Diagnostic Use
Cholinesterase (plasma/serum) or Pseudocholinesterase:
Assay of plasma cholestinerase requires a scientist to be called in to the laboratory, and so is not available after hours except in specific circumstances where short term management will be influenced by the result. The table below sets out relevant clinical scenarios, and more detailed explanation follows.
| Pseudocholinesterase (or butyryl) test |
Out of hours -offered with specific clinical justification |
Out of hours – not routinely offered other than in specific circumstances |
| Indication(s): |
– Recent suspected toxic exposure to organophosphate or related compounds where an urgent level will affect patient management
– Serial monitoring (even out of hours) when required to closely monitor recovery from acute exposure, especially if the patient is in critical condition
– If carbamate exposure/toxicity is suspected, however note important caveats in interpretation (see below)
– Note that a low pseudocholinesterase level may or may not be specific for organophosphate poisoning due to multiple reasons (see below) |
– Assessment of liver function (unless specifically requested by hepatologist)
– For investigation of apnoea post muscle relaxant (a more than 24 hrs post exposure blood should be collected) – not normally offered out of hours unless the patient is about to have urgent surgery. May also be available with strong suspicion for pre-existing Scoline apnoea condition AND there is no other choice but to use depolarising muscle relaxant like succinylcholine.
– RBC cholinesterase (or acetyl) test will not be offered out of hours as it is normally more for detection and monitoring of chronic occupational exposure to organophosphates. |
More detailed related information:
Requests for Plasma cholinesterase (or pseudo or butyryl) and RBC cholinesterase (or acetyl) out of hours:
Indications for measuring plasma cholinesterase:
- Monitoring of long-term occupational organophosphate exposure/toxicity (RBC and/or plasma)
– While RBC cholinesterase reflects more on the nature of chronic occupational exposure (and neurotoxicity), plasma cholinesterase is a good exposure marker especially at subclinical level.
– Both not offered out of hours for this reason
- As part of assessment for liver function
– Plasma cholinesterase is not offered out of hours (in very rare circumstances post liver transplant – if hepatologist ever requires it urgently, they will need to discuss with chemical pathologist)
- As part of investigation for Scoline Apnoea
– A low level of activity could indicate an abnormal phenotype. Genetic cholinesterase variants may be classified by measuring their sensitivity to inhibitors. This is termed cholinesterase phenotyping. Genotyping (available on provision of an EDTA whole blood specimen) provides supportive information. For interpretive information on phenotyping results, see cholinesterase phenotyping.
– Plasma cholinesterase sample for total activity and phenotyping should only be taken at least 24hours after the muscle relaxant was last used to allow its effect to wean off from the circulation.
– Not normally offered out of hours
- Suspected acute organophosphate (OP) or carbamate (or ?? Novichok) toxicity
– Plasma cholinesterase activity is suppressed rapidly (within minutes to hours) from significant exposure, and recovers in days to weeks, thus is a reasonably good marker for recent exposure. Following exposure to organophiosphates, serum cholinesterase is expected to increase by 20% in 5 days.
However, the level is both not very specific and sensitive for OP exposure. Reasons include:
Sensitivity:
– Intra- and interindividual biological variation (<20% change from ‘baseline’ is considered equivocal). A change of:
>30% or more is likely to be clinically
<50% – latent poisoning, no clinical manifestation
50-60% reduction is mild
60-90% reduction is moderate (weakness, impair ability to walk)
>=90% -severe exposure (unconsciousness, paralysis, death)
– There is variation in the dose response relationship in vivo from different OPs with respect to absorption,
distribution, metabolism, excretion. This means a single level cannot indicate worsening inhibition or a
recovery trend
– For carbamate exposure, unlike OP exposure, binding is reversible in-vivo as well as post collection, in terms of hours. Thus a normal measured plasma cholinesterase level does not exclude prior carbamate toxicity and a suppressed level when measured a few hours later might already mean a recovered level in the patient.
Specificity:
Other causes of low results:
– Hepatitis, cirrhosis, malnutrition, chronic alcoholism and dermatomyosis can have low plasma cholinesterase activities
– Toxicants like cocaine, carbon disulphide, benzalkonium salts, organic Hg compounds, ciguatoxins and
solanines may reduce plasma cholinesterase activity
– Early pregnancy, birth control pills, metoclopramide may also cause depression in plasma cholinesterase.
RBC cholinesterase activity – although a better marker to correlate with neurotoxicity, RBC cholinesterase may not reach minimum level for several days and not recover until after 1-3 months, thus is not a good exposure marker in the emergency setting. Although its reduction in activity is more specific than plasma for OP exposure, beware of increased RBC turnover (e.g. due to haemolytic anaemia). Also the test a lot more labour intensive for the laboratory. Thus not normally offered out of hours.
Whether the test should be offered urgently out of hours as a one off or for monitoring depends on above factors and should also consider clinical symptoms, availability of critical care and whether the level will affect specific management plan e.g. to use pralidoxime or more experimental therapy like Magnesium.
Do not confuse this test with Acetyl Choline Receptor Antibodies (Acetylcholinesterase antibodies) which is performed in VIM. Clinical details for the antibody test may include Myasthenia Gravis
Test Method
Principle: Kinetic rate reaction
Reagent: In-house reagents
Analyser: Indiko Plus