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Gentamicin, Trough (GENTT)

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EPIC Test Name

GENTAMICIN LEVEL, TROUGH

EPIC Code

LAB26

Specimen Requirements

plasma
Minimum Volume:0.5 mL
Collection:Collect specimens using standard laboratory procedures.
Trough level should be drawn 30 minutes prior to next dose.
Transport:Room Temperature ASAP
Stability:Refrigerated: 7 days at 2-8 degrees C
Frozen: 2 weeks at <-10 degrees C
Container:LT GRN
Rejection Causes:Hemolysis,
Insufficient Sample Volume

Methods

Homogeneous particle-enhanced turbidimetric immunoassay

Turnaround Time

SpecimenTurnaround TimeFrequency
plasmaStat: 90 minutes Routine: 4 hours24/7

Reference Ranges

Homogeneous particle-enhanced turbidimetric immunoassay
UnitCritical ValuesTherapeutic Levels
ug/mL>2.0 ug/Ml<2.0 ug/mL

Clinical Indications

Gentamicin, a member of aminoglycoside antibiotics, kills or is bactericidal to aerobic gram-negative bacteria through binding to the 30S ribosomal subunit of bacterial RNA to inhibit protein synthesis. Gentamicin is inactive under anaerobic conditions due to oxygen-dependent active transport mechanism that delivers the drug across the bacterial cell wall.

The blood gentamicin concentration has better correlation with antibacterial activity than the drug's dosage. Thus, monitoring the blood gentamicin concentration with concurrent clinical assessment is necessary to ensure adequate therapeutic effect while minimizing the potential for adverse effects. The mode of administration, the volume of extracellular fluid, renal retention, and physiological change influence the volume of distribution and drug elimination during therapy. As a result, gentamicin has a narrow range of safe and effective concentrations.

Gentamicin is routinely administered intravenously or intramuscularly to achieve a high degree of bioavailability. Clearance is highly dependent on renal function. With normal renal function, elimination half lives are short ranging from 2-3 hours.

The goal of gentamicin therapy is to achieve a plasma concentration that kills the bacteria without causing toxicity to the patient. Renal tubular necrosis, and auditory and vestibular (inner ear) adverse effects occur after exposure to high concentrations. In the setting of nephrotoxicity due to gentamicin, renal findings may include mild proteinuria and reduced glomerular filtration rate. In the setting of ototoxicity due to gentamicin, patients may experience high-pitched tinnitus, high-frequency hearing loss, nausea, vomiting, balance disorder, and vertigo.

Additional Information

Patient samples containing the drug sisomicin will yield falsely elevated values for gentamicin. However, this drug is not usually coadministered with gentamicin.

High concentrations of penicillins or cephalosporins inactivate gentamicin in vitro. The degree of inactivation depends on the particular aminoglycoside measured, the type and concentration of the penicillin or cephalosporin that is also present, and the storage conditions of the sample. Frozen samples are much less affected than those left at room temperatures; therefore, for patients receiving additional antibiotics of these types, should be assayed immediately or stored frozen. Placing the specimen on ice may be helpful to slow inactivation.

A small number of patients experience adverse effects regardless of the concentration. Most adverse effects are reversible with discontinuation or dose adjustment. Irreparable vestibular, cochlear, or renal damage to the patient usually occurs with an increased aminoglycoside blood concentration for longer than two weeks.

Patients with impaired renal function receiving gentamicin will have a prolonged half-life and slower elimination; therefore, quantitative gentamicin testing of the blood is necessary to guide dose corrections. Nephrotoxicity caused by gentamicin may also be difficult to distinguish from symptoms of underlying renal disease.

In very rare cases, patient samples may contain heterophile antibodies, which may produce low results with the QMS Gentamicin assay. Interfering heterophile antibodies occur at low frequency in the general population. These antibodies can cause autoagglutination of the microparticle reagent leading to undetected erroneously low results.

A serious, although rare, adverse effect is neuromuscular blockade. Patients at greater risk have concurrent conditions, such as myasthenia gravis, or on medications, such as vecuronium, that interfere with the neuromuscular junction. Peak and trough specimens are required to monitor toxicity.

Performed

Lab
Chemistry - Downtown

Interpretative Information

The goal level of blood gentamicin concentration depends on the type of infection being treated. The blood draw for trough gentamicin concentration should be collected prior to the administration of the next dose. Trough target levels should be less than 2 mcg/mL. Prolonged exposure to trough levels exceeding 2.0 mcg/mL may lead to toxicity.

CPT

80170

LOINC

3665-7

References

1. Chaves, BJ, Tadi P. Gentamicin.Steatorrhea. StatPearls. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. https://www.ncbi.nlm.nih.gov/books/NBK541055/.

2. Milone MC, Shaw LM: Therapeutic Drugs and Their Management. In: Rifai N, Horvath AR, Wittwer CT, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier;2018: 800-31.

3. Riff LJ, Thomason JL. Comparative aminoglycoside inactivation by beta-lactam antibiotics. Effects of a cephalosporin and six penicillins on five aminoglycosides. J Antibiot. 1982 Jul;35(7):850-7. doi: 10.7164/antibiotics.35.850. https://pubmed.ncbi.nlm.nih.gov/7174538/

4. Tindula RJ, AMbrose PJ, Harralso AF. Aminoglycoside inactivation by penicillins and cephalosporins and its impact on drug-level monitoring. Drug Intell Clin Pharm. 1983 Dec;17(12):906-8. doi: 10.1177/106002808301701210. https://pubmed.ncbi.nlm.nih.gov/6653408/

Contact Information

Chemistry - Downtown: (315)464-4460
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