Theophylline Dosing Calculator (Infusion and Loading Dose)
Compute weight-based theophylline infusion rates by age, smoking status, and organ function, with loading doses from the volume of distribution, aminophylline salt conversion, and labelling maximum infusion limits. Use the calculator above, then read the guide below to interpret your result and its limitations.
This theophylline dosing calculator gives the intravenous infusion rate and loading dose. Enter age, weight, clinical condition, target level, current level, salt form, and smoking status, and you get the maintenance infusion in mg/hour for your patient group, the loading dose IV over 20 to 30 minutes, the oral equivalent daily dose, and a maximum-infusion check. Theophylline has one of the narrowest therapeutic windows in routine prescribing, and its clearance varies several-fold between a smoking teenager and an elderly patient with heart failure. Guessing the infusion rate is not acceptable; the labelling table this calculator implements exists precisely because the differences matter.
Calculate the theophylline infusion and loading dose
A narrow therapeutic index demands precision
Theophylline relaxes bronchial smooth muscle and is used in severe asthma and chronic obstructive pulmonary disease, usually when inhaled therapy is insufficient. Its therapeutic range is 10 to 20 microgram/mL, and the distance from helpful to harmful is uncomfortably short: nausea, vomiting, and tremor appear just above 20, tachycardia and arrhythmias follow, and seizures can occur above 30. Deaths from theophylline toxicity, though rare now, were historically associated with dosing that ignored individual clearance differences.
The core problem is variability. Theophylline is cleared by the liver enzymes CYP1A2 and CYP2E1, whose activity differs enormously between people. A healthy young smoker clears the drug nearly twice as fast as a non-smoker; a patient with heart failure or liver disease clears it at a quarter of the normal rate; a premature infant barely clears it at all. A single standard infusion rate cannot serve all of these patients, which is why the labelling gives a table of rates by age and condition rather than one number.
This calculator implements that labelling table. Each group has a reference infusion rate in mg/kg/hour that achieves about 10 microgram/mL at steady state, and the calculator scales it linearly to your target level, multiplies by weight, and checks the result against the group's maximum daily infusion. It is arithmetic support for a prescribing decision, not a substitute for therapeutic drug monitoring: a level must still be checked after starting.
Infusion rates by patient group
The reference rates, all in mg/kg/hour of theophylline for a 10 microgram/mL target, are: children 1 to 9 years 0.8; children 9 to 12 years 0.7; adolescents 12 to 16 years 0.5 (0.7 if smoking); healthy non-smoking adults 16 to 60 years 0.4 (0.64 if smoking); adults over 60 years 0.3 (0.48 if smoking); cystic fibrosis 0.5; and the impaired-clearance group, which includes congestive heart failure, cor pulmonale, liver dysfunction, sepsis with multi-organ failure, and shock, just 0.2. Maximum daily infusions are 400 mg for the impaired and elderly groups, 600 mg for young children, and 900 mg for adolescents and adults.
Smoking deserves emphasis because it is common and commonly forgotten. Polycyclic aromatic hydrocarbons in tobacco smoke induce CYP1A2, raising theophylline clearance by about 60 percent. A 70 kg smoking adult targeting 10 microgram/mL needs 0.64 times 70, about 44.8 mg/hour, versus 28 mg/hour for the non-smoker. When a smoker stops smoking in hospital, clearance falls over several days and the previously correct infusion becomes toxic: levels must be rechecked after smoking cessation, a trap that catches teams regularly.
The impaired-clearance group is where most serious toxicity originates. In liver dysfunction the half-life stretches to about 32 hours, versus under 9 in healthy adults, so a standard infusion accumulates relentlessly. Heart failure and shock reduce hepatic blood flow with similar effect. For these patients the calculator's 0.2 mg/kg/hour starting point is deliberately conservative, and levels should be checked early, well before the usual 12 to 24 hours, because accumulation is faster than the extended half-life suggests when clearance is still falling.
Loading doses and the volume of distribution
An infusion takes three to five half-lives to reach steady state, which in a healthy adult means most of a day. When the patient is acutely unwell, a loading dose fills the volume of distribution immediately. Theophylline's volume of distribution is about 0.5 L/kg, so the loading dose in mg of theophylline is 0.5 times weight in kg times the gap between target and current level in microgram/mL. For a 70 kg patient with no theophylline on board targeting 12, that is 0.5 times 70 times 12, which is 420 mg, given IV over 20 to 30 minutes.
The current level matters: loading into a patient who already has a level of 8 toward a target of 12 needs only the 4 microgram/mL gap, or 140 mg for the same 70 kg patient. Loading a patient whose level is unknown is discouraged; if the history is unreliable, a level should be drawn first or the loading dose withheld, because stacking a full load on top of an existing therapeutic level invites toxicity. The calculator gives no loading dose when the current level already meets the target.
Aminophylline, the salt form most often stocked for infusion, is 80 percent theophylline by weight, so aminophylline doses are the theophylline doses divided by 0.8. The 420 mg theophylline load becomes 525 mg of aminophylline, and a 42 mg/hour theophylline infusion becomes 52.5 mg/hour of aminophylline. The calculator handles this conversion automatically when the salt form is selected, so the number on screen matches the ampoule in hand.
