Investigations
1st investigations to order
clinical diagnosis
Test
No tests are usually necessary.
In most cases, organophosphate poisoning is a clinical diagnosis based on a history of exposure with characteristic signs of cholinergic excess.
Result
clinical features of organophosphate poisoning
Investigations to consider
atropine challenge
Test
Rapid administration of atropine to patients with organophosphate poisoning who present with rhinorrhoea and bronchorrhoea may be life saving so early recognition is important.[13] If organophosphate poisoning is suspected (by signs and symptoms) but history of exposure is not known, some clinicians recommend considering use of atropine to confirm the diagnosis.
The likelihood of organophosphate poisoning is greatly increased if few or none of the following features are seen following atropine administration: dry skin and mucous membranes, increased heart rate, moderately dilated pupils, and decreased bowel sounds.
Result
lack of anticholinergic effects
plasma cholinesterase
Test
Where this test is available, measurement of decreased plasma cholinesterase activity can be used to confirm diagnosis; however, results are rarely available in time to aid decisions on management.[5]
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
Where this test is available, measurement of decreased red blood cell acetylcholinesterase (RBC-AChE) activity can be used to confirm diagnosis; however, results are rarely available in time to aid decisions on management.[5]
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.[16]
There are a few organophosphates where the correlation with clinical severity is very poor. For example, profenofos may cause undetectable activity in asymptomatic patients.[15]
Result
decreased cholinesterase activity
CXR
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.[17] Metabolic acidosis is also common in organophosphate poisoning.[18]
Result
metabolic acidosis, hypoxia, hypercapnia
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