Evaluation of performance characteristics of automated measurement systems for allergy testing

Reliability of test results, convenient handling and flexibility are major requirements on automated immunoassays systems. To investigate to what extent these requirements were met by the Pharmacia CAP and DPC IMMULITE and DPC Microplate Systems, we evaluated several performance characteristics of assays of specific IgE against some common inhalant allergens as well as the atopy tests Phadiatop (Pharmacia CAP System) and AlaTOP (DPC IMMULITE and Microplate System). Comparing Phadiatop and AlaTOP results (n = 95) to clinical data, the sensitivity was found to be 97% in the Pharmacia CAP System and 82% in the AlaTOP-DPC Microplate System and 88% with AlaTOP-IMMULITE. Specificity was in all cases higher than 90%. The pooled total variation was more than twice as high with the DPC Microplate System as compared to the Pharmacia CAP System in our first investigation. A second investigation showed similar values. The investigation of systematic differences showed that the error contribution of sample related differences between the systems was even larger and far exceeded the analytical variation. Thus the two methods do not seem to be measuring the same specific IgE antibodies. In 8 out of 8 cases with the Pharmacia CAP System positive and DPC negative results and in 2 out of 2 cases with DPC positive and Pharmacia CAP System negative results, the presence of IgE antibodies could be confirmed by IgE immunoblotting. Serum dilutions showed very irregular O/E patterns for the DPC Microplate System. There was no effect when adding non-specific IgE to serum samples. Addition of competing IgG antibodies showed a moderate decrease in binding of specific IgE in the Pharmacia CAP System when increasing amounts of IgG were added. The effect in the DPC Microplate System was more pronounced with large decreases, or increases of measured values even at lower concentration of the competing antibody. The results may indicate insufficient allergen concentration in the DPC assay and draw attention to the risk for undesirable complex formation between allergen and antibody in solution. The combination of the two DPC systems did not offer any advantages over Pharmacia CAP System from the handling or work flow point of view.