D-dimer is a by-product of clots formed in blood vessels. Measurement of D-dimer concentrations is used routinely in human medicine to exclude syndromes in which blood clots form in the vessels, including pulmonary thromboembolism (PTE) and deep vein thrombosis (DVT). Most commercially available D-dimer tests are specific for use in humans. Veterinary specific tests are lacking, however some human tests appear to work well in dogs and horses. So far, none have proven useful in cats, and the reasons for this are unknown. This study aims to separate D-dimer out of canine, equine and feline blood samples to identify the characteristics of this protein, and then use it to investigate if human tests can be reliably used in cats and dogs.