BackgroundTransient loss of consciousness (TLOC) is a common primary care presentation. 90% are due to syncope (S), epilepsy (E), or psychogenic non-epileptic seizures (PNES). Misdiagnosis and delayed diagnosis is common. We explore symptoms and witness observations that can classify patients with likely diagnoses of E, S, or PNES.MethodsPatients with objectively-documented diagnoses of E, S, or PNES, and an attack witness, were invited to complete a questionnaire (capturing medical history, 86 peri-episodal experiences, and 31 witness observations). Iterative feature selection identified questions strongly predictive of diagnosis; a random forest trained on these classified patients into likely diagnoses of E, S, or PNES.Results249 patients (86 E, 79 s, 84 PNES) were randomly assigned to training or validation in a 2:1 ratio. Feature selection identified 36 highly-predictive questionnaire items. The classifier correctly diagnosed 86% of patients in validation. 100% of S were correctly diagnosed, 85.7% E and 75% PNES. A simpler 12-feature model correctly classified 76.7% of cases (E: 75%; S: 92.3%; PNES: 65.6%).ConclusionsTLOC-associated symptoms and manifestations can contribute to a decision rule for primary/emergency care, assisting triage and referral. Determining a diagnostic pre-test probability from TLOC features can aid interpretation of investigation abnormalities of uncertain significance.