Good question. Laboratory tests can be quantitative or qualitative. Qualitative tests give you a yes or no answer to whether something (like HIV) is present of not. Quantitative tests for HIV tell you how much virus is present. We use quantitative tests in caring for patients with HIV to judge how therapy is doing (it should reduce the amount of HIV virus present in blood), as well as, on some occasions, to monitor the progression of disease. For figuring out if a person has HIV or not, qualitative tests are the way to go. Just as you cannot be a little bit pregnