Interpreting Feline Immunodeficiency Virus (FIV) Tests

Smoky came to see us this morning. He has been hanging around since just after Hurricane Katrina, but it took four-plus years to get him sufficiently domesticated to bring him to the clinic.

Georgia, his new owner, had been under the impression that “he” was a “she” and that he had never seen a doctor before. When we determined that Smoky had been neutered it was apparent that he had, indeed, been to a doctor, but there was no way to know what else had been done.

So, we started from scratch, testing a stool sample for intestinal parasites and a blood sample for Feline Leukemia Virus (FeLV) and Feline Immunodeficiency Virus (FIV).

The stool test was negative, as was the FeLV test, but the FIV test was positive. Which leaves us sifting through the possible interpretations below and still not knowing everything we would like to know.


  • The FIV test is an antibody test, so it doesn’t check for the actual virus in the body, as the FeLV test does. Antibodies are proteins that the immune system produces in response to disease. For example, if you are bitten by a tick carrying Lyme Disease today, in a few weeks your blood will test positive for antibodies to the Lyme bacteria, Borrelia burgdorferi. So, when a cat is bitten by another cat and becomes infected with FIV, the immune system begins an always-futile attempt to rid the body of the virus and, in the process, creates antibodies. While a positive antibody test doesn’t actually demonstrate the presence of the virus in the body, elimination of infection is virtually unheard-of, so the correlation is, practically speaking, 100%.
  • A wrench gets thrown directly into the main gears of the works when we are presented with a cat of unknown vaccination history. Vaccination with the FIV vaccine converts a kitty to positive-test status, whether or not he is infected with the disease. In the case of a cat that stays in one’s practice, it’s easy to look at the medical record and know whether a cat has received the vaccine. In cases like Smoky’s, we have no way of knowing whether he has ever been vaccinated against FIV. Similarly, if a cat is vaccinated with the FIV vaccine, is bitten by an FIV-infected cat and the vaccine fails to protect him, his test result will still be positive, but we have no way to know that his infection status has changed. At this writing there is no test, at any price, that will differentiate vaccine antibodies from natural disease antibodies.

What does the future hold for Smoky? We have no way of determining that, either.

  • If his test is positive from a previous vaccination and he is free of Feline Immunodeficiency Virus, he should live as long as any other cat.
  • During the 4½ years that Georgia has been feeding Smoky he has not grown, indicating he was already an adult then. However, he has gained weight and thrived, so we know he hasn’t been clinically ill prior to today. Which leaves us asking, “If vaccine did not turn Smoky’s test positive, was he infected this month, last month, last year, two, or even three years ago?” We know that cats will eventually exhibit evidence of a damaged immune system, developing infections, cancers, and/or other maladies the immune system should protect them from. Not knowing when Smoky was infected leaves us also not knowing when he may begin to fail.

We are taking the approach that Smoky is infected. That is predicated on the results of a mandatory followup test in two months. We never fully interpret a positive FeLV or FIV test based on a single positive or negative. It also means we will be monitoring Smoky for signs of illness more closely than we would other six-year-old cats. Perhaps most importantly, by treating Smoky as an infected FIV-positive cat, we are making him a totally-indoor cat, doing our best to reduce the risk of transmitting FIV to other cats in the neighborhood.

We hope this helps you to better interpret FIV test results in your own kitty.

NOTE:  There are many parameters to consider in the interpretation of the FIV test, so don’t be intimidated.  If you have to read the post multiple times, that’s completely understandable.

See you Monday, Dr. Randolph.

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