Definately, if a cat has been exposed in close contact
to one that has feline leukemia, have the test. However,
there are false positives. The test that vets can do
in their office is called the Elisa test and is can have
false positives, so any positive must be followed up
with an IFA test done in a lab. When anyone says a
cat has tested positive for Felv, make sure they mean
the IFA test.

For more info, try this link:
http://www2.dgsys.com/~ermiller/FeLV.html