RE: from Dr. Susan RE: PCR test and FeLV answer

[AnnLmla2]

<<<what about the western Inkblot tests and all that stuff..if you dont mind
the history lesson :)

I thought the IFA and Dr Hardy's tests were different too....? >>>


Dr. Hardy's test is an IFA test (immunofluorescent antibody) and was the
first one offered. It detects cell-associated virus. The in-clinic ELISA
tests detect soluble viral antigen. Therefore, results from these tests mean
different things and give us different information about the viral status of
the cat.

Western blot (often called immunoblot) is an antibody test and is not
usually performed for FeLV, but is used as a confirmatory test for FIV.

We need different things from screening tests versus confirmatory tests. For
a screening test, we want a method that is very sensitive. Sensitivity is
defined as the likelihood the test will be positive when the disease is
actually present. Sensitive tests produce a low number of false negatives,
but they can produce some false positives. Therefore, positive test results
should be confirmed by a second test methodology. ELISA or lateral flow
tests (immunochromatographic tests) for in-clinic use are the preferred
screening tests for FeLV.

Confirmatory tests should be highly specific. Specificity is defined as the
likelihood the test will be negative when the disease is not present. Highly
specific tests will produce a low number of false positives and are good
confirmatory tests. IFA is the preferred confirmatory test for FeLV.

It is a statistical truth that it is very hard to make a test that is both
highly sensitive and highly specific. One test usually cannot be both.
Therefore, we have different expectations of screening tests versus
confirmatory tests, which means we have to understand the limitations of
each type.

Sensitivity and specificity of a test are strongly influenced by the
prevalence of the disease in the population you are testing. Simply put, if
the disease is common in the population you are testing, a positive test
result is more likely to be a true positive. If the disease is at low
prevalence in your test population, a positive test result is more likely to
be false. When we do the statistical calculations, a positive result on a
screening test for FeLV could be wrong 2 out of 3 times in a population with
low disease prevalence. That is why we need very specific confirmatory
tests. And that is why it is crucial to understand these concepts of
testing.

Sensitivity and specificity are statistical calculations (based on
comparison to a gold-standard), and well-researched tests have independent
measures of the sensitivity and specificity published in the veterinary
literature. Unfortunately, there are no up to date evaluations in the
literature of the sens/spec for the commonly available FeLV tests, both
screening and confirmatory. There are no published evaluations of the
sens/spec of commercially available PCR tests for FeLV. Most labs doing PCR
do not evaluate the sens/spec or negative/positive predictive values (see
below) of their own tests, and this makes it very hard for veterinarians to
choose the best test for their purpose. The commercial labs are rarely
interested in encouraging independent evaluation of their tests. Given that
there is no requirement for oversight or regulation of commercial labs, we
should all be sceptical about new test methodologies and we should be
looking for independent evaluation of them before we pay for them and make
life or death decisions based on them.

The other statistical calculations that give us good information about
diagnostic tests are positive and negative predictive values. The positive
predictive value (PPV) of a test is the probability of a positive result in
a truly infected individual. The negative predictive value (NPV) is the
probability of a negative result in an uninfected individual. Crucial
information to know about a test.

We are beginning to get independent measures of the sens/spec of FIV tests,
and this is the area I have been working in. Our data has been published in
major peer-review journals in the last year, and presented at the annual
meeting of the American College of Veterinary Internal Medicine last June in
Minneapolis as well as the meeting of the International Veterinary Vaccines
and Diagnostics Conference in 2003. Research for FIV testing is crucial
right now because of the introduction of the FIV vaccine, and several
projects are ongoing. Just yesterday, I drew blood from our cats for
projects at 3 different labs.

With all due respect to the person who said it, I don't think I am out of
date.


Dr. Susan
Chapter Author, A Home Veterinary Guide, in:
The CFA Complete Cat Book
http://www.cfainc.org/catalog/books.html#completecatbook

Feline Reproduction Manual:
http://catvet.homestead.com/ReproCD.html

__^^__^^__^^__^^__^^__^^
Susan Little, DVM
Diplomate ABVP (Feline)
Bytown Cat Hospital
Ottawa, Canada
http://catvet.homestead.com
__^^__^^__^^__^^__^^__^^


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Sent: February 2, 2005 1:35 AM
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Subject: Fwd: from Dr. Susan RE: PCR test and FeLV answer