Gentamicin, Serum, Peak

CPT: 80170
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Test Details

Synonyms

  • Garamycin®
  • Gentak®

Use

This aminoglycoside is used to treat serious infections caused by aerobic gram-negative bacilli (eg, a number of the Enterobacteriaceae, P aeruginosa). These include lower respiratory tract, intra-abdominal, soft tissue, bone or joint, wound, and complicated urinary tract infections; bacteremias; and meningitis (by intrathecal administration).

Gentamicin often is the preferred aminoglycoside for general use in hospitals where the prevalence of bacterial resistance to this agent is low (based on locally generated antimicrobial susceptibility profiles). Its major advantage over tobramycin, amikacin, and netilmicin is lower cost. The empiric use of gentamicin or tobramycin is not justified in hospitals where gentamicin resistance is common; amikacin is the aminoglycoside of choice in these locales.

The combination of gentamicin plus penicillin G (or ampicillin) exhibits synergistic activity against enterococci (eg, Enterococcus faecalis) and viridans streptococci and frequently is the regimen of choice for the treatment of endocarditis caused by these organisms. Streptomycin is an alternative to gentamicin for streptomycin-susceptible strains of enterococci. Enterococcal clinical isolates with high-level resistance to gentamicin and other aminoglycosides (minimum inhibitory concentrations, >2000 μg/mL) have been reported. Therapy for endocarditis caused by these gentamicin-resistant enterococci is not established, but some isolates do remain susceptible to streptomycin, which may be given with penicillin to treat this infection.

Gentamicin is recommended by the American Heart Association for parenteral prophylactic use with ampicillin or vancomycin in patients who are at high risk of developing bacterial endocarditis but are unable to take the standard oral regimen prior to undergoing invasive procedures involving the oral cavity and upper respiratory, gastrointestinal, or genitourinary tract.

Methodology

Immunoassay

Reference Interval

Therapeutic: peak: 6.0−10.0 μg/mL, trough: 0.5−1.5 μg/mL

Critical Value

Potentially toxic: peak: >12.0 μg/mL, trough: >2.0 μg/mL

Additional Information

Gentamicin is cleared by the kidney and accumulates in renal tubular cells. Nephrotoxicity is most closely related to the length of time that trough levels are >2 μg/mL. Creatinine levels should be monitored every two to three days as an indicator of impending renal toxicity. The initial toxic result is nonoliguric renal failure that is usually reversible if the drug is discontinued. Continued administration of gentamicin may produce oliguric renal failure. Nephrotoxicity may occur in as many as 10% to 25% of patients receiving aminoglycosides; most of this toxicity can be avoided by monitoring levels and adjusting dosing schedules accordingly.

Aminoglycosides may also cause irreversible ototoxicity that manifests itself clinically as hearing loss. Aminoglycoside ototoxicity is relatively uncommon and clinical trials where levels were carefully monitored and dosing adjusted failed to show a correlation between auditory toxicity and plasma aminoglycoside levels. In situations where dosing is not monitored and adjusted, however, sustained high levels may be associated with ototoxicity. This association is far from clear cut, and new once-daily dosing regimens (and associated high peak serum concentrations) that fail to enhance toxicity further complicate this issue.

Specimen Requirements

Specimen

Serum or plasma

Volume

1 mL

Minimum Volume

0.3 mL

Container

Red-top tube or green-top (heparin) tube. Do not use a gel-barrier tube. The use of gel-barrier tubes is not recommended due to slow absorption of the drug by the gel. Depending on the specimen volume and storage time, the decrease in drug level due to absorption may be clinically significant.

Collection

Transfer separated serum or plasma to a plastic transport tube. Both peak and trough concentrations should be monitored. The trough sample is drawn immediately prior to the next dose. Peak samples should be drawn 45 to 60 minutes after an IM injection, 30 minutes after the end of a 30-minute IV infusion, or immediately after a 60-minute IV infusion.

Storage Instructions

Room temperature

Stability Requirements

Temperature

Period

Room temperature

14 days

Refrigerated

14 days

Frozen

14 days

Freeze/thaw cycles

Stable x3

Causes for Rejection

Gel-barrier tube; hemolysis; lipemia

Clinical Information

Special Instructions

Peak levels should be ordered using test 007162. Trough levels should be ordered using test 007163. Peak and trough levels may be ordered together as a profile on the same test request form using test 717348. Please label tubes appropriately as “peak” and “trough.”

References

American Medical Association, Division of Drugs and Toxicology. Drug Evaluations Subscription. Chicago, Ill: AMA, Winter 1993.

LOINC® Map

Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
007162 Gentamicin Peak, Serum 3663-2 007162 Gentamicin Peak, Serum ug/mL 3663-2
Reflex Table for Gentamicin Peak, Serum
Order Code Order Name Result Code Result Name UofM Result LOINC
Reflex 1 007000 See below: 007000 See below: N/A

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