How confidential are HIV test results?

Depends. If you opt to use an anonymous number instead of your name, then it is completely confidential (granted you go to a free clinic). If you use a family doctor, or a doctor in general where you have to fill out all the forms, chances are pretty slim. AND, if you test positive, your health insurance gets alerted and it goes in your file.

completely, any medical information is protected under doctor/patient confidentiality.

HIV tests results are the same as any test results. they are confidential. there is a Federal law called HIPPAA which prevents anyone in health care from giving your test results to another person. HIV results are treated with even more confidentiality.
if someone gives out your protected health information, you need to get an attorney immediately as that would be a large financial settlement.

Actually, there are situations in which your HIV status is not covered under confidentiality clauses. One of these is covered under the Ryan White Law. This requires the hopital or lab where the test is performed to notify any healthcare workers of your status if they have been exposed so that they can begin treatment and rapid retroviral therapy.
The Health Insurance Portability and Accountability Act (HIPAA) also requires disclosure if the patient's actions endanger the welfare of the community in general. For instance, if the local Department of Health became aware that an HIV patient was intentionally infecting other patients.
HIV as well as other disease information is required to be given to the victim of a sexual assault if the perpartrator is found and convicted.

They should not be CONFIDENTIAL AT ALL. your family should know, your spouse (boyfriend) should know and so should be the health department. Because if you don't do it, this can be hazardous not only for you, but also for the community you live in. ........(And here with the word YOU, i don't exactly mean "You", its a broad term used for EVERYBODY.)