Police custody is detention in response to a suspicion of crime. A person in custody has the right to be examined by a physician, who is responsible for determining whether the person's medical condition allows him/her to continue to be held in detention. However, there is no consensus on the definition of compatibility of the mental state with custody. Our objective was to determine the relevant criteria for compatibility and incompatibility of the mental state with detention in police custody according to medical examiners and psychiatrists. A descriptive study using a semi-structured questionnaire was conducted from November 2016 to May 2017 with medical examiners and psychiatrists who examined detainees in police custody in Marseille. We recruited 17 medical examiners and 43 psychiatrists. We identified three sets of criteria used to determine a mental state compatible with custody: care, pathology/disorder, and the context of police custody. The primary determinant of incompatibility was a need for immediate hospitalization, followed by a high suicide risk, psychiatric dangerousness, delusional symptomatology, an inability to understand questions, and an inability for the detainee to defend him-/herself. The psychiatrists and medical examiners differed significantly in their views of suicide risk, delusional symptomatology, and psychiatric diagnosis. Our work has permitted identification of the main determinants of incompatibility of the mental state with custody according to psychiatrists and medical examiners in Marseille. It offers preliminary results to reach a consensus and provide an inventory for physicians to use regarding police custody.