Proportional Maintenance Dose Adjustment Calculator

Overview

Calculates a proportional new maintenance dose from the current dose, measured concentration and desired concentration.

Clinical Significance

The result assumes linear, dose-proportional steady-state pharmacokinetics. It is unsuitable for medicines with nonlinear kinetics or when the sample is mistimed.

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

• New dose = current dose × target concentration ÷ measured concentration.
• MedicalC validates that all required inputs are numeric and rejects internally impossible time or concentration relationships where applicable.

Interpretation

• The result assumes linear, dose-proportional steady-state pharmacokinetics. It is unsuitable for medicines with nonlinear kinetics or when the sample is mistimed.
• 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: New dose = current dose × target concentration ÷ measured concentration. Recheck the source chart and sampling times before using the result.

Patient Considerations

• Applicable only when the medicine, patient and sampling conditions satisfy linear steady-state assumptions.
• 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 concentration ratio can suggest a dose mathematically, but clinical response, toxicity and nonlinear kinetics can make that answer unsafe.
• 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: Rowland M, Tozer TN. Clinical Pharmacokinetics and Pharmacodynamics: Concepts and Applications. 4th ed.

References

1. Rowland M, Tozer TN. Clinical Pharmacokinetics and Pharmacodynamics: Concepts and Applications. 4th ed. — Primary publication, consensus guideline or authoritative pharmacokinetic source.