Results of a quality control scheme for the assay of theophylline in serum

Twenty-eight laboratories in the United Kingdom, Europe, and Australia participated in pilot study to assess interlaboratory variability in determining serum theophylline levels. Each laboratory received two samples of pooled serum containing theophylline each month and was asked to report their results and method to us. Data obtained over a period of 12 months were collated, enabling certain conclusions to be drawn. Five methods of analysis were used: high pressure liquid chromatography (HPLC), gas--liquid chromatography, EMIT, thin-layer chromatography, and ultraviolet spectroscopy (UV). All of these methods, with the exception of UV, were found to be satisfactory, with HPLC marginally superior. Several laboratories improved their assay techniques over the period of operation of the scheme, and all found the scheme helpful in assessing the performance of their methods.