Anti-thyroid peroxidase (TPO) antibodies - Comparative analysis of two automatic methods, ECLIA and CMIA

Introduction: Anti-thyroid peroxidase autoantibodies (TPO) is an essential diagnostic tool for autoimmune disorders of the thyroid gland. However, TPO results are not always comparable due to differences between methods. Here, we aimed to investigate the differences between two modern laboratory methods for TPO measurement: electrochemiluminescence (ECLIA) and chemiluminescence microparticle (CMIA) immunoassays. Methods: A total of 234 serum samples were tested on two methods: Cobas-e601 (ECLIA) and Alinity i (CMIA). TPO results were compared statistically both quantitatively and qualitatively (results were coded as positive/negative, according to ECLIA/CMIA reference ranges. Results: The precisions of both methods were acceptable compared with the claims of the manufacturer. There was a very strong, but unsatisfactory correlation between the two methods (Pearson r=0.85). Passing-Bablok regression revealed a significant deviation from linearity (Cusum p<0.01) and an unacceptable quantitative relationship: intercept -7.61, slope 1.10. Moreover, a visual analysis of overall and medical decision level-focused Bland-Altman plots confirmed the lack of quantitative agreement. As for the qualitative analysis, the concordance rate between methods was 218/234 (93.1%). The agreement was considered good to very good according to the inter-rater agreement test: weighted Cohen kappa = 0.805. Conclusions: The qualitative agreement between Cobas-e601 (ECLIA) and Alinity i (CMIA) was good, therefore the two methods may be used indiscriminately for initial testing of patients suspected of thyroid gland autoimmune diseases. However, due to poor quantitative agreement, the two methods should not be used interchangeably for monitoring as the results may mislead both physicians and patients, possibly leading to medical errors.