Retrospective antibody testing should be viewed carefully, given the relatively low specificity (i.e., high rate of false positives).
If there’s a high clinical suspicion (could be relevant in this case, though it may also represent the normal flu), the utility of the testing improves.
Ah, I'd be quite interested seeing your 98+% specificity antibody!
Looking at the three out of four FDA EUA-approved serology tests (see https://www.fda.gov/medical-devices/emergency-situations-med...) most do not report sensitivities up to 98%, with the notable exception of Ortho Vitro, though the diversity of the samples they test isn't described.
If there’s a high clinical suspicion (could be relevant in this case, though it may also represent the normal flu), the utility of the testing improves.