Evaluation of an office method of measuring theophylline serum concentrations

A strip test for theophylline, with an immunoassay with a colorimetric indicator, has been developed for use in the physician's office (Seralyzer, Ames Division, Miles Laboratories, Elkhart, Ind.). Diluted serum is applied to the plastic reagent strip, which is then inserted into a reflectance photometer that quantitates the color change and displays the serum concentration in 80 seconds. To determine the reliability of this method, control sera (5.0, 10.0, 15.0, 20.0, and 25.0 micrograms/ml of theophylline) and 78 specimens, each from a different patient, were analyzed by Seralyzer and a reference method (EMIT, Syva Co., Palo Alto, Calif.). Nine replicate measurements of a 15.0 micrograms/ml control serum were performed on one day, an all five control sera were analyzed once daily for 20 days. Patient specimens were analyzed at the rate of eight per day. The coefficient of variation (mean/SD X 100) for within day precision of the 15 micrograms/ml control was 5.2% for Seralyzer and 4.1% for EMIT (p less than 0.05). Coefficients of variation for between-day precision of the Seralyzer were less than 10% at concentrations greater than 5 micrograms/ml, but were significantly greater than EMIT measurements at 5 and 10 micrograms/ml (p less than 0.05). There was a strong correlation between the two methods for the 78 patients' specimens (r2 = 0.95). The Seralyzer demonstrated a positive bias of 0.9 microgram/ml. Results analyzed by the two methods differed by greater than 25% in 10 of the 78 specimens (mean error, 3.3 micrograms/ml over the range of 2.9 to 22.2 micrograms/ml).(ABSTRACT TRUNCATED AT 250 WORDS)