were studied in seven tetraplegic and six paraplegic volunteers. The volume of distribution of gentamicin in l/kg of body weight varied in a statistically significant way from values of this parameter measured in normal subjects. The elimination of gentamicin in spinal man proceeded in a log-linear fashion accurately characterized by a one compartment open-model with a half-life of approximately 2 hours. The clinical significance of altered disposition kinetics and an increased intersubject variability in gentamicin disposition in spinal man as compared to normal subjects is unknown. The existence of these observed differences in pharmacokinetic parameters, however, emphasizes the need to define individual pharmacokinetic profiles and individualize dosing regimens in spinal man. The data presented are supportive of the hypothesis that spinal man constitutes a discreet therapeutic population.

Comparison of the Beckman Auto ICS and the Syva Autolab 6000 for determination of gentamicin levels in serum

Two fully automated drug-monitoring systems, the Syva Autolab 6000 and the Beckman Auto ICS, were compared in terms of accuracy, precision, speed of operation, and cost effectiveness in the determination of gentamicin levels in serum. Within-run and between-run precision for both systems were acceptable (coefficient of variation, 2.0 to 6.9%), and patient sample comparisons resulted in an intermethod correlation coefficient of 0.96. When reference samples (prepared to contain 1.2 to 10 micrograms of gentamicin per ml) were assayed, the Syva Autolab 6000 obtained concentrations within 8% of expected values, whereas the Beckman Auto ICS reported values up to 17% higher than target values. In a time and cost comparison, reagent costs for the Beckman system were ca. 50% less than for the Syva Autolab 6000; the Syva system, however, determined patient results two to three times faster than the Beckman Auto ICS.

Drug interferences with plasma assays in therapeutic drug monitoring

In this article, many of the reports which describe the various assay procedures for 8 of the most commonly monitored drugs in plasma (digoxin, gentamicin, phenobarbitone, phenytoin, procainamide, quinidine, salicylates and theophylline) are reviewed, together with studies dealing with interferences of other drugs with these assays. Factors which are evaluated include whether the interference was studied when the drug was taken by a patient or a volunteer or by adding it to serum in vitro, the concentration or dose of the interfering drug (when reported), and the clinical implications of the interference. Suggestions as to how to eliminate some of these potential sources of interference are made.

Stability of gentamicin, tobramycin, and amikacin in combination with four beta-lactam antibiotics

The stability of the aminoglycosides gentamicin, tobramycin, and amikacin stored in combination with carbenicillin, piperacillin, cefotaxime, and moxalactam was evaluated at four temperatures (25, 4, -8, and -70 degrees C) over a 3-week period. Amikacin was the most stable of the aminoglycosides and demonstrated no loss of activity when stored with either carbenicillin or piperacillin. Gentamicin and tobramycin were inactivated by carbenicillin and piperacillin at 25 and 4 degrees C, with aminoglycoside activity declining substantially after 8 to 48 h of storage; virtually no loss of gentamicin or tobramycin activity occurred with storage at -8 or -70 degrees C. Cefotaxime and moxalactam produced no degradation of any of the three aminoglycosides.

Comparison of a simplified liquid chromatographic assay of gentamicin in serum with enzyme immunoassay and bioassay

A sensitive and reproducible high-performance liquid chromatographic (HPLC) method is described for assay of gentamicin in serum using ultrafiltration of serum samples and pre-column derivatization with o-phthalaldehyde. The procedure was compared with a bioassay and an enzyme immunoassay. Coefficients of correlation were 0.98 for HPLC vs. bioassay and 0.97 for HPLC vs. immunoassay. The between-day imprecision (n = 5) was estimated from the coefficients of variation at a concentration of 6 mg/l. Values were 9.4% for the bioassay, 6.5% for the immunoassay and 2.4% for the chromatographic method. The major advantage of the method described is simplified sample preparation and optimal serum extraction. The latter is important in routine use because it prolongs column life and thus reduces costs.

Homogeneous enzyme immunoassay for netilmicin

A newly developed homogeneous enzyme immunoassay for the determination of netilmicin in serum was evaluated and compared with a radioenzymatic assay. A total of 102 serum samples from patients treated with netilmicin were measured by both methods. This comparison showed an excellent correlation (r = 0.993). The enzyme immunoassay has proved to be precise, accurate, and specific. Because of its rapidity and the ease of performance, this method is a useful alternative to current assays for monitoring serum netilmicin concentrations.

Enzyme immunoassay of gentamicin with the Abbott ABA-100 analyzer

The enzyme immunoassay of gentamicin using "EMIT" reagents was adapted to take advantage of the small reagent volume required by the Abbott ABA-100. The method increases fourfold the maximum number of tests per kit. The correlation with the bacterial inhibition method was 98%.

Comparison of different tobramycin assays

Three commercial available tobramycin assays, the fluorescent immunoassay (AMES), the Diagnostic Products Corp. radioimmunoassay, and the Nuclear Medical System radioimmunoassay, were compared with the microbiological assay and the chemical assay involving high-performance liquid chromatography. The results obtained with each assay were evaluated in terms of usefulness, reproducibility, and cost-effectiveness.

Comparison of an HPLC method with a RIA, EMIT and FIA method for the assay of serum gentamicin with extensive statistical evaluation

A high performance liquid chromatography (HPLC) assay, a radioimmunoassay (RIA), an enzyme immunoassay (EMIT) and a fluorescence immunoassay (FIA) method for assaying serum gentamicin were compared in terms of accuracy and precision. Spiked human serum samples in the range of 0-11.6 mg.1(-1) gentamicin were used in all the tests. Regression analysis and analysis of variance were performed. The Performance Index (PI) was used to qualify the different methods. Other aspects of performance were also compared: simplicity, speed, cost, application in relation to workload. After extensive statistical evaluation the FIA method gives the best results.

Performance characteristics of bioassay, radioenzymatic assay, homogeneous enzyme immunoassay, and high-performance liquid chromatographic determination of serum gentamicin

We compared the accuracy, precision, and between-method error of the microbiological assay, the radioenzymatic assay, the homogeneous enzyme immunoassay, and the high-performance liquid chromatographic assay for the quantitation of gentamicin in serum. Precision and accuracy were evaluated by reference samples prepared to contain 0.0 to 32.7 micrograms of gentamicin per ml. Correlations between the methods utilized patient sera with gentamicin concentrations ranging from 0.6 to 13.3 micrograms/ml. All methods were reliable within acceptable limits for routine clinical use; intermethod correlation coefficients exceeded 0.96. Relative to the microbiological assay, the alternative methods offer the advantage of rapid analysis. The elapsed times for acquiring data on a set of 10 specimens under routine operating conditions were 0.5 h by the enzyme immunoassay, 4 h by the radioenzymatic assay, 5 h by the high-performance liquid chromatographic assay, and 10 h by the microbiological assay.

Potential liabilities of gentamicin homogeneous enzyme immunoassay

We report two potential liabilities of the homogeneous enzyme immunoassay for the determination of serum gentamicin. One is a considerable loss of precision and accuracy at the ends of the calibration curve, and the other is an apparent loss of gentamicin with storage at -60 degrees C.

Comparison of five methods for the assay of serum gentamicin

The microbiological bioassay, the adenylation method, and the radiometric, enzyme, and fluorescent immunoassay methods for assaying serum gentamicin were compared. The precision, reproducibility, and specificity of each method was assessed and proved satisfactory, with the exception of the radioimmunoassay, which gave artificially high results. Good correlations between the other four methods was obtained in a comparison involving 103 patient sera. The other aspects of performance were also compared, namely, simplicity, speed, cost, ease of automation, and application to large and small workloads. The enzyme immunoassay performed best in this comparison, being accurate, specific, rapid, and very simple to perform. However, other laboratories might find that workload, staffing, and available equipment make other methods more attractive.

Evaluation of five gentamicin assay procedures for clinical microbiology laboratories

Five gentamicin assay procedures (a bioassay, an enzyme immunoassay, a latex agglutination inhibition test, a fluorescence immunoassay, and a radioimmunoassay) were evaluated to determine which was optimal for our laboratory. The evaluation was based on recovery and precision studies and results of analyses of patient samples, as well as technical assay performance factors. The latex agglutination inhibition test appears useful for laboratories performing only occasional assays for gentamicin; however, the fact that some rheumatoid factor-positive sera, as well as some other sera for unknown reasons, may give falsely low values is a potential drawback to this procedure. Because of its accuracy, precision, rapid turn-around time, and relative simplicity of performance, we selected the enzyme immunoassay procedure for routine use for gentamicin assays in our laboratory.