Serum theophylline and phenytoin levels: can we afford to do them? Can we afford not to?

The following factors must be considered when determining the need for theophylline and diphenylhydantoin (DPH) levels: accuracy and reproducibility, turnaround time, and cost-benefit ratio for the patient. Serum levels must bear a relationship to the therapeutic effects of the drugs. In addition, the pharmacology of the drugs must be of such an order of complexity that there is not a simple relationship between dosage and serum levels.