Health & Medicine · Pharmacokinetics
Maintenance Dose Calculator
Calculates the maintenance dose required to sustain a target steady-state plasma drug concentration based on pharmacokinetic parameters.
Calculator
Formula
MD = Maintenance dose (mg per dose); C_{ss} = Target steady-state plasma concentration (mg/L); CL = Drug clearance (L/h); \tau = Dosing interval (h); F = Bioavailability fraction (0–1, use 1 for IV routes).
Source: Rowland M, Tozer TN. Clinical Pharmacokinetics and Pharmacodynamics: Concepts and Applications. 4th ed. Lippincott Williams & Wilkins, 2011.
How it works
When a drug is administered repeatedly, plasma concentrations rise and fall with each dose until a steady state is reached — typically after 4–5 half-lives. At steady state, the rate of drug administration equals the rate of drug elimination. The maintenance dose is the precise amount needed to replenish what is cleared by the body during each dosing interval, keeping concentrations within the therapeutic window.
The core formula is MD = (Css × CL × τ) / F. Here, Css is the target average steady-state concentration, CL is total body drug clearance (reflecting hepatic, renal, and other elimination routes), τ (tau) is the dosing interval in hours, and F is bioavailability — the fraction of the administered dose that reaches systemic circulation. For intravenous administration, F = 1. For oral drugs, F is typically 0.4–0.99 depending on first-pass metabolism and absorption efficiency.
This calculation is applied across virtually every therapeutic area: antibiotics (maintaining bactericidal concentrations), antiepileptics (seizure prophylaxis), cardiac glycosides (digoxin), immunosuppressants, and many others. Adjustments are frequently required in patients with renal impairment (reduced CL), hepatic disease, obesity (altered volume of distribution), or drug–drug interactions that inhibit or induce CYP enzymes.
Worked example
Clinical Scenario: A patient requires theophylline therapy for asthma. The target steady-state concentration is 10 mg/L. The patient's estimated clearance is 2.8 L/h, the dosing interval is 12 hours, and theophylline has an oral bioavailability of 0.96.
Step 1 — Apply the formula:
MD = (Css × CL × τ) / F
MD = (10 mg/L × 2.8 L/h × 12 h) / 0.96
Step 2 — Numerator:
10 × 2.8 × 12 = 336 mg
Step 3 — Divide by bioavailability:
336 / 0.96 = 350 mg per dose
Step 4 — Total daily dose:
With a 12-hour interval, 2 doses per day: 350 × 2 = 700 mg/day
This result suggests administering a 350 mg oral dose every 12 hours. In practice, a clinician would round to the nearest available tablet strength (e.g., 300 mg or 400 mg) and confirm with therapeutic drug monitoring.
Limitations & notes
This calculator assumes a one-compartment pharmacokinetic model with linear (first-order) elimination. Drugs with non-linear kinetics — such as phenytoin, which follows Michaelis-Menten saturation kinetics — require different dosing equations and should not be calculated with this tool. The formula yields an average steady-state concentration; peak and trough values will differ, which may be critical for narrow therapeutic index drugs like aminoglycosides or vancomycin. Clearance estimates from population pharmacokinetic models carry inherent uncertainty; individualized values from measured drug levels are more reliable. Bioavailability can vary significantly between formulations (immediate vs. extended release), between manufacturers (generic substitution), and within patients due to food interactions, GI conditions, or transporter polymorphisms. This calculator does not account for renal or hepatic dose adjustment tables — use specialized renal dosing tools for patients with impaired clearance. Always verify results against clinical guidelines and consult a licensed pharmacist or physician before implementing any dosing regimen.
Frequently asked questions
What is the difference between a loading dose and a maintenance dose?
A loading dose rapidly achieves the target therapeutic concentration at the start of treatment by saturating the volume of distribution. The maintenance dose is then given repeatedly to replace the drug eliminated during each dosing interval, sustaining steady-state concentrations over time. Together, they allow faster onset without exceeding toxic thresholds.
How does renal impairment affect the maintenance dose?
Many drugs are primarily eliminated by the kidneys, so reduced renal function decreases total clearance. A lower clearance means less drug is removed per unit time, requiring either a lower maintenance dose or an extended dosing interval to prevent drug accumulation and toxicity. GFR-based adjustment equations or renal dosing guidelines should be applied in these patients.
What bioavailability value should I use for intravenous drugs?
For intravenous administration, the drug is delivered directly into systemic circulation, so bioavailability (F) equals 1. This means the full administered dose is available, and the formula simplifies to MD = C<sub>ss</sub> × CL × τ. Always use F = 1 for IV bolus and IV infusion routes.
Can this calculator be used for pediatric patients?
The formula itself is valid for pediatric patients, but clearance and volume of distribution values differ substantially from adults due to developmental changes in organ function, body composition, and enzyme maturation. Pediatric pharmacokinetic parameters should be sourced from pediatric-specific references or population models, and weight-based dosing (mg/kg) is typically used alongside this calculation.
How do I know if my dosing interval (τ) is appropriate?
The dosing interval is ideally chosen relative to the drug's half-life. A common rule is τ ≤ 1–2 half-lives to prevent excessive peak-to-trough fluctuation. For drugs with wide therapeutic windows, longer intervals may be acceptable. For narrow therapeutic index drugs (digoxin, lithium, aminoglycosides), more frequent dosing or sustained-release formulations reduce dangerous fluctuation.
Last updated: 2025-01-15 · Formula verified against primary sources.