Investigations
1st investigations to order
atropine therapeutic trial
Test
Atropine (administered intravenously as a single dose) is given as a therapeutic trial in all suspected cases, or when diagnosis is in doubt.
The likelihood of organophosphate poisoning is greatly increased if few or none of the following features are seen: dry skin and mucous membranes, increased heart rate, moderately dilated pupils, and decreased bowel sounds.
If no response, the dose may be increased; the higher the dose that has no effect, the more the diagnosis is supported.
The trial is unlikely to cause severe toxicity if there is no organophosphate poisoning.
Result
lack of anticholinergic effects
plasma cholinesterase
Test
Used to confirm diagnosis. A normal result is not consistent with significant poisoning by any organophosphate.
Less useful than an atropine trial because treatment needs to be initiated before the test results are available.
A highly sensitive test; however, it is far more sensitive for exposure to some organophosphates than others, and correlates poorly with severity.
Result
decreased cholinesterase activity
red blood cell cholinesterase
Test
Used to confirm diagnosis, but is less useful than an atropine trial because treatment needs to be initiated before the test results are available.
Decreased activity should correlate well with the extent of neuronal inhibition of acetylcholinesterase (AChE). However, reactions between AChE, organophosphate, and oxime (antidote) continue for a few hours after collection in vitro if the sample is not tested immediately or rapidly diluted and cooled.
Thus, the reported result may be difficult to interpret if the time to analysis and specimen handling is variable.
Two RBC-AChE point of care devices that provide reliable results within minutes are available.[13]
There are a few organophosphates where the correlation with clinical severity is very poor. For example, profenofos may cause undetectable activity in asymptomatic patients.[11]
Result
decreased cholinesterase activity
Investigations to consider
chest x-ray
Test
Used to rule out diagnosis of aspiration pneumonia and should be ordered if chest signs are focal or not responsive to atropine.
Result
normal; consolidation if concomitant aspiration
ECG
Test
Should be ordered in symptomatic patients and repeated if an abnormal heart rate or hypotension is persistent.
Result
QT prolongation; arrhythmia
blood gases
Test
Ordered to monitor respiratory failure. Respiratory failure due to excessive secretions, bronchospasm, aspiration, paralysis, respiratory centre dysfunction, and/or sedation is the leading cause of death, but is also the one most amenable to treatment. Metabolic acidosis is also common in organophosphate poisoning.
Result
metabolic acidosis, hypoxia, hypercapnia
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