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Assessment of test results when using a commercial enzyme-linked immunosorbent assay for diagnosis of paratuberculosis in repeated samples collected from adult dairy cattle

Heather L. Hirst DVM, MS1, Franklyn B. Garry DVM, MS, DACVIM2, and M. D. Salman BVMS, PhD, DACVPM3,4
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  • 1 Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523.
  • | 2 Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523.
  • | 3 Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523.
  • | 4 Department of Environmental Health, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523.

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.

Design—Cross-sectional observational study.

Animals—3,757 adult dairy cattle.

Procedure—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 positivecontrol serum [S/P] ≥ 0.25), 62 (39.5%) had negative results for the second sample. Of 71 cattle with an initial S/P value ≥ 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 ≥ 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 and Clinical Relevance—Many cows changed ELISA status between samples collected at variable intervals. Cows with an initial high S/P value (≥ 0.70) were more likely to maintain positive status than cows classified as positive on the basis of cutoff values of ≥ 0.25 or ≥ 0.40. Veterinarians should expect variability in ELISA results when repeated testing of cattle is used as part of an MAP control program. (J Am Vet Med Assoc 2002;220:1685–1689)

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.

Design—Cross-sectional observational study.

Animals—3,757 adult dairy cattle.

Procedure—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 positivecontrol serum [S/P] ≥ 0.25), 62 (39.5%) had negative results for the second sample. Of 71 cattle with an initial S/P value ≥ 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 ≥ 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 and Clinical Relevance—Many cows changed ELISA status between samples collected at variable intervals. Cows with an initial high S/P value (≥ 0.70) were more likely to maintain positive status than cows classified as positive on the basis of cutoff values of ≥ 0.25 or ≥ 0.40. Veterinarians should expect variability in ELISA results when repeated testing of cattle is used as part of an MAP control program. (J Am Vet Med Assoc 2002;220:1685–1689)