Standards of laboratory practice: theophylline and caffeine monitoring. National Academy of Clinical Biochemistry

Theophylline is used in the treatment of asthma and chronic obstructive pulmonary disease. The use of theophylline has declined with the advent of potent steroid inhalants. Because of the therapeutic index of this drug, monitoring of theophylline concentrations in plasma is essential. Monitoring should be done on trough specimens after steady-state has been reached. Non-steady-state concentrations may be indicated in selected situations. Caffeine is used to treat apnea of the newborn because of its low toxicity. Monitoring is often by clinical effect. Monitoring of serum concentrations should be performed in cases where there is no clinical response or if there is suspected toxicity.

Published by Clinical chemistry, 1998. The guideline itself is at the source linked here.

pubmed.ncbi.nlm.nih.gov/9590396/