Simple SummaryMeasuring left atrial size has important prognostic and therapeutic consequences in cardiac diseases. An enlarged left atrium indicates that the cardiac disease is severe. Though cardiac ultrasound examination is the best way to determine left atrial size, this technique is often unavailable in veterinary practices, as it is expensive and requires specific expertise. Therefore, chest X-rays are usually used to decide whether the left atrium is enlarged. With that, practicing veterinarians aim to differentiate cardiac from respiratory diseases, which both can lead to similar clinical signs. However, determining left atrial enlargement on X-rays can be challenging. The authors of this study came up with a simple and reproducible method that might make determining left atrial size on X-rays easier. The study aimed to compare two methods. Dogs with and without cardiac disease were included. The left atrial size of the included dogs had also been determined with ultrasonography to serve as a reference. First, 39 veterinarians and veterinary students interpreted 90 canine chest X-rays subjectively. At least two weeks later, the same observers applied the new method on the same radiographs. The new standardized method did not give a more accurate diagnosis than subjective assessment.AbstractBackground: Left atrial enlargement indicates severe cardiac disease. Although the gold standard for determining left atrial size is echocardiography, many veterinary practices lack the necessary equipment and expertise. Therefore, thoracic radiography is often used to differentiate cardiogenic pulmonary edema from primary respiratory diseases and to facilitate distinguishing dogs with stage B1 and B2 mitral valve degeneration. Methods: The goal was to test a new standardized method for identifying radiographic left atrial enlargement. On a lateral radiograph, a straight line was drawn from the dorsal border of the tracheal bifurcation to the crossing point of the dorsal border of the caudal vena cava and the most cranial crus of the diaphragm. If a part of the left atrium extended this line dorsally, it was considered enlarged. Echocardiographic left atrial to aortic ratio (LA:Ao) was used as a reference. Thirty-nine observers with various levels of experience evaluated 90 radiographs, first subjectively, then applying the new method. Results: The new method moderately correlated with LA:Ao (r = 0.56–0.66) in all groups. The diagnostic accuracy (72–74%) of the subjective assessment and the new method showed no difference. Conclusions: Though the new method was not superior to subjective assessment, it may facilitate learning and subjective interpretation.