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"
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)
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)
is another test, known as a PCR which actually tests for
the dna of the virus itself, 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
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 - that's 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 - that's 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
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.