Gentamicin Projected Trough Concentration Calculator

Overview

Projects a post-distribution Gentamicin concentration forward to the end of the dosing interval.

Clinical Significance

The projected trough assumes no intervening dose and unchanged elimination. Low trough exposure is commonly sought, but the applicable target depends on indication and regimen.

When to Use

• Use when the required concentration, timing and dosing information are reliable and the stated pharmacokinetic assumptions are reasonable.
• Use within an approved therapeutic-drug-monitoring or medicines-governance pathway.
• Use to support transparent review, not to automate prescribing.

How It Is Calculated

• C(trough) = C(measured) × e^[−k × (dose interval − sample time)].
• MedicalC validates that all required inputs are numeric and rejects internally impossible time or concentration relationships where applicable.

Interpretation

• The projected trough assumes no intervening dose and unchanged elimination. Low trough exposure is commonly sought, but the applicable target depends on indication and regimen.
• Interpret the result with the clinical indication, microbiology or seizure control, organ function, assay timing and local targets.

Worked Example

Enter a verified set of source values in the displayed units. MedicalC applies: C(trough) = C(measured) × e^[−k × (dose interval − sample time)]. Recheck the source chart and sampling times before using the result.

Patient Considerations

• Primarily intended for clinically stable adults receiving intermittent intravenous aminoglycoside therapy. Paediatric, neonatal, pregnancy, obesity, burns, cystic fibrosis, dialysis and rapidly changing renal function require specialist protocols.
• Consider acute kidney injury, augmented renal clearance, fluid shifts, obesity, burns, pregnancy, critical illness and interacting medicines.

Limitations

• One-compartment equations may not describe distribution or multicompartment behaviour accurately.
• Derived values can appear precise despite uncertainty in sampling time, assay result and clearance stability.
• A calculated exposure or dose does not establish clinical benefit or safety.

Clinical Pearls

• A mathematically acceptable projected trough can still be clinically misleading when kidney function is changing between the sample and the next dose.
• Plot concentrations against actual elapsed time before accepting any pharmacokinetic slope.
• When the result conflicts with the patient, verify the data rather than forcing a dose change.

Common Mistakes

• Using scheduled rather than actual administration and blood-sampling times.
• Treating a level drawn during distribution as a post-distribution concentration.
• Mixing mg/L, micrograms/mL and micrograms/L.
• Applying steady-state equations before steady state or during changing clearance.
• Using a generic concentration target outside the relevant indication or local protocol.

Evidence Base

Principal source: Bauer LA. Applied Clinical Pharmacokinetics. 3rd ed. McGraw-Hill; 2015.

References

1. Bauer LA. Applied Clinical Pharmacokinetics. 3rd ed. McGraw-Hill; 2015. — Primary publication, consensus guideline or authoritative pharmacokinetic source.