Development of a radioimmunoassay for theophylline. Application to studies in premature infants.

The use of theophylline in premature infants has necessitated the development of an assay applicable to samples under 0.1 ml. To accomplish this we have developed a radioimmunoassay for theophylline. The antiserum to theophylline has been characterized and found to be highly specific for theophylline and sensitive to 2 ng. The affinity of our antibody and the titer developed in the goat allowed the assay to be performed routinely with the use of a plasma sample size of only 2 to 5 micronl. Theophylline levels of 6 to 20 microng/ml were found 2 hr after maintenance doses of aminophylline, 4 mg/kg per rectum in 12 premature infants. Terminal serum half-lives ranged from 12 to 54 hr.