Thrombocytopenia occurs frequently. We will illustrate, through the presentation of a clinical case, the difficulties encountered to identify and characterize thrombocytopenia. The clinicobiological validation of a low platelet count implies, at the same time, the biologist, who must assume the validation of numeration while mentioning the morphological characteristics of the platelets and other blood cells, as well as the clinician who must interpret these data according to the clinical context. Firstly, we will detail the basic rules to correctly ensure this validation. Secondly, we will see which are the arguments which that make it possible to direct the diagnosis towards an acquired or inherited thrombocytopenia. Lastly, we will approach the classification of inherited thrombocytopenias.