The interpretation of an antibody test result can be complex because antibody testing is used for many reasons. Depending on the antibodies tested for, antibody testing can be used for (1) diagnosis of infection, (2) identification of previous exposure to pathogens (particularly in unvaccinated animals), and (3) assessment of immunity prior to or following vaccination (Table 7). Clinicians should understand when and why to perform antibody testing and use this knowledge to make evidence-based decisions prior to vaccination.

Hemagglutination inhibition (for FPV) and serum neutralization (for FHV-1, FCV, and rabies) are the reference standards to determine the presence of effective antibody-mediated immunity. These test methodologies can only be performed in a laboratory setting using live cell cultures (i.e., they cannot be performed in a practice using rapid patient-side test kits). These diagnostic tests are predominantly research tools used in vaccine efficacy and prevalence studies.

It is important when attempting to demonstrate protective immunity in a patient using an in-clinic antibody test kit that the performance of the kit be compared against the appropriate reference standard in order to demonstrate correlationwith protective immunity.

The presence of anti-FPV antibodies correlates strongly with protection (Table 7). Currently, experts recommend antibody testing for FPV to assess immunity and inform decisions about whether to vaccinate.^6,40 Rapid in-clinic test kits to detect antibodies to FPV, FHV-1, and FCVare available to veterinarians in North America and have been validated in two different studies using the appropriate reference tests.^47,48 Of concern, however, was the occurrence of some anti-FPV antibody false-positive results in one study, which in practice would lead to some unprotected cats not being vaccinated.⁴⁸