Elisa (Enzyme-Linked ImmunoSorbent Assay)
The elisa test is the most common, in that it is the "quick" test that vets can carry out in-house, and therefore get results within minutes.
The elisa is sometimes called a "snap", "cite" or "combo" test.
It tests for both FIV and FeLV at the same time, although different in what is tested - antibodies for FIV and antigens for FeLV, the test is a good "general indicator", but is not always to be relied upon as it is a sensitive test which is therefore liable to give a false positive, so should be confirmed with one of the other (slower) tests.
You will see various claims to the accuracy of the Elisa test; in fact it is not a simple thing to give a percentage of accuracy - (see explanation at foot of page)
IFA (immunoFluorescent assay)
The IFA test is performed in a specialist laboratory, so your vet will need to send a blood sample away for the test, and this can take around a week to give results.
Western Blot (protein immunoblot)
The Western Blot is another test that needs your vet to send a sample of blood away
Each of these tests uses slightly different techniques, the details of which are not important to the cat owner, other than to know that a positive result from an IFA or Western Blot test is considered very reliable, assuming that potential infection took place more than 3 weeks previously, to allow time for the antibodies to develop.
PCR (Polymerase Chain Reaction)
There is another test, known as a PCR which actually tests for the dna of the virus itelf, so can be used effectively for kittens as well as adults; it can also be used at an earlier stage after infection, as it does not need to wait for the antibodies to form (which can take 3 weeks). There are not many labs that do the pcr test, so, depending on where you are, it may be difficult to arrange.
The PCR test can also be useful to test for the 'amount' of virus present (proviral load), if an FIV cat is having unexpected responses to treatment, this may help with planning future treatment.
Accuracy of Elisa testing for FIV
The Elisa (in-house) test is not 100% accurate, but it is not possible to give a simple percentage as to its actual accuracy in use - for example:
If the test itself is only slightly inaccurate, say 99% accurate; then if 100 cats are tested, there will statistically be one false positive.
Now, what this means in practice, is dependent on the prevalence of the virus in the local cat population.
Estimates vary from 1% to 14% of healthy cats in an area that might be FIV+, and up to 44% of ill cats might be FIV+
Assuming for the purpose of this example that these estimates are accurate, then in the least infected populations where there is just 1% prevalence, there will statistically be one true positive in 100 cats tested; but then there will also be the one false positive (as described above); so 2 cats will test positive in the 100 tested cats, but only one will be true - thats an accuracy of only 50%
However, if on the other hand, there are 14% infected cats, then the same test of 100 cats will show 14 true positives and one false positive - - 14 out of 15 will be accurate - thats an accuracy rate of 93%.
And if we take the quoted 44% of ill cats being positive, the same test of 100 would give 44 true positives and one false positive - 44 out of 45 is nearly 98% accurate.
So you can see that any accuracy figure is highly variable depending on circumstances.
Remember also, that the way the test is carried out (timing etc), can affect the result - so a busy surgery might potentially not give the degree of attention required, which could then lead to further false positives.
What does this mean?
Not a lot more than common sense tells us - that the test is more likely to be accurate in high FIV-risk cats, and less so in low FIV-risk cats.
This is why you should always have a confirmatory lab test done on a positive result.