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Comparison of different diagnostic tools for the detection of Anaplasma phagocytophilum in dogs.

BACKGROUND
Anaplasma phagocytophilum is common in dogs, but the best way to diagnose an infection is still not determined. Antibody detection assays are frequently used in veterinary practice. Additionally, polymerase chain reaction (PCR) is available for detection of A phagocytophilum DNA. It is still unknown, how well different diagnostic methods correlate.


OBJECTIVES
The aim of the study was to compare 2 antibody detection assays, an immunofluorescence assay (IFA) and the ELISA SNAP4Dx, and to determine the correlation of these assays by evaluating the sensitivity and specificity compared with PCR as a direct detection method of the organism.


METHODS
Sera of 200 prospectively included dogs were tested for antibodies to A phagocytophilum using IFA and SNAP4Dx. DNA of the organism was detected by PCR on whole blood. Sensitivity, specificity, and predictive values, including their 95% confidence intervals, were calculated for IFA and SNAP4Dx in relation to PCR.


RESULTS
Four of 200 animals were PCR-positive. Sensitivity of IFA and SNAP4Dx was 100% (95% CI 51.01-100). Specificity of IFA was 52.9% (95% CI 50.42-64.17) and that of SNAP4Dx, 57.4% (95% CI 45.83-59.70). Agreement of the 2 antibody tests was fair (κ 0.334).


CONCLUSIONS
Immunofluorescence assay and SNAP4Dx were very sensitive and therefore can be useful as screening tests for A phagocytophilum infection. However, the specificity was low, and agreement between both antibody tests was insufficient. This could be due to either false-positive antibody test results, or false-negative PCR results in dogs that were actually infected.