Organophosphate Toxicity Workup

Updated: Sep 05, 2017
  • Author: Kenneth D Katz, MD, FAAEM, ABMT; Chief Editor: Sage W Wiener, MD  more...
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Workup

Laboratory Studies

Organophosphate (OP) toxicity is a clinical diagnosis. Confirmation of organophosphate poisoning is based on the measurement of cholinesterase activity; typically, these results are not readily available in a clinically relevant timeframe. Although red blood cell (RBC) and plasma (pseudo) cholinesterase (PChE) levels can both be used, RBC cholinesterase correlates better with central nervous system (CNS) acetylcholinesterase (AChE) and is, therefore, a more useful marker of organophosphate poisoning.

The portable Test-mate ChE field test measures RBC AChE and PChE within 4 minutes. A study of patients with acute organophosphorus poisoning compared Test-mate ChE results with those of a reference laboratory test and found good agreement between the two. Results show the Test-mate ChE field kit is a reliable test that provides rapid measurement of RBC AChE in acute organophosphorus poisoning. [13]

If possible, draw blood for measurement of RBC and plasma cholinesterase levels prior to treatment with pralidoxime (2-PAM). Monitoring serial levels can be used to determine a response to therapy.

RBC AChE represents the AChE found on RBC membranes, similar to that found in neuronal tissue. Therefore, measurement more accurately reflects nervous system OP AChE inhibition.

Plasma cholinesterase is a liver acute-phase protein that circulates in the blood plasma. It is found in CNS white matter, the pancreas, and the heart. It can be affected by many factors, including pregnancy, infection, and medical illness. Additionally, a patient's levels can vary up to 50% with repeated testing.

RBC cholinesterase is the more accurate of the two measurements, but plasma cholinesterase is easier to assay and is more readily available.

Cholinesterase levels do not always correlate with severity of clinical illness. Moreover, a variety of conditions can result in falsely lowered cholinesterase levels (see Diagnostic Considerations).

The level of cholinesterase activity is relative and is based on population estimates. Neonates and infants have baseline levels that are lower than adults. Because an individual patient's baseline levels are rarely available, the diagnosis can be confirmed by observing a progressive increase in the cholinesterase value until the values plateau over time.

Other laboratory findings include the following:

  • Leukocytosis
  • Hemoconcentration
  • Metabolic and/or respiratory acidosis
  • Hyperglycemia
  • Hypokalemia
  • Hypomagnesemia
  • Elevated troponin levels [14]
  • Elevated amylase levels
  • Elevated liver function test results

A retrospective analysis of OP-poisoned patients by Liu et al found a direct correlation between the severity of poisoning and mortality and the presence of pretreatment metabolic and respiratory acidosis. [15]

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Imaging Studies

A chest radiograph may reveal pulmonary edema but typically adds little to the clinical management of a poisoned patient.

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Electrocardiography

ECG findings include prolonged QTc interval, elevated ST segments, and inverted T waves. [14] Although sinus tachycardia is the most common finding in the poisoned patient, sinus bradycardia with PR prolongation can develop with increasing toxicity due to excessive parasympathetic activation.

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