Abstract: The present invention relates to a diagnostic method for the detection of acute leptospirosis. The method of the invention allows for a quick, specific and sensitive detection of the disease caused by pathogenic Leptospira. The described method is based on the immunological detection of the protein LipL21, a membrane protein expressed specifically by pathogenic strains of Leptospira. Furthermore, the present invention relates to the development of a diagnostic device that allows positive diagnosis to be conducted in the early and critical stage of the infection. The preferred diagnostic device of the invention is a lateral flow test, which as a hand-held assay, is easy to use and facilitates diagnosis of leptospirosis in the first 5 days of infection. Further disclosed is the antibody raised against the LipL21 protein as well as its respective uses in the diagnostic methods and devices.