Unit Converter
Tobramycin
(Aminoglycoside Antibiotic - Requires Therapeutic Drug Monitoring for Efficacy & Nephro/Ototoxicity Prevention)
Synonyms
- Tobramycin
- Nebcin®
- Aminoglycoside antibiotic
- Tobra
- TDM for tobramycin
Units of Measurement
- µmol/L
- mg/L
- mg/dL
- mg/100 mL
- mg%
- µg/mL
Unit Conversions
Molecular Weight of Tobramycin = 467.52 g/mol
µmol/L ↔ mg/L
1 µmol/L=0.4675 mg/L1\ \text{µmol/L} = 0.4675\ \text{mg/L}1 µmol/L=0.4675 mg/L 1 mg/L=2.14 µmol/L1\ \text{mg/L} = 2.14\ \text{µmol/L}1 mg/L=2.14 µmol/L
mg/dL → mg/L
1 mg/dL=10 mg/L1\ \text{mg/dL} = 10\ \text{mg/L}1 mg/dL=10 mg/L
µg/mL → mg/L
1 µg/mL=1 mg/L1\ \text{µg/mL} = 1\ \text{mg/L}1 µg/mL=1 mg/L
mg% = mg/dL
Description
Tobramycin is an aminoglycoside antibiotic used primarily against Gram-negative bacteria, including Pseudomonas aeruginosa.
Because aminoglycosides have:
- Narrow therapeutic index
- Dose-dependent bactericidal action
- Potential nephrotoxicity & ototoxicity
→ Therapeutic Drug Monitoring (TDM) is essential.
Mechanism of action:
- Irreversible inhibition of the 30S ribosomal subunit
- Causes misreading of mRNA
- Rapid bactericidal effect
Used in:
- Sepsis
- Hospital-acquired pneumonia
- Ventilator-associated pneumonia
- Complicated UTI
- CF exacerbations (inhaled form)
- Bacteremia
Pharmacology & Physiology
- 100% renal elimination via glomerular filtration
- Half-life prolonged in renal impairment
- Poor oral absorption
- Volume of distribution approximates extracellular fluid
Clinical Significance
1. Elevated Tobramycin
Occurs when trough levels are high.
Major Toxicities
- Nephrotoxicity
- Rising creatinine
- Acute tubular necrosis
- Rising creatinine
- Ototoxicity
- Hearing loss
- Tinnitus
- Vestibular dysfunction
- Hearing loss
Symptoms of Toxicity
- Dizziness
- Imbalance
- Hearing impairment
- Rise in serum creatinine
- Electrolyte abnormalities
Risk higher in:
- Elderly
- Renal impairment
- Septic shock
- Dehydration
- Concomitant nephrotoxic drugs (vancomycin, amphotericin B, cisplatin)
2. Low Tobramycin Levels
→ Subtherapeutic, risk of treatment failure.
Causes:
- Under-dosing
- Cystic fibrosis (high clearance)
- Burns (large Vd)
- Pregnancy (increased GFR)
- Drug interactions
- Improper sampling time
Reference Intervals
(Aligned with ACCP + IDSA + Mayo + ARUP)
Traditional Multiple-Daily Dosing
| Level | Target Range |
| Peak | 5 – 10 mg/L |
| Trough | < 2 mg/L |
Extended-Interval Dosing (Once-Daily / High-Peak Strategy)
| Level | Target |
| Random level (6–14 hr post-dose) | Nomogram-based (Hartford nomogram) |
| Peak target | 20 – 30 mg/L |
Inhaled Tobramycin
Not routinely monitored; systemic absorption minimal.
Diagnostic Uses
- Monitor therapy in severe infections
- Adjust dosing in renal failure
- Avoid nephrotoxicity & ototoxicity
- Evaluate altered pharmacokinetics (burns, CF, ICU patients)
- Guide extended-interval dosing
Analytical Notes
- Peak sample: 30 minutes after end of IV infusion
- Trough sample: Immediately before next dose
- Avoid drawing during infusion
- Measure using immunoassay or LC-MS/MS
- Consider renal function (GFR, creatinine clearance) for dose adjustments
Clinical Pearls
- Tobramycin is more potent against Pseudomonas than gentamicin.
- Once-daily dosing reduces toxicity due to drug-free interval.
- Always check baseline and serial creatinine.
- Avoid concurrent nephrotoxins if possible.
- In CF patients, clearance is high → higher doses needed.
- Ototoxicity may be irreversible-monitor closely.
Interesting Fact
Aminoglycosides like tobramycin exhibit concentration-dependent killing - the higher the peak, the more effective the bacterial kill rate, which is the scientific basis for high-peak, once-daily dosing.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Therapeutic Drug Monitoring
- ACCP: Aminoglycoside Dosing & Monitoring Guidelines
- IDSA Guidelines - Hospital-acquired Pneumonia
- Mayo Clinic Laboratories - Tobramycin
- ARUP Consult - Aminoglycoside TDM
- NIH / MedlinePlus - Tobramycin