A good explanation of discordant test results
From The University of Glasgow: Companion Animal Diagnostics
(http://www.gla.ac.uk/companion/felv.htm#felvdiscordant)
"FeLV discordant test results
In veterinary practice, screening tests for FeLV use either an enzyme-linked immunosorbent assay (ELISA) or a rapid immunomigration (RIM) technology, both of which detect FeLV capsid (p27) antigen in the plasma. Most responsible professional authorities recommend that when a cat is found to be FeLV-positive by either of these methods, the diagnosis should be confirmed by a second, different type of test. This is particularly important for healthy cats that are found to be antigen-positive. In our laboratory we use immunofluorescence or virus isolation as confirmatory tests, which demonstrate the presence of infectious virus, not just antigen, in the sample.In most cases FeLV diagnosis is straightforward, particularly in sick cats. However, in a proportion of samples, the screening test will give a positive result for FeLV antigen but the result of the confirmatory test for virus will be negative.
Why should we find a blood sample to be antigen-positive but virus-negative? A crucial fact is that the antigen that is found in the plasma is not obtained from the virus particles themselves, but from free antigen that is secreted by FeLV-infected cells. There are three possible reasons.
The first reason is that there may be a technical problem with the in-practice test, or the laboratory ELISA. This might be overcome by repeating the test. However, the result may be a false positive. In-practice tests have a specificity of around 98-99%. While this is high, in a situation where the prevalence of FeLV is only 1%, more than half of the positive results obtained with these tests will be false.
A second reason for this state may be that the cat is in the early stages of infection, either at the beginning of an infection or in the course of recovery. In either of these phases there may be sufficient antigen in the blood to be detected but insufficient virus. In this case the discordant state is transient and on re-testing 4-12 weeks later, the cat will usually be found to be negative by both tests.
A third reason may be that following apparent recovery from FeLV infection, the virus is sequestered in a particular tissue in the body from which only antigen, but not virus, is released into the blood. A well-documented example of this situation was the localisation of virus to the mammary gland in a lactating queen. In this case, a cat may remain in a discordant state for many months or even years. The danger in this situation is that at any time the virus may be reactivated and the cat may become viraemic, so that it excretes virus and has a high risk of subsequently developing a serious FeLV-related disease.
Recommendations·
Cats found to be positive for FeLV antigen in a screening test should be re-tested by a confirmatory test, such as immunofluorescence or virus isolation in our laboratory.· As a precaution, the discordant cat should be kept in isolation from other cats and a further blood sample should be taken 4 weeks after the initial sample and sent to us for testing. This, and further samples are tested free of charge.· If this second sample is found to be negative for both antigen and virus, the cat is likely to be virus-free. To confirm the result, a third sample should be taken after a further period of 8 weeks.· If the second sample is found still to be discordant, a sample should be tested after a further 8 weeks to establish whether this is likely to be a permanent state.
Our research shows that more than half of cats that are found to be discordant on a first test become negative for antigen, and remain negative for virus, after 12 weeks."
(http://www.gla.ac.uk/companion/felv.htm#felvdiscordant)
"FeLV discordant test results
In veterinary practice, screening tests for FeLV use either an enzyme-linked immunosorbent assay (ELISA) or a rapid immunomigration (RIM) technology, both of which detect FeLV capsid (p27) antigen in the plasma. Most responsible professional authorities recommend that when a cat is found to be FeLV-positive by either of these methods, the diagnosis should be confirmed by a second, different type of test. This is particularly important for healthy cats that are found to be antigen-positive. In our laboratory we use immunofluorescence or virus isolation as confirmatory tests, which demonstrate the presence of infectious virus, not just antigen, in the sample.In most cases FeLV diagnosis is straightforward, particularly in sick cats. However, in a proportion of samples, the screening test will give a positive result for FeLV antigen but the result of the confirmatory test for virus will be negative.
Why should we find a blood sample to be antigen-positive but virus-negative? A crucial fact is that the antigen that is found in the plasma is not obtained from the virus particles themselves, but from free antigen that is secreted by FeLV-infected cells. There are three possible reasons.
The first reason is that there may be a technical problem with the in-practice test, or the laboratory ELISA. This might be overcome by repeating the test. However, the result may be a false positive. In-practice tests have a specificity of around 98-99%. While this is high, in a situation where the prevalence of FeLV is only 1%, more than half of the positive results obtained with these tests will be false.
A second reason for this state may be that the cat is in the early stages of infection, either at the beginning of an infection or in the course of recovery. In either of these phases there may be sufficient antigen in the blood to be detected but insufficient virus. In this case the discordant state is transient and on re-testing 4-12 weeks later, the cat will usually be found to be negative by both tests.
A third reason may be that following apparent recovery from FeLV infection, the virus is sequestered in a particular tissue in the body from which only antigen, but not virus, is released into the blood. A well-documented example of this situation was the localisation of virus to the mammary gland in a lactating queen. In this case, a cat may remain in a discordant state for many months or even years. The danger in this situation is that at any time the virus may be reactivated and the cat may become viraemic, so that it excretes virus and has a high risk of subsequently developing a serious FeLV-related disease.
Recommendations·
Cats found to be positive for FeLV antigen in a screening test should be re-tested by a confirmatory test, such as immunofluorescence or virus isolation in our laboratory.· As a precaution, the discordant cat should be kept in isolation from other cats and a further blood sample should be taken 4 weeks after the initial sample and sent to us for testing. This, and further samples are tested free of charge.· If this second sample is found to be negative for both antigen and virus, the cat is likely to be virus-free. To confirm the result, a third sample should be taken after a further period of 8 weeks.· If the second sample is found still to be discordant, a sample should be tested after a further 8 weeks to establish whether this is likely to be a permanent state.
Our research shows that more than half of cats that are found to be discordant on a first test become negative for antigen, and remain negative for virus, after 12 weeks."
0 Comments:
Post a Comment
<< Home