In blood, what you are checking for is viral antibodies not HIV RNA. Your body produces antibodies to antigens (e.g. HIV proteins) to combat foreign bodies.

The first test for HIV is called ELISA. This is a sensitive test that will be positive for many people that are not infected. The reason for this is to capture everybody that could POSSIBLY be infected. This is done to rule out those that do not have HIV.

The next test that follows ELISA is a Western Blot test. This is a specific test for HIV viral proteins. This test is to rule in those that do have HIV.

If you have an acute infection of HIV, you may not have the antibodies that are produced for the tests above. It takes approximately 2 months for antibodies to show up in your blood. In this case you can do a viral load count using PCR.