For a 179.4‑lb patient, how should I calculate and administer tobramycin (aminoglycoside) at a total dose of 10 mg/kg per day, including dose per infusion, dosing interval, and required therapeutic drug monitoring?

Tobramycin Dosing at 10 mg/kg/day for a 179.4-lb Patient

For a 179.4-lb (81.4 kg) patient receiving tobramycin at 10 mg/kg/day, administer 814 mg divided into three doses of approximately 271 mg (6.8 mL of 40 mg/mL solution) every 8 hours by IV infusion over 20–60 minutes, with mandatory therapeutic drug monitoring to maintain peak levels of 4–10 mg/L and trough levels below 2 mg/L. 1

Weight-Based Dose Calculation

• Patient weight: 179.4 lbs = 81.4 kg (divide pounds by 2.2)
• Total daily dose: 10 mg/kg/day × 81.4 kg = 814 mg/day 1
• Dose per infusion: 814 mg ÷ 3 doses = 271 mg per dose (approximately 6.8 mL of reconstituted 40 mg/mL solution) 1

Dosing Interval and Administration

• Frequency: Every 8 hours (three times daily) 1, 2
• Route: Intravenous infusion 1
• Infusion duration: 20–60 minutes (never less than 20 minutes to avoid peak concentrations exceeding 12 mcg/mL) 1
• Dilution: Add the calculated dose to 50–100 mL of 0.9% sodium chloride or 5% dextrose 1

The 10 mg/kg/day regimen at this weight falls within the established therapeutic range for aminoglycosides in urinary tract infections, where once-daily or divided dosing has proven effective. 3, 2

Mandatory Therapeutic Drug Monitoring

Baseline assessments (before starting therapy):

• Serum creatinine 4
• Audiometry and vestibular testing 4

Peak concentration monitoring:

• Target: 4–10 mg/L 4
• Timing: Draw sample 30 minutes after IV infusion ends 1
• Frequency: Measure after 2–3 doses initially, then every 3–4 days during therapy 1

Trough concentration monitoring:

• Target: < 2 mg/L (rising trough levels above 2 mcg/mL indicate tissue accumulation and risk of toxicity) 1, 4
• Timing: Draw sample immediately before the next dose 1
• Frequency: At 8-hour intervals or just prior to each dose 1

Renal function monitoring:

• Check serum creatinine twice weekly during the first month 4
• Weekly in the second month 4
• Every two weeks thereafter 4
• Increase frequency if renal impairment develops 1

Duration of Treatment

• Standard duration: 7–10 days 1
• Extended therapy: If treatment exceeds 10 days, intensify monitoring of renal, auditory, and vestibular functions because neurotoxicity risk increases significantly 1

Critical Dosing Adjustments

For renal impairment (if creatinine clearance < 70 mL/min or elevated serum creatinine):

• Do NOT reduce the mg/kg dose—this compromises efficacy 5, 6
• Instead, extend the dosing interval: Multiply the patient's serum creatinine (mg/dL) by 6 to calculate the new interval in hours 2
• Example: If creatinine = 2.0 mg/dL, dose every 12 hours instead of every 8 hours 2

For elderly patients (> 59 years):

• Reduce total daily dose to 10 mg/kg/day maximum (750 mg) due to increased ototoxicity and nephrotoxicity risk 4, 5
• This patient's calculated dose of 814 mg/day would need reduction to 750 mg/day if over age 59 4

For obese patients:

• Calculate dosing weight as ideal body weight (IBW) + 40% of excess weight 1
• Male IBW = 50 kg + 2.3 × (height in inches above 60 inches) 6
• Female IBW = 45.5 kg + 2.3 × (height in inches above 60 inches) 6

Common Pitfalls to Avoid

• Never use fixed 500 mg doses regardless of patient weight—this causes underdosing in heavier patients and overdosing in lighter patients 4
• Never infuse in less than 20 minutes—rapid infusion causes dangerously high peak concentrations (> 12 mcg/mL) 1
• Never reduce the mg/kg dose in renal impairment—always extend the interval instead to preserve concentration-dependent bactericidal activity 5, 6
• Never administer before dialysis in dialysis patients—this removes the drug prematurely and causes treatment failure 5

Absolute Contraindications

• Pregnancy: Tobramycin is absolutely contraindicated due to risk of fetal nephrotoxicity and congenital hearing loss 4
• Concurrent loop diuretics (furosemide, ethacrynic acid): Markedly increase ototoxicity risk and should be avoided 4

Special Clinical Contexts

Cystic fibrosis or extensive burns:

• Altered pharmacokinetics may reduce serum concentrations 1
• Consider initial dosing of 10 mg/kg/day in 4 equally divided doses 1
• Therapeutic drug monitoring is especially critical in these populations due to wide inter-patient variability 1

Urinary tract infections:

• The 10 mg/kg/day regimen has demonstrated excellent clinical and bacteriologic response with round-the-clock urinary levels exceeding the minimal inhibitory concentration of pathogens 3
• Single daily dosing has been successfully used in UTIs, though the FDA-approved regimen remains divided dosing 3