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Clinical diagnosis is based on history of ingestion, clinical presentation and improvement or resolution of muscarinic signs after atropine administration. If the diagnosis is uncertain, a test dose of atropine (0.02 mg/kg IV) can be administered after evaluating the baseline heart rate. If the heart rate increases, the pupils dilate and hypersalivation stops in 10–15 min, the animal is unlikely to have organophosphate or carbamate intoxication as it takes approximately 10 times this test dose to resolve clinical signs caused by these compounds.

Laboratory findings of heparinized whole blood cholinesterase (ChE) activity reduced by 50% of normal (based on the normal range for that laboratory) suggest exposure, whereas ChE activity less than 25% of normal indicates toxicosis in animals with characteristic clinical signs (Wismer, 2012). ChE activity of heparinized whole blood is a combination of true AChE activity of RBCs and pseudo-ChE activity of serum. Packed cell volume should be checked on blood samples for AChE testing as anaemia can result in decreased AChE activity. Blood should be kept refrigerated to prevent the loss of enzyme activity. ChE activity can remain decreased for 6 to 8 weeks following organophosphate exposure, whereas it may be normal in animals with carbamate intoxication. Carbamates bind reversibly with AChE in the body and they may also dissociate from AChE or pseudo-ChE in a blood tube or other specimen during transit. Stomach content, vomitus, hair, or suspected baits can be submitted to the laboratory for an organophosphate or carbamate residue screen.