What does it mean? Is it really FIP?
Elevated titer levels in and by themselves mean almost nothing in regards to FIP. All the levels indicate is SOMETHING is going on. It may be a current vaccination. It may mean exposure to the corona virus. Without further examination and testing and research, the elevated levels are not a death sentence. They may be a great cause for concern, though, and not to be taken lightly.
Where do you find these levels? Depending who did the testing, at the bottom of the blood test you have had done by now will possibly be FCV-IFA 1:400 POSITIVE, and FCV-IFA 1:1600 POSITIVE. These are the levels we will be discussing here.
A titer is a measurement of the dilution level at which antibodies to a disease are still detectable during testing. The titer values are used to determine the likelihood of a disease. Titers for the same disease can vary from laboratory to laboratory depending on how they run the test. Titers for different diseases are not comparable at all -- a titer of 1:20 is significant in some diseases while a titer of 1:6400 may be "normal" for another disease. When interpreting titers it is very important to know what the testing lab considers the normal levels to be and to consider factors about the disease that may influence the titer.
For feline infectious peritonitis (FIP) there are several factors to consider when looking at titer levels. In most labs, titer levels of 1:1600 or less are considered to be normal (or below the range where active infection is likely based on serology alone). It is also important to consider the number of cats living in the household with a cat you are testing for FIP. In multi-cat households the "background" titer level is likely to be higher in a cat for FIP than it is is a one or two cat household, since exposure to coronavirus (the type of virus that causes FIP and other enteric infections in cats) is going to be more likely. So a cat with a titer of 1:6400 in a household with many cats is less likely to actually have FIP than a cat living alone with a titer this high. In catteries it is possible that 1:6400 should be considered the "suspicious" level for titers. The last thing you have to consider is how sick the cat is. In a very sick cat with a lot of signs of FIP (dehydration, fever, weight loss, lethargy, enlarged abdomen, etc.) a low titer level has to be taken more seriously than in a cat that isn't sick. It can be a difficult interpretation in this situation but that is what you pay your vet for, so make him or her tell you what it means to them. It can be useful to check the titer again in two or three weeks in these cases. A rising titer would be meaningful and a falling titer in the face of the illness worsening may be indicate a very bad short term prognosis.
Titer levels above 1:3200 are usually considered to be sufficient to suspect FIP so you do have to take the titer seriously but this test is inaccurate enough that you should almost never base a diagnosis on titer alone. Interpreting the result in light of your cat's individual situation is very important. Again, this is something you should talk to your vet about and your vet should take the time to tell you what the result means for your individual cat.
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