*2.2. Antibody Detection*

A total of 17 datasets were received from the 17 labs (100% response) for the generic detection of WNV antibodies using commercial competition ELISAs. Sixteen labs used the recommended method (INgezim West Nile Compac ELISA) and one lab (#8) applied an alternative commercial kit (ID Screen West Nile Competition Multi-species, IDvet).

All the labs that used the Ingenasa kit reported 100% concordant results, except for one negative sample that was assigned as doubtful by lab #7. The only lab that used the IDVet kit also reported 100% correct results.

For the specific detection of WNV IgM antibodies, 19 datasets were received from 17 labs (100% response). Sixteen labs used the recommended method (INgezim WNV IgM ELISA), and two (#14 and #16) also analyzed the panel with an alternative MAC ELISA, the ID Screen West Nile IgM Capture kit from IDvet. Laboratory #8 used this kit instead of the recommended one.

The results of the IgM antibody detection using the recommended method are presented in Table 3. Correct results in the whole panel were reported by 56.2% of the labs. It is important to note that, of the 4 IgM positive sera, two (W2 and W8) had low IgM antibody titers, which might explain the di fficulties of some labs in identifying these samples. In fact, these sera displayed OD values close to the reference limits producing a higher percentage of false negative results (Table 3). The serum with high IgM antibody titer (W4) was successfully identified by all labs except one (#4). The six IgM negative sera were correctly assigned by all the labs except for one sample (W6) that was reported as IgM doubtful by lab #15.

As explained earlier, three labs used an alternative ELISA kit to analyze the panel. In all of them this kit failed to detect three out of the four IgM positive sera of the panel. Only the sample with high IgM antibody titer (W4) could be correctly identified by the IDVet IgM capture ELISA in the three labs.


Grey: incorrect result; NA: not analyzed; D: doubtful.

*Pathogens* **2020**, *9*,

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