Comparison of theophylline blood levels measured by the standard TDx assay and a new patient-side immunoassay cartridge system.

Plasma steady-state concentrations of theophylline were measured by the standard TDx assay and a novel patient-side immunoassay system with a single-use cartridge (Biotrack 516) in 100 samples from 93 patients. Intra- and interassay coefficients of variation for the new cartridge technique averaged 4.3 and 9.8%, respectively. In the concentration range of 2 to 40 micrograms/ml, there was a linear correlation (r = 0.96, p < 0.0001) between both assays [slope of 0.86; y-intercept (TDx values) = -0.14 micrograms/ml], indicating that the two methods can be used interchangeably. Since the completely automated cartridge system is very simple and extremely rapid, this technique offers some advantages for routine clinical use, especially under ambulatory conditions.