Comparative evaluation of canine and human ELISA assays for myeloperoxidase quantification in canine urine and serum

Myeloperoxidase (MPO), primarily produced by neutrophils, is highly conserved between species and exists as a glycosylated monomeric pro-MPO prior to storage as dimeric MPO in azurophilic granules. MPO is increasingly recognized as a biomarker of inflammatory diseases and is commonly measured using antibody-based assays. We compared two commercial enzyme linked immunosorbent assays (ELISAs): a canine assay detecting MPO based on antibodies produced against pro-MPO, and a human assay detecting MPO based on antibodies produced against the MPO monomer (MPO-intermediate), in the detection of MPO in canine serum and urine samples. Neither assay demonstrated cross-reactivity with the MPO-standard of the other assay. In urine samples (n = 87), median MPO concentrations were 78.0 pg/mL (IQR:78.0218.0) using the canine assay and 99.6 pg/mL (IQR:31.25107.4) using the human assay, with no correlation observed between the two assays. When segregating urine samples based on the presence of bacteriuria, only the human MPO-assay showed differences between groups, with higher and more variable concentrations in bacteriuric samples and consistently low concentrations in non-bacteriuric samples. Only the concentrations obtained with the human MPO-assays demonstrated an association with urinary IL-8 concentrations and urinary leukocyte degree. In serum samples (n = 56), the canine MPO-assay measured the highest median concentrations, 86.85 ng/mL (IQR:75.17 97.11), compared to the human MPO-assay, 1.2 ng/mL (IQR: 0.761.54), with no correlation between the two assays. These findings highlight the complexity of MPO measurements and underscore the importance of assay selection and biochemical understanding when investigating MPO as a biomarker in different biological matrices.