title: Repeatability and validity of a field kit for estimation of cholinesterase in whole blood.
abstract: OBJECTIVE
  To evaluate a spectrophotometric field kit (Test-Mate-OP) for repeatability and validity in comparison with reference laboratory methods and to model its anticipated sensitivity and specificity based on these findings.
  
  
  METHODS
  76 farm workers between the age of 20 and 55, of whom 30 were pesticide applicators exposed to a range of organophosphates in the preceding 10 days, had blood taken for plasma cholinesterase (PCE) and erythrocyte cholinesterase (ECE) measurement by field kit or laboratory methods. Paired blinded duplicate samples were taken from subgroups in the sample to assess repeatability of laboratory and field kit methods. Field kits were also used to test venous blood in one subgroup. The variance obtained for the field kit tests was then applied to two hypothetical scenarios that used published action guidelines to model the kit's sensitivity and specificity.
  
  
  RESULTS
  Repeatability for PCE was much poorer and for ECE slightly poorer than that of laboratory measures. A substantial upward bias for field kit ECE relative to laboratory measurements was found. Sensitivity of the kit to a 40% drop in PCE was 67%, whereas that for ECE was 89%. Specificity of the kit with no change in mean of the population was 100% for ECE and 91% for PCE.
  
  
  CONCLUSIONS
  Field kit ECE estimation seems to be sufficiently repeatable for surveillance activities, whereas PCE does not. Repeatability of both tests seems to be too low for use in epidemiological dose-response investigations. Further research is indicated to characterise the upward bias in ECE estimation on the kit.
authors.affiliation: Department of Community Health, University of Cape Town, Observatory, South Africa.
authors.forename: L
authors.initials: L
authors.lastname: London
authors.forename: M L
authors.initials: ML
authors.lastname: Thompson
authors.forename: S
authors.initials: S
authors.lastname: Sacks
authors.forename: B
authors.initials: B
authors.lastname: Fuller
authors.forename: O M
authors.initials: OM
authors.lastname: Bachmann
authors.forename: J E
authors.initials: JE
authors.lastname: Myers
mesh_terms: D000328:Adult; D000382:Agricultural Workers' Diseases / Q000175:diagnosis*; D002802:Cholinesterases / Q000097:blood*; D004796:Clinical Enzyme Tests / Q000295:instrumentation*; D004912:Erythrocytes / Q000737:chemistry; D006801:Humans; D007306:Insecticides; D008875:Middle Aged; D016273:Occupational Exposure; D009943:Organophosphorus Compounds; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013053:Spectrophotometry / Q000592:standards
chemical_list: D007306:Insecticides; D009943:Organophosphorus Compounds; D002802:Cholinesterases