Monitoring, levels, and interactions
A level should be checked 12 to 24 hours after starting the infusion, roughly three half-lives in a healthy adult, and sooner in impaired-clearance patients. The result is interpreted against the 10 to 20 range and the infusion adjusted proportionally: new rate equals current rate times (target divided by measured), the same linear logic as other TDM drugs, valid here because theophylline kinetics are approximately linear in the therapeutic range. Recheck after each adjustment until stable, then periodically.
Interactions are frequent and clinically important. Ciprofloxacin, erythromycin, fluvoxamine, cimetidine, and oral contraceptives raise theophylline levels; rifampin, carbamazepine, phenytoin, St John's wort, and smoking lower them. Starting or stopping any of these in a patient on theophylline should trigger a level within days. Febrile illness and viral infections can also reduce clearance unpredictably, which is worth remembering in winter respiratory wards.
Signs of toxicity should be sought actively, not awaited. Early toxicity is gastrointestinal and neurological: nausea, vomiting, tremor, irritability, and headache. Cardiac toxicity follows: sinus tachycardia, then supraventricular and ventricular arrhythmias. Seizures can occur without much warning at high levels and may be the first dramatic sign in a patient whose earlier symptoms were missed. Any of these with a level above 20 means stopping or reducing the infusion and seeking urgent senior review; activated charcoal and haemodialysis have roles in severe overdose managed by toxicology.
Oral therapy and stepping down
Intravenous theophylline is for acute severe disease; stable patients use oral sustained-release formulations. The calculator gives the oral equivalent as the infusion's daily total converted to the chosen salt, in divided doses. In practice the oral dose is individualised to the same 10 to 20 target with trough monitoring, and the sustained-release kinetics mean once- or twice-daily dosing is feasible. Food can affect absorption of some formulations, so patients should be consistent in how they take it relative to meals.
Stepping down from infusion to oral needs overlap planning: because the infusion washes out over several half-lives, the first oral doses can be started while the infusion continues, with the infusion stopped once oral absorption is established. Levels guide the transition. Abrupt cessation without oral cover risks rebound bronchospasm in a patient whose airways were only just stabilised.
Long-term theophylline has fallen out of favour in many guidelines because inhaled therapies achieve more with less risk, but it retains a niche in severe disease and in patients who cannot use inhalers effectively. Anyone on chronic theophylline needs periodic levels, a medication review for interactions at every encounter, and counselling about smoking: starting or stopping smoking changes the dose requirement substantially, and the patient should be told to report either change before the next level surprises everyone.
Sources
- Theophylline Summary of Product Characteristics: intravenous infusion rates by age and condition; loading dose 5 mg/kg (as aminophylline) over 20 to 30 minutes. Electronic medicines compendium. medicines.org.uk
- Barnes PJ. Theophylline. American Journal of Respiratory and Critical Care Medicine. 2013;188(8):901-906. doi:10.1164/rccm.201302-0388PP
Key takeaways
- Theophylline's therapeutic range is 10 to 20 microgram/mL; toxicity begins just above, with seizures possible above 30.
- Infusion rates vary several-fold: 0.8 mg/kg/hr in young children, 0.4 in healthy adults, 0.64 in smokers, and 0.2 in heart failure, liver disease, or shock.
- Loading dose is 0.5 L/kg times body weight times the gap to target; aminophylline doses are theophylline divided by 0.8.
- Check a level 12 to 24 hours after starting, recheck with any interacting drug change or smoking change, and never guess in impaired-clearance patients.
Frequently asked questions
What is the target theophylline level?
The usual therapeutic range is 10 to 20 microgram/mL. Infusion rates in the labelling are quoted for a 10 microgram/mL reference and scale linearly to your target. Levels above 20 bring nausea, tremor, and tachycardia; above 30 there is a seizure risk.
How do I calculate a theophylline infusion rate?
Take the reference rate for the patient's group in mg/kg/hour (for example 0.4 for a healthy non-smoking adult), multiply by (target level divided by 10), then multiply by weight in kg. A 70 kg non-smoking adult targeting 15 needs 0.4 times 1.5 times 70, which is 42 mg/hour of theophylline.
Why do smokers need higher theophylline doses?
Chemicals in tobacco smoke induce the liver enzyme CYP1A2 that clears theophylline, raising clearance by about 60 percent. When a smoker is admitted and stops smoking, clearance falls over several days, so the infusion that was correct on admission can become toxic: recheck the level after smoking cessation.
What is the difference between theophylline and aminophylline?
Aminophylline is the ethylenediamine salt of theophylline and is 80 percent theophylline by weight. To convert, divide the theophylline dose by 0.8: 420 mg of theophylline is 525 mg of aminophylline. The calculator does this automatically when aminophylline is selected.
How is the loading dose worked out?
Loading dose equals 0.5 L/kg (the volume of distribution) times body weight times the gap between target and current level. A 70 kg patient with no theophylline on board targeting 12 needs 420 mg IV over 20 to 30 minutes. No loading dose is needed if the current level already meets the target.
Which drugs interact with theophylline?
Levels are raised by ciprofloxacin, erythromycin, fluvoxamine, cimetidine, and oral contraceptives, and lowered by rifampin, carbamazepine, phenytoin, St John's wort, and smoking. Any start or stop of these in a patient on theophylline should prompt a level within days.
Clinical sources: Theophylline Summary of Product Characteristics (electronic medicines compendium); Barnes PJ. Am J Respir Crit Care Med. 2013;188(8):901-906.