The most specific test for organophosphorus poisoning-

Both RBC cholinesterase and plasma cholinesterase are measured in organophosphorus poisoning. RBC cholinesterase represents the acetyl-cholinesterase found on RBC membranes, similar to that found in neuronal tissue. Thus, its measurement more accurately reflects nervous system organophosphorus acetyl cholinesterase inhibition. Plasma cholinesterase is a liver acute-phase protein, circulates in the blood plasma. It is found in CNS white mater, pancreas and heart. It can be affected by many factors, including pregnancy, infection and medical illness. Additionally, a patient's level can vary up to 50% with repeated testing. Thus, RBC cholinesterase is more accurate of the two measurements, but plasma cholinesterase is easier to assay and is more readily available.