By Ashleigh Newman
The Clinical Pathology laboratory is excited to announce two new tests that we are offering: amikacin and gentamicin. Amikacin and gentamicin are aminoglycoside antibiotics, which target gram negative bacteria and are most commonly administered to equine patients. This class of antibiotics is considered “concentration-dependent,” meaning that their rate of bacterial killing increases as the drug concentration surpasses the minimum inhibitory concentration (MIC) for a given pathogen. They exhibit a “post-antibiotic” effect, whereby there is continued bacterial suppression after limited exposure to the drug. This is in contrast to “time-dependent” antibiotics, which are bactericidal only while their concentration is above the MIC. Dosing of concentration- versus time-dependent antibiotics varies due to this difference. Concentration-dependent antibiotics are typically administered once daily (long dosing interval), whereas time-dependent antibiotics are often administered two to three times per day (short dosing interval).
For concentration-dependent antibiotics, such as amikacin and gentamicin, measurement of drug levels are important to determine if an adequate drug level has been reached. This ensures adequate bacterial killing, as well as a decrease in the selection of resistant bacteria. Due to individual variations in drug pharmacokinetics, therapeutic drug monitoring should also be performed when administering aminoglycosides to minimize the risk of nephrotoxicity.1 Measurements of peak (CMAX) and trough (CMIN) concentrations allow the determination of the highest and lowest drug concentrations, respectively. There is no established protocol for the timing of these samples. In horses, CMAX is typically collected 30 minutes to 1 hour after an intravenous bolus administration, but some authors recommend measurement at 1.5-2hrs post administration.1,2 CMIN can be measured at 16-19 hours (foals)1, at 22 hours or just prior to the next dose (closer to 24 hours).1,2
In horses, amikacin and gentamicin are reportedly most effective when the ratio of CMAX to MIC is 8-12:1.1,2 Considering an MIC of 4 µg/mL for most equine pathogens, including those of intermediate susceptibility, this yields a desired peak drug concentration of 32-48 µg/mL.1,2 Proving the value of therapeutic drug monitoring, in one study, 66% of CMAX measurements were below the minimum target of 32 µg/mL, leading to a dose adjustment in 69% of the cases.2 CMAX has also been shown to be significantly higher in horses with focal disease, as compared to horses with systemic disease.2 The CMIN allows for the evaluation for adequate drug clearance helping to reduce the risk of nephrotoxicity. There is no consensus on trough concentrations, however in the human and veterinary literature recommendations range from <0.5-2 µg/mL.2–4
Both tests are a particle enhanced-immunoturbidimetric assay. In this methodology, antibodies against amikacin or gentamicin cause aggregation of particles coated by amikacin or gentamicin, respectively. The degree of aggregation results in a proportional decrease in light transmission, which correlates to the drug concentration. Although these tests are done most frequently in horses (on which published data on peak & trough values are available), the assay can detect the drugs in other species.
A minimum of 0.5 mL of serum or plasma is needed for each test. Serum from serum separator tubes (containing gel) should be avoided, as the gel may interfere with the testing. After sample collection, the sample should be kept refrigerated and sent to our laboratory with a cold pack. However, if testing will be delayed more than a week past collection, the sample should be frozen and sent on dry ice. Per the manufacturer of the assay, frozen samples are stable for two weeks
The current cost of testing is below: