Theophylline Dosing Calculator

The Theophylline Dosing Calculator estimates loading and maintenance doses for theophylline (aminophylline) based on patient-specific pharmacokinetic parameters. Theophylline is a methylxanthine bronchodilator that has been used for decades in the management of asthma and chronic obstructive pulmonary disease (COPD), though its role has diminished with the availability of safer and more effective inhaled therapies.

Theophylline produces bronchodilation through phosphodiesterase inhibition (increasing intracellular cAMP) and adenosine receptor antagonism. At therapeutic concentrations, it also has anti-inflammatory properties, improves diaphragmatic contractility, stimulates the respiratory center, and has mild diuretic effects. These pleiotropic actions make it useful as add-on therapy in patients with refractory asthma or COPD who remain symptomatic despite optimal inhaler therapy.

The narrow therapeutic index of theophylline (target range 5-15 mcg/mL, with toxicity common >20 mcg/mL) necessitates careful individualized dosing and therapeutic drug monitoring. Theophylline clearance varies widely among individuals, influenced by age, smoking status, heart failure, liver disease, concurrent medications, and genetic polymorphisms in CYP1A2 (the primary metabolizing enzyme). Smokers have approximately 50-80% higher clearance than non-smokers, while heart failure and liver disease reduce clearance by 50% or more.

Aminophylline, the IV formulation, contains approximately 80% theophylline by weight (the remainder is ethylenediamine). Loading doses for aminophylline are therefore adjusted by dividing by 0.8 to convert from theophylline equivalents. The standard loading dose is 5-6 mg/kg (aminophylline) in patients not currently taking theophylline, expected to raise serum levels by approximately 10 mcg/mL. In patients already on theophylline, each 1 mg/kg of aminophylline raises the level by approximately 2 mcg/mL.

Oral sustained-release theophylline formulations are used for chronic maintenance therapy, typically dosed every 12 hours. Bioavailability of oral formulations is approximately 96%, and food can affect absorption depending on the specific formulation. Starting doses are typically conservative (10 mg/kg/day or 400 mg/day, whichever is less) with gradual titration guided by serum levels.

Drug interactions with theophylline are numerous and clinically significant. CYP1A2 inhibitors (ciprofloxacin, erythromycin, cimetidine, fluvoxamine) can dramatically increase theophylline levels. CYP1A2 inducers (rifampin, phenytoin, high-protein diet, smoking) decrease levels. The clinician must review all medications when initiating or adjusting theophylline therapy.

Theophylline toxicity produces dose-dependent symptoms: GI effects (nausea, vomiting) at 15-20 mcg/mL; cardiac effects (tachycardia, arrhythmias) at 20-40 mcg/mL; and seizures at >40 mcg/mL (may occur at lower levels in elderly). Seizures are often refractory to standard treatment and carry high morbidity/mortality.

This calculator integrates patient-specific factors to provide individualized theophylline dosing recommendations with appropriate loading and maintenance dose calculations.