Charting methods to monitor the operational performance of ELISA method for the detection of antibodies against trypanosomes

Four indirect enzyme-linked immunosorbent assays (ELISAs) for the detection of antibody against trypanosomes using antigen-precoated plates (Trypanosoma congolense and T. vivax) were used in 15 veterinary diagnostic laboratories in Africa and Europe. The study provided data allowing an evaluation of charting methods with respect to the operational performance of each ELISA. Data from standardised internal quality control (IQC) samples were plotted on charts and used as the assay performance indicators with reference to expected upper and lower control limits. Based on unprocessed (optical density) and normalised absorbance values (calculated as a percentage positivity of a control), dispersion of values from the expected data range was estimated plotting the location and deviation of the values. In addition, assay precision was estimated plotting the distribution of coefficients of variation<10% of the IQCs. Binding ratios of controls were calculated to estimate the assay proficiency with respect to the accuracy of assessing that the IQC samples tested positive or negative in the test proper. The graphical analysis of dispersion of absorbance values in combination with assay precision and proficiency criteria was considered fully satisfactory to evaluate the operational performance of the ELISAs and provided useful decision criteria for plate acceptance and rejection. The establishment of standardised and transparent IQC data charting methods for the indirect ELISAs provided an increased measure of confidence to national laboratories with respect to their reports on disease occurrence. Moreover, the relative assay performances between all laboratories were examined using summary data charts with reference to the performance criteria described. The IQC data were also examined using modified Youden plot analysis demonstrating that indirect ELISA methods can be successfully applied at diagnostic laboratories in the tropics for monitoring trypanosomosis control programmes.