Plasmatic levels of theophylline in asthmatic patients. Comparative evaluations of two different methods.

BACKGROUND There are several criteria to choose an analytical method for drug monitoring. Such methods have to comply with standard values and quality control as well as other subjective features such as cost and the time consumed to obtain quantification (TCOQ). The purpose of this work was to compare two methods used to quantify plasmatic levels of theophylline in asthmatic patients as support to choose the best method. METHODS We analyzed plasma samples from 30 asthmatic pediatric patients at the pediatric service of the Hospital General de México, who were under treatment with theophylline and whose monitoring of drug levels was indicated. Plasma samples were analyzed by liquid chromatography (HPLC) and by enzyme immunoassay (EMIT), and were then compared with respect to reliability, as well as cost and TCOQ. RESULTS The difference of the plasmatic levels of theophylline quantified by both methods was not significant (p > 0.05); both showed a good correlation index (r = 0.995), and both were reliable based on other validity parameters. However, TCOQ for HPLC was 20.0 +/- 5.5 min (mean +/- SD) for each sample analyzed, and 2.3 +/- 0.5 for EMIT. With respect to the cost of each analysis, HPLC required 2.3 +/- 0.5 USD (mean +/- SD) and EMIT 4.5 +/- 0.3 USD. CONCLUSIONS Analytical methods used to quantify plasmatic levels of theophylline based on HPLC and EMIT proved to be suitable, because they fulfilled the criteria and standard values regarding quality control, although laboratorists have to select subjectively the best method according to cost and TCOQ, since HPLC was less expensive, and EMIT was more rapid.