Reproducibility of enzyme‐linked immunosorbent assay and immunofluorescence for detecting Clavibacter michiganensis subsp. sepedonicus in multiple laboratories*

Results from enzyme-linked immunosorbent assays (ELISA) and immunofluorescence (IF) tests for the detection of Clavibacter michiganensis subsp. sepedonicus in potato tissue were analysed to determine the variation that occurs when different analysts perform the test. Data generated in accredited laboratories from sets of proficiency panel samples were used for the analysis. Sensitivity and specificity for both the ELISA and IF tests were very high as very few false positive and false negative results occurred. Analysis of z-scores for the positive samples in the proficiency panel sets showed, for both serological tests, that about 90% of the results were within the acceptable range around the assigned values for the samples. Rescaled sum of scores for individual analysts who had false positive or negative results in ELISA were generally outside the acceptable range, although most analysts with high or low rescaled sums of z-scores still identified each sample correctly as being positive or negative for the ring-rot pathogen. For IF there were no false positive results, and the false negative results were not associated with aberrant rescaled sums of z-score for the analysts, suggesting that perhaps an error in testing occurred rather than a problem with quantifying cell numbers.