The high sensitivity and NPV of S100A8/A9USG observed in this study suggest that measuring S100A8/A9USG could be a good screening test for TCC/PCA in dogs, especially patients where a UTI has been ruled out as a cause of clinical signs of lower urinary tract disease, and that a negative test result (i.e., a S100A8/A9USG of <109.9) essentially excludes a diagnosis of TCC/PCA in dogs. This evidence concerns the gene S100A8 and posterior cortical atrophy.