Leptospirosis is a common condition in Wallis and Futuna, and the definitive diagnosis needs to be established urgently at the first patient consultation, which is usually one to two days after the onset of clinical signs. As a diagnostic aid, a composite index was established based on data from 338 patients seen by the Wallis and Futuna admissions services between 2008 and 2015. The data taken into account include: age and sex of the patient, their home island, the consultation period and the results of leukocytes, platelets, CRP, creatinine and GGT tests combined with 2 major clinical signs, headache and conjunctival suffusion. Then 5 threshold limits were defined for this index, which indicates from very low risk to almost certain biologically confirmed leptospirosis. Other febrile diseases responsible for thrombocytopenia are not found in Wallis and Futuna, which probably explains the good statistical qualities of this index with a value of area under the curve equal to 0.821.