The invention relates to a method of determining feline or canine proBNP or fragments thereof for the diagnosis of heart disease, comprising the steps of providing a feline or canine sample, contacting the sample with at least one species-specific antibody for binding to at least one epitope of the respective species proBNP, determining the concentration of at least one fragment of proBNP, and diagnosing the animal with heart disease if the concentration is elevated compared to healthy subjects.