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Cholinesterase Biomonitoring Program



Biological monitoring of employees should be performed in occupations where employees may be exposed to toxic substances which may have an adverse health effect. Exposure to organophosphate (OP) chemicals may result in the inhibition of acetyl-cholinesterase, an enzyme vital to maintaining effective nerve and muscle functions. Periodic cholinesterase testing is therefore useful in assessing the extent of exposure before the occurrence of clinical illness. Although thiocarbamate compounds may also inhibit the acetyl-cholinesterase enzyme, any inhibition which occurs tends to be short in duration since there is a rapid reactivation of the enzyme. Thus, while it is important to recognize that thiocarbamates may inhibit cholinesterase and to be alert for symptoms of illness, periodic monitoring of cholinesterase levels among individuals working safely with thiocarbamates is not recommended as part of occupational health biomonitoring.

Criteria for Participating in Biomonitoring

All employees having a potential for significant exposure to toxic or highly toxic cholinesterase inhibiting OP should participate in periodic monitoring of cholinesterase levels. Cholinesterase inhibiting OP compounds should be considered toxic or highly toxic if the oral LD50 is less than 500/kg or the dermal LD50 is less than 1000 mg/kg.

Frequency of Monitoring

If an employee has worked all or part of 7 days in the prior calendar month with a potential for significant exposure to toxic or highly toxic OP compounds, he or she should be tested monthly. If three or more consecutive monthly tests show no evidence of cholinesterase inhibition, the physician may elect to increase the interval between tests for the employee.


  • Blood samples should be obtained by venipuncture only.
  • Both RBC and plasma cholinesterase levels should be determined by the methodology described in the cholinesterase determination method defined in this protocol.
  • Employees should be informed of the test results.

Baseline Cholinesterase

All employees in the biomonitoring program must have both a baseline RBC and plasma cholinesterase values established against which subsequent values are to be compared. Such values may be classified as pre-exposure or working baselines, depending on whether or not the individual has been exposed to any cholinesterase inhibiting compounds in the 30 days prior to testing. Baselines should also be established for employees not participating in the biomonitoring program, who have potential for significant exposure in the event of an accidental spill or other release. Pre-exposure baseline: Defined as the mean of two RBC and plasma cholinesterase activity determinations taken at least 3 days, but preferably not more than 14 days apart, after a period of at least 30 days without any known exposure to cholinesterase inhibiting compounds. If the difference between the two determinations exceeds 15%, a third should be performed. The pre-exposure baseline RBC or plasma cholinesterase is the mean of the two closest cholinesterase determinations. Working baseline: Defined as the mean of two cholinesterase activity determinations, taken at least 3 days apart. These two successive samples should differ by no more than 15% from each other or from the mean of normal values for any appropriate control population of non-exposed individuals. Should the laboratory performing the determinations change, or if the laboratory changes testing methods or reagents, new baseline values must be determined.
Baselines should also be established for employees not participating in the biomonitoring program, who have a potential for significant exposure in the event of an accidental spill or other release.

Criteria for exclusion from cholinesterase inhibitors

The finding of a reduction of activity of 30% or more of either RBC or plasma cholinesterase from baseline is evidence that the employee should be promptly removed from any exposure to cholinesterase inhibitors including thiocarbamates. At this time, another blood sample should be obtained for confirmation of the test result suggesting exposure. The employee shall be interviewed by the supervisor to determine the cause and extent of exposure. In addition, if possible, the work site should be inspected, the operation reviewed, sources of possible exposure evaluated, and further exposure of personnel prevented.

Criteria for return to work

When an employee has been removed from the exposure because his/her cholinesterase activity has fallen below the recommended limits, appropriate follow-up cholinesterase tests should be repeated after 7 days and, if necessary, 7 days thereafter. The employee should not be allowed to return to his/her job until the cholinesterase activity has returned to 90% of the pre-exposure or working baseline level and the return to work is approved by the physician involved in the monitoring.

Cholinesterase determination method

It is highly desirable that cholinesterase determinations be done by a single laboratory to allow for unbiased comparisons and quality control of the results . For this reason, Student Health Service, should be contacted at 919-513-0277, before any testing is done. The laboratory selected should be using the modified Ellman or Michel method for determining RBC and plasma cholinesterase.