Agency for Toxic Substances and Disease Registry
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Learning Objectives |
Upon completion of this section, you will be able to:
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Misconception | Cholinesterase levels play an important role in the emergency diagnosis and treatment of cholinesterase inhibitor toxicity. |
Reality | Emergency assessment and treatment should be based on the patient’s history, signs, and symptoms. (Wiener and Hoffman 2004) It should not be delayed to await the results of these laboratory tests, which are rarely available in time to guide emergency treatment! (Tareg et al. 2001) |
Introduction |
Circulating RBC and serum (uninhibited) cholinesterase levels are used to approximate levels in neural tissue, since the latter are impractical to obtain. (Clark 2002) |
The Two Types of Cholinesterase Levels |
There are two types of cholinesterase levels
In general, the key differences between RBC cholinesterase and serum cholinesterase are shown in the following table. |
RBC cholinesterase | Serum cholinesterase | |
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Accuracy* |
Greater** |
Less |
Availability |
Less |
Greater |
Duration of depression (Reigart and Roberts 1999) |
Several days to a few weeks |
1-3 months |
Onset of depression |
Later |
Early |
*That is, it is thought to more closely approximate cholinesterase levels in the neurosynapse. **Although one author contends that there is no evidence that this is the case for nerve agents. (Wiener and Hoffman 2004) |
Sources of Error in the Measurement of Cholinesterase Levels (Optional Reading) |
Listed below are some of the sources of error in the measurement and interpretation of cholinesterase levels.
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Case Example (Optional Reading) |
In one study of 29 farm workers who were symptomatic after exposure to pesticides, none had RBC or serum cholinesterase levels below the lower limit of normal. (Midtling, Barnett et al. 1985) |
Variations among Laboratories (Optional Reading) |
Due to differences in techniques, the absolute cholinesterase values vary from laboratory to laboratory. (Minton and Murray 1988) Laboratories may report their findings as percentages of average or normal (in unexposed subjects) rather than as absolute values. |
Levels Correlated with Toxicity (Optional Reading) |
Even with the more accurate RBC cholinesterase, the point at which various authors have suggested that toxicity begins to appear ranges from 40% to 75% of normal values. (Carlton, Simpson et al. 1998) (Romano, McDonough et al. 2001; Clark 2002) |
The Role of Baseline Cholinesterase Levels (Optional Reading) |
Some authors suggest that, when compared to an individual’s baseline value, changes in RBC cholinesterase levels correlate well with cholinesterase inhibitor toxicity. (Tareg et al. 2001; Clark 2002) However, such baseline values are rarely available except, perhaps, in occupational settings where workers are monitored for ongoing exposures. |
Other Causes of Cholinesterase Level Abnormalities |
Other conditions, besides exposure to cholinesterase inhibitors can cause abnormalities in cholinesterase levels. |
RBC Cholinesterase | Serum Cholinesterase | |
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Low levels |
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High levels |
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Key Points |
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Progress Check |