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I assume that you mean a test for herpes simplex virus (HSV), although a similar issue is present with many of the ELISA tests.
When performing an ELISA test for HSV, there can be a small amount of cross-reactivity with other antibodies that are present in the body. Therefore, the result is an index (or ratio) between what is measured and what could be measured from cross-reactivity. When there is an index report of 0.3, that does not mean that there is a small amount of antibodies against the HSV, it means that there is a small amount of other antibodies that cross-react with the ELISA test.
In reality, a value that is 0.9 - 1.1 s considered borderline and requires a recheck after a period of time to see if the value is increasing. There also is some evidence that someone with a mildly positive result also may be a false positive from cross-reactivity, and the only way to determine this is to perform a more specific test, a western blot test.
The primary limitation with this test is the recognition that it takes some time (months) for antibodies to form, so if someone were recently exposed or concerned that they might have been recently exposed, a test that has been done early would not be meaningful. However, in your case, it does not sound like you had a recent worrisome exposure, so this should not be an issue.
Therefore, in your situation, a value of 0.3 is clearly negative and is due to cross-reactivity, and there is no reason for you to be concerned about the possibility of infection based on this lab result.
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