Serology and Diagnostics

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


Possible Uses of In-Clinic Serology Testing

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PATHOGEN Usefulness of Antibody Testing
FPV Useful for assessment of immunity because presence of antibodies correlates strongly with protection.6,40 Result can be used to decide whether to vaccinate (i.e., only vaccinate antibody-negative cats)
FHV-1 Not reliable for assessment of immunity.1,40 Effective immunity against FHV-1 requires both an antibody and cell-mediated immune response. Result should not be used to decide whether to vaccinate
FCV Not reliable for assessment of immunity.1,40 Effective immunity against FCV requires both an antibody and cell-mediated immune response. Result should not be used to decide whether to vaccinate
FeLV Useful for assessment of exposure and/or diagnosis of infection (in combination with other testing methodologies). Recently a rapid in-clinic test kit to detect antibodies to FeLV transmembrane protein (p15E) was released in Europe. A positive p15E antibody result cannot differentiate between exposure and infection. FeLV-vaccinated cats usually have low levels of antibodies to p15E.41 Results from FeLV antigen testing (and not antibody testing) should be used to decide whether to vaccinate. Rapid in-clinic FeLV test kits detect soluble p27 antigen in whole blood, serum, or plasma and are not affected by FeLV vaccination. The AAFP recommends testing all cats for FeLV p27 antigen prior to initial vaccination. There is no proven benefit to vaccinating infected cats14,34,42
FIV Useful for diagnosis of infection.34 Between 2002 and 2015, an inactivated whole-virus vaccine was available in North America that interferes with antibody results using some test kits.43,44 FIV-vaccinated cats may test antibody positive for more than 7 years after the last vaccination.45 Additionally, cats may travel from locations where the vaccine is still in use to the USA, Canada, and other countries where the vaccine is not available. Rapid in-clinic test kits able to differentiate between FIV-infected and FIV-vaccinated cats are available46
Rabies Vaccination against rabies is essential in regions where it is required by statute/law or where the virus is endemic and should follow label recommendations. Serum neutralization results cannot be used to decide whether to vaccinate against rabies


Boehringer Ingelheim Animal Health USA Inc., Elanco Animal Health, Merck Animal Health, and Zoetis Petcare supported the development of the
2020 AAHA/AAFP Feline Vaccination Guidelines and resources through an educational grant to AAHA.