I took an HIV RNA test 19 day after I had what I thought could have been an exposure.  The test came back “not detected”.  I would like to know how accurate this test is at 19 days in a healthy middle age male.  I looked at several websites but it is a little confusing.

Welcome back but I'm sorry you found it necessary, especially since last time I said the RNA tests are "valid any time 11 days or more after exposure" and therefore your negative test at 19 days is conclusive proof you do not have HIV. As also discussed last time, you were not exposed by the event you described and therefore at no risk for HIV. As you suspected,"not detected" is a negative RNA result. You're fine.---
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Thank you for your response.  I do have a follow up question.  Is there a concern for syphilis? I was tested at 45 days and the results were negative..  however I’ve noticed in the last couple days a lot of itching and possible rashes and itching on my feet.  

No. There was no risk for syphilis. You did not need syphilis testing, and the negative result at 45 proves you do not have it. Syphilis does not cause the kind of skin symptoms you report.---

Does taking antibiotics for an abscess two weeks before affect the results of syphilis test?

Some antibiotics are active against syphilis; others have no effect at all. Most likely, the antibiotic(s) used to treat an abscess would be effective. If so, it would have cured early syphilis if you had it, preventing the blood test from ever becoming positive. If your blood test remains negative, it will prove either that you never had it; or that it was cured. Either way, a negative test result will mean you can go forward with complete confidence you do not have syphilis.---