5.2.1 OECD Test Guideline 423 — Acute Toxic Class Method (ATM)

The Acute Toxic Class Method evaluates acute lethality using sequential dosing of small groups of three animals at fixed, internationally standardised dose...

The Acute Toxic Class Method evaluates acute lethality using sequential dosing of small groups of three animals at fixed, internationally standardised dose levels of 5, 50, 300, and 2000 mg/kg. The Wistar rat is the standard species, with female animals generally preferred owing to their marginally higher sensitivity in most toxicity classes, and each dose level uses a group of three animals. The oral (gavage) route is most common, although intraperitoneal, intravenous, dermal, and inhalation routes may also be used depending on the intended clinical route. Animals are observed for fourteen days following dosing, with body weight and clinical signs — piloerection, tremors, convulsions, and mortality — recorded systematically. Rather than yielding a precise LD50 value, OECD 423 classifies the substance into an acute toxicity class (approximately 5, 50, 300, 2000, or greater than 2000 mg/kg), sufficient for hazard classification and labelling purposes. A defined humane endpoint — severe distress, loss of righting reflex, or cyanosis — mandates immediate euthanasia to minimise unnecessary suffering, consistent with the Refinement principle of the 3Rs.