Abstract: The invention provides a determination method for determining the presence of a target microorganism in a patient from a sample of said patient's stools, the method being characterized in that it comprises the following operations: obtaining a sample of liquid stools of said patient or a liquid sample obtained from stools of said patient, referred to as the liquid sample; pretreating the liquid sample with activated carbon; and using immunochromatography to detect in the resulting pretreated liquid sample the possible presence of at least one antigen of the target microorganism so as to come to a conclusion about the presence or the absence of the target microorganism in said patient. The invention also provides a device (1) for detecting an antigen of a target microorganism in the liquid sample (3) by immunochromatography, the device including a zone (20) for purification with activated carbon (21).