Transient loss of consciousness (TLoC) is a leading cause of referrals to acute neurological services. A witness account is often lacking, and ancillary investigations are a critical diagnostic adjunct. Hypophosphatemia was recently identified as a potential marker of epileptic seizures in ward and emergency department presentations. We evaluated the real-world utility of checking phosphate in an unselected cohort of people presenting with TLoC. We retrospectively reviewed 182 episodes (91 referrals to the first seizure clinic and 91 consults) from 170 patients. We assessed nine prespecified serological markers frequently measured in people presenting with TLoC. Raw p-values comparing mean levels showed a significant difference between epileptic seizures and other causes of TLoC only for phosphate (0.98 vs 1.17 mmol/L, p = 0.01) and lactate (2.93 vs 1.77 mmol/L, p = 0.001). Mean lactate level, but not phosphate, remained significant after multiple comparison correction (p = 0.01). However, a phosphate below 0.8 mmol/L was significantly more likely to be associated with epileptic seizures (17/60; 28%) than episodes not thought to be epileptic (2/30; 6.6%; p = 0.03). Stepwise regression showed a model including lactate, phosphate, and calcium predicted convulsive seizures with an area under the ROC curve of 0.825 (95% CI 0.718-0.931), superseding lactate alone. Checking serum phosphate may be valuable in helping to determine the etiology of an episode of TLoC. PLAIN LANGUAGE SUMMARY: We studied blood test results of 170 people suspected of having had an epileptic seizure who presented to a UK hospital neurology service. Although blood phosphate tests were infrequently requested, our results suggest a low phosphate level could be useful to help distinguish between epileptic seizures and other causes of collapse.