How to interpret urine toxicology tests.

A 13-year-old girl presents to the emergency department for the second time with an unresponsive episode. She has a GCS (Glasgow Coma Scale) score of 11 on arrival and all other observations are normal. The story is unclear, but there are ongoing safeguarding concerns and the family are known to social services. All investigations are normal. After a period of observation on the ward, her GCS returns to normal and she appears well. Both on the first presentation and this presentation ingestion of a toxin was suspected. However, this was denied by the patient and urine toxicology screen was negative. Does this rule out toxin ingestion? Will this change your management?