Evaluation of whole blood theophylline enzyme immunochromatography assay

A new whole blood enzyme immunochromatographic (EIC) theophylline assay was evaluated in 18 low (25 to 37 percent) and 15 high hematocrit (49 to 56 percent) samples. A good correlation was observed between EIC and fluorescence polarization methods for plasma samples (r = 0.95). However, comparison of results between EIC whole blood and plasma values demonstrates a significant proportional bias that is inversely related to the sample's hematocrit. The EIC method for whole blood samples may substantially underestimate theophylline levels in polycythemic patients with theophylline values near or above the toxic range and underestimate levels in those with anemia, if a correction is not made for the sample's hematocrit. A correction formula to approximate plasma theophylline concentrations from whole blood measurements is described.