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Herpes blood tests are not entirely reliable, and it's possible for a person to have herpes (either 1 or 2), but not to test positive. There are many uncertainty in testing for herpes, and the results are often difficult to interpret, which often leads to false results. This is because the blood tests don't actually look for the virus itself, but for antibodies to this virus. It can take several months to develop enough antibodies for a positive herpes blood test after being infected, and even then, without frequent infections, the level of antibodies tends to decrease significantly over time. False positives are also possible on these tests. Thus, a person without herpes can sometimes test positive with the blood tests, and people with herpes can test negative. In fact, as many as 20% of these tests are wrong. Also, the blood tests cannot distinguish between genital and oral herpes (although oral forms of type 2 is fairly rare), or whether a current lesion is due to herpes. The best way to determine the true results, in the absence of an outbreak, is to have multiple tests. If lesions are present, the best way to see if it's herpes-related is through a swab of the actual fluids.
On the other hand, the cultures are fairly accurate. The lesion you have may very well not be related to herpes, but to something else, such as a fungal or bacterial infection, or an allergic reaction or contact dermatitis. This may be the case even if you do have herpes simplex virus. A bacterial/fungal culture of the area is needed to determine the cause, and treatment depends on this cause, but might include cortisone creams, antibiotics, or antifungals, accordingly.
I hope this helps!
Do you have any specific questions I can help you with? More tests are necessary because herpes blood tests are not always accurate. With the blood tests for heres, there's about a 5% chance of a false positive, meaning that the blood test says a person has herpes when he really doesn't, and there's a 15% chance of false negatives (where the test indicates that a person doesn't have herpes when, in fact, they do).
The most accurate way to test for herpes is with viral cultures from the actual fluid in the sores. However, these can give false negatives on some occasions too, especially if the sore was in the process of healing, or it's not the first time you've had symptoms. Your swab test was either negative as a result of a false negative (meaning you DO have herpes, but the test was wrong), or because the lesion was caused by something else entirely.
Most likely, you do have herpes, either type 1 or 2, or both. But because of the high risk of false results, you need additional tests to determine whether it's from type 1 or 2 herpes. If you're asking WHY this distinction is necessary, it's really not when it affects the genitals. Treatment is not different for the two different types, but does indicate the type of severity and frequency of outbreaks you can expect, and gives clues about how you contracted the disease (type 2 is usually more severe and frequent). Also, a positive result for type one herpes does not mean the person has genital herpes, as about 80% of the population has oral herpes which does not affect the genitals at all.
Concerning your partner, he may or may not have herpes... it's impossible to tell accurately with a blood test alone. A negative blood test is reassuring, but again, there's about a 15% chance of a false negative due to the length of time it takes for antibodies to develop, or a decrease in antibody level in the absence of outbreaks. Some people have the virus and never have symptoms at all, in fact.
So again, the only way to clarify the cause of your lesion, and if herpes, whether it's from type 1 or 2 is to use more serological tests. You can read more about the different tests here: http://www.webmd.com/genital-herpes/Herpes-Tests and http://www.webmd.com/genital-herpes/guide/genital-herpes-diagnosis
And more about herpes testing in general here (including false test results): http://www.stdservices.on.net/std/herpes/serology.htm
Please let me know if I can clarify things for you further.