Pharmacology · Year 2 · from Pharmacology
Case 3: Gentamicin Therapeutic Drug Monitoring
Clinical Scenario
A 62-year-old male with infective endocarditis is being treated with gentamicin and requires dose adjustment based on therapeutic drug monitoring.
Patient Demographics
- Age: 62 years
- Sex: Male
- Weight: 80 kg
- Height: 175 cm
Chief Complaint
Follow-up for gentamicin level monitoring during treatment for endocarditis
History of Present Illness
The patient was admitted 5 days ago with Enterococcus faecalis endocarditis and started on ampicillin plus gentamicin for synergy. He has mild chronic kidney disease at baseline. Current regimen: Gentamicin 120 mg IV every 8 hours (started 3 days ago).
Physical Examination
- Vital Signs: BP 130/75 mmHg, HR 82 bpm, RR 14, T 37.2C
- General: Improving, less fatigued
- Cardiovascular: Regular rhythm, grade II/VI systolic murmur
- Neurological: No vestibular symptoms, hearing intact
Workup and Results
| Test | Result | Reference Range |
|---|---|---|
| Gentamicin peak (30 min post-dose) | 8.5 mcg/mL | 3-5 mcg/mL (synergy dosing) |
| Gentamicin trough (pre-dose) | 2.1 mcg/mL | <1 mcg/mL |
| Creatinine | 1.6 mg/dL | 0.6-1.2 mg/dL |
| BUN | 28 mg/dL | 7-20 mg/dL |
| Audiometry | Normal | - |
Diagnosis
Supratherapeutic gentamicin levels with elevated trough indicating accumulation and nephrotoxicity risk
Pharmacokinetic Principles Illustrated
- Therapeutic drug monitoring: Aminoglycosides require monitoring due to narrow therapeutic index and concentration-dependent toxicity.
- Peak levels: Correlate with bactericidal efficacy (concentration-dependent killing).
- Trough levels: Correlate with nephrotoxicity and ototoxicity risk; should be <1 mcg/mL for synergy dosing.
- Renal elimination: Gentamicin is eliminated almost entirely by glomerular filtration; dose adjustment required in renal impairment.
- Steady-state: Reached after 4-5 half-lives; levels drawn after reaching steady state.
Treatment
- Extend dosing interval to every 12 hours
- Reduce dose to 100 mg IV every 12 hours
- Repeat levels after 3 doses at new regimen
- Monitor serum creatinine daily
- Obtain weekly audiometry during prolonged therapy
- Target trough <1 mcg/mL for synergy dosing
Key Learning Points
- Aminoglycoside dosing requires careful PK/PD optimization
- Extended-interval (once-daily) dosing reduces nephrotoxicity while maintaining efficacy for standard indications
- Synergy dosing uses lower doses with different target levels
- Trough levels are better predictors of toxicity than peak levels