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Feline coronavirus antibody tests

Addie DD, Le Poder S, et al. Utility of feline corona antibody tests. J Feline Med Surg. 2015 Feb; 17(2):152-162.

Feline coronavirus (FCoV) antibody tests have been available for almost 40 years. Their clinical application has been in question in recent years. This study looked at several reasons for testing cats for antibodies to FCoV and evaluated 8 different tests that may be used for that purpose. The conclusions offer ideas on how each might be more appropriately used in a clinical setting.

Currently, commercially available FCoV antibody tests are categorized in three ways: indirect immunofluorescent antibody tests (IFAT) using cells infected with FCoV or the related porcine transmissible gastroenteritis virus (TGEV) as the antigen; enzyme-linked immunosorbent assays (ELISA); or rapid immunochromatographic (RIM) tests. Both the ELISA and RIM test formats are for in-house use. A fourth method, immunoblotting, is only available through specialized labs.

When choosing which of the above tests for use, the decision should include a number of factors, especially the reason for determining the FCoV status of a particular patient. In house tests are usually preferred when a fast result is desired (eg, for eliminating FIP as a diagnosis for a sick cat, or for screening a breeding queen immediately before mating). A test offering antibody titers is preferred when sequential tests are required (eg, to determine if a cat is no longer infected).

One of the major misunderstandings related to FCoV antibody tests is that many clinicians consider the test to be a test for FIP, especially since many tests are labeled as FIP tests versus FCoV tests. A FCoV test should be used more for consideration of elimination of FIP as a diagnosis than for other reasons. One fact for additional attention is that a large amount of virus in a sample can reduce, or even, block antibody production.

The authors list what they consider 5 desirable qualities for a FCoV test: high sensitivity; high specificity; able to test a small quantity of sample; ability to use effusion to test along with whole blood, serum, or plasma; and the sensitivity of the test in the presence of virus. There are two other qualities previously mentioned to consider: speed of the test results and determination of an antibody titer.

Of the 8 tests evaluated, 4 were IFATs, 1 was ELISA, and 3 were RIM formats. The specificity was 100% for all the tests except for the two IFATs based on TGEV. IFATs and the ELISA test were best in obtaining an antibody titer and working in the presence of virus. RIM tests were the fastest format in obtaining results; the sensitivities though ranged from 92.4% to 64.1% among the 3 tests evaluated. Sensitivity was 100% for the ELISA test, one FCoV IFAT, and one TGEV IFAT; the sensitivity was slight lower for the other three IFATs. All tests worked with effusions, even when blood products were specified for use.

Due to the fact that large amounts of virus can affect all of the types of FCoV antibody tests, creating false-negative results or a reduced antibody titer, the recommendation was to screen samples for viral RNA when samples are negative. It was noted that 43% of effusions of cats with FIP were negative by RT-PCR and the specific reason is not known though the authors did speculate on a possible cause.

Potentially, both FCoV RT-PCR and serology in the diagnosis of FIP could be used together or as sequential tests: a negative antibody test (if a highly sensitive test format) allows FIP to be ruled out of a differential list for an effusion, and a negative RT-PCR test would not rule out FIP. As has been noted in the past, positive serology is not diagnostic of FIP. But, large amounts of virus, detected by RT-PCR would indicate FIP is highly likely. These would be additional tools in building toward a diagnosis of FIP.

In conclusion, it was recommended to be flexible in selecting the type of test in this situation most appropriate for the reason for testing and not selecting one test and using just that one type of test. The most sensitive in-house test was the FCoV Immunocomb; the best RIM test was the Speed F-Corona. The FCoV Immunocomb was the best test overall in exhibiting the desirable qualities mentioned by the authors. The hope is that this information will reduce the misunderstanding surrounding FCoV test results, often attributable to the mislabeling of FCoV tests as FIP tests. Unfortunately, the FDA determined that the FCoV Immunocomb test be labeled FIP Immunocomb in the United States. (VT)

See also:
Kipar A, Meli ML. Feline infectious peritonitis: still an enigma? Vet Pathol. 2014 Mar, 51(2):505-526.