title: Assessment of test results when using a commercial enzyme-linked immunosorbent assay for diagnosis of paratuberculosis in repeated samples collected from adult dairy cattle.
abstract: OBJECTIVE
  To determine the proportion of adult cattle that change test status when an ELISA for antibodies against Mycobacterium avium subsp paratuberculosis (MAP) is used to assay samples collected twice at variable intervals and to determine whether cows with an initial strong positive result were more likely to maintain positive status, compared with all cows with an initial positive result.
  
  
  METHODS
  Cross-sectional observational study.
  
  
  METHODS
  3,757 adult dairy cattle.
  
  
  METHODS
  Serum samples were obtained twice from cattle at intervals ranging from 77 to 600 days between collections. Samples were tested with an ELISA for detection of antibodies to MAP.
  
  
  RESULTS
  Of 157 cattle with initial positive results (value for the sample divided by the value for positive-control serum [S/P] > or = 0.25), 62 (39.5%) had negative results for the second sample. Of 71 cattle with an initial S/P value > or = 0.40, 13 (18.3%) had a negative result (S/P < 0.25) for the second sample. Of 33 cattle with an initial S/P > or = 0.70, 3 (9.1%) had a negative result (S/P value < 0.25) for the second sample. Interval between collection of samples did not affect results.
  
  
  CONCLUSIONS
  Many cows changed ELISA status between samples collected at variable intervals. Cows with an initial high S/P value (> or = 0.70) were more likely to maintain positive status than cows classified as positive on the basis of cutoff values of > or = 0.25 or > or = 0.40. Veterinarians should expect variability in ELISA results when repeated testing of cattle is used as part of an MAP control program.
authors.affiliation: Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins 80523, USA.
authors.forename: Heather L
authors.initials: HL
authors.lastname: Hirst
authors.forename: Franklyn B
authors.initials: FB
authors.lastname: Garry
authors.forename: M D
authors.initials: MD
authors.lastname: Salman
mesh_terms: D000818:Animals; D000907:Antibodies, Bacterial / Q000097:blood*; D002417:Cattle; D002418:Cattle Diseases / Q000175:diagnosis* / Q000276:immunology / Q000382:microbiology; D003430:Cross-Sectional Studies; D004797:Enzyme-Linked Immunosorbent Assay / Q000379:methods / Q000662:veterinary; D005189:False Positive Reactions; D005260:Female; D016927:Mycobacterium avium subsp. paratuberculosis / Q000276:immunology*; D010283:Paratuberculosis / Q000175:diagnosis* / Q000276:immunology
chemical_list: D000907:Antibodies, Bacterial