• Suspected theophylline toxicity
• Although toxicity has been observed at levels as low as 15 mg/L (15 microgram/mL), toxicity is most commonly observed at levels >20 mg/L (20 microgram/mL).  The risk of toxicity is greater in patients with levels above 25 microgram/mL or with significant risk factors including ICU admission, prior seizures, or arrhythmias.  Critically ill patients may manifest signs of toxicity with only mildly elevated serum concentrations.
• Patients with acute theophylline intoxication have a 50% probability of major toxicity at peak serum theophylline concentration of 611 micromole/L (110 mg/L). 

• Theophylline monitoring
• The established therapeutic range should be used as a guide only, due to theophylline's narrow therapeutic index and daily variations in plasma theophylline clearance.  Although large variability in theophylline elimination occurs, individual patient variability is usually relatively small in the absence of confounding factors such as drug interactions. 
• In otherwise healthy patients, levels <25 microgram/mL are usually not associated with a substantial risk of major theophylline toxicity.
• Bronchodilatory, anti-inflammatory, and immunomodulatory effects of theophylline may be adequate for some patients at serum levels <10 microgram/mL.
• Tine to steady state is as follows:
• Adults:  2 to 3 days
• Children:  1 to 2 days
• Infants:  1 to 5 days
• Neonates/Premature infants:  About 6 days
• Healthy newborns:  About 5 days

In an epidemiologic investigation of 36,000 ambulatory patients, the overall risk for serious toxic reactions to theophylline was rare; however risk was five times greater among elderly patients and those taking cimetidine. 

Serum concentrations increased in:

Serum concentrations reduced in: