Therapeutic drug monitoring of gentamicin: evaluation of five nomograms for initial dosing at Al-Amiri Hospital in Kuwait

Appropriateness of gentamicin therapeutic drug monitoring at a Middle Eastern tertiary hospital setting: a retrospective evaluation and quality audit

Introduction

The use of gentamicin in the treatment of infectious diseases requires frequent monitoring to attain the best treatment outcomes.

Objective

This study aimed to evaluate the appropriateness of gentamicin therapeutic drug monitoring (TDM) at a tertiary care hospital in Qatar.

Methods

A one-year quantitative retrospective chart review of all gentamicin TDM records was conducted. Evidence-based criteria were applied to evaluate the appropriateness of gentamicin TDM in terms of indication, sampling times, and post-analytical actions.

Results

Out of 59 captured gentamicin TDM records, 58 gentamicin samples were eligible for evaluation. Overall, gentamicin TDM appropriateness was achieved in 50% (n = 29) of the evaluated records. However, 12% (n = 7) of gentamicin drug concentrations were below the assay quantification limits or were not sampled appropriately. Inappropriate post-analytical actions (22.4%, n = 13) and inappropriate sampling times (44.8%, n = 26) were recorded. Most of the gentamicin blood samples (n = 43; 74.2%) were taken appropriately at steady-state. Inappropriate sampling time relative to the last dose was captured in 31% (n = 18) of the cases. Although 27.6% (n = 16) of gentamicin concentrations were non-therapeutic, continuing gentamicin dosing without adjustment was the most frequent post-analytical action (69.8%, n = 37). Gentamicin dose regimen continuations, dose regimen decreases and dose regimen discontinuations were inappropriately applied in 27% (n = 10), 25% (n = 2) and 14% (n = 1) of the times, respectively.

Conclusion

Suboptimal gentamicin TDM practices exist in relation to sampling time and post-analytical actions. Studies exploring setting-specific reasons behind inappropriate TDM practices and methods of its optimisation are needed.

Predictive performance of gentamicin dosing nomograms

Background

Several nomograms have been proposed to facilitate the determination of initial gentamicin dosing regimens in clinical settings. This study aimed to assess the predictive performance of these nomograms in Korean patients.

Methods

Gentamicin concentrations were determined in 84 patients with infective endocarditis (IE) and in 95 patients with other infections. All patients underwent therapeutic drug monitoring in Seoul National University Hospital from 2006 to 2012. Individual pharmacokinetic parameters were estimated using a Bayesian method, which predicted steady state peak and trough serum concentrations. Six nomograms were evaluated in patients with “other” infections: the Thomson guidelines, Hull-Sarubbi table, and Rule of Eights, for multiple daily dosing; and the Hartford nomogram, Barnes-Jewish Hospital nomogram, and Sanford Guide, for extended-interval dosing. In IE patients, synergistic combination dosing nomograms, based on the American Heart Association dosing interval guidelines, were evaluated.

Results

Gentamicin dosing nomograms performed poorly in attaining the target peak serum concentrations. Multiple-daily dosing nomograms predicted peak serum gentamicin concentrations better than did the extended-interval dosing nomograms (31.9%–72.3% vs 4.3%–45.7%, respectively). Similarly, in patients with IE, the once-daily dosing nomogram resulted in a significantly lower percentage of patients achieving target peak gentamicin concentrations than that associated with the thrice-daily dosing nomogram (P=0.0015). All of the multiple-daily dosing, extended-interval dosing, and synergistic combination dosing nomograms predicted the nontoxic target trough concentrations in >80% of patients.

Conclusion

Gentamicin dosing nomograms performed poorly in achieving the target peak serum concentrations. New gentamicin nomograms may be required in patients with IE, particularly for once-daily dosing. Therapeutic drug monitoring is highly recommended for gentamicin to ensure that the target concentrations are achieved.

Factors Affecting Gentamicin Penetration in Lower Extremity Ischemic Tissues With Ulcers

The aims of the study were to analyze the penetration of gentamicin in foot ulcers in patients with different severities of peripheral arterial disease (PAD) and to determine significant parameters affecting lower limb tissue concentrations. Patients undergoing debridement of a wound or an amputation procedure were included. All patients received a 120 mg or 240 mg intravenous dose of gentamicin prior to the procedure. Patients were classified according to the degree of PAD. Tissue and serum samples were collected at the time of intervention, and gentamicin concentrations were determined by fluorescence polarization immunoassay. Blood and tissue samples were taken from 61 patients, 41 males and 20 females with a mean age of 66 years. Nineteen patients had nil or borderline PAD, 9 patients had mild or moderate PAD, and 26 patients had severe PAD. Forty-eight patients had type 2 diabetes, 8 patients had type 1 diabetes, and 5 patients were nondiabetic. The concentration of gentamicin in peripheral skeletal muscle tissue was dependent on the serum concentration, degree of PAD, gender, and age. For patients with ischemic lower extremity wounds (patients with mild, moderate, and severe PAD), the concentration of gentamicin was significantly lower ( P = .010) than the concentration in nonischemic wounds, and the concentration in female patients was also significantly lower than in male patients ( P = .047). The concentration in peripheral subcutaneous tissue was 0.663 times the concentration in skeletal muscle tissue ( P < .00001). Gentamicin showed greatest penetration in male patients without PAD. For patients with severe PAD, higher doses of gentamicin may be required to achieve the same effect.

Current use for old antibacterial agents: polymyxins, rifamycins, and aminoglycosides

This article reviews three classes of antibacterial agents that are uncommonly used in bacterial infections and therefore can be thought of as special-use agents. The polymyxins are reserved for gram-negative bacilli that are resistant to virtually all other classes of drugs. Rifampin is used therapeutically, occasionally as a companion drug in treatment of refractory gram-positive coccal infections, especially those involving foreign bodies. Rifaximin is a new rifamycin that is a strict enteric antibiotic approved for treatment of traveler's diarrhea and is showing promise as a possible agent for refractory Clostridium difficile infections. The aminoglycosides are used mainly as companion drugs for the treatment of resistant gram-negative bacillary infections and for gram-positive coccal endocarditis.

Current use for old antibacterial agents: polymyxins, rifamycins, and aminoglycosides

This article reviews three classes of antibacterial agents that are uncommonly used in bacterial infections and therefore can be thought of as special-use agents. The polymyxins are reserved for gram-negative bacilli that are resistant to virtually all other classes of drugs. Rifampin is used therapeutically, occasionally as a companion drug in treatment of refractory gram-positive coccal infections, especially those involving foreign bodies. Rifaximin is a new rifamycin that is a strict enteric antibiotic approved for treatment of traveler's diarrhea and is showing promise as a possible agent for refractory Clostridium difficile infections. The aminoglycosides are used mainly as companion drugs for the treatment of resistant gram-negative bacillary infections and for gram-positive coccal endocarditis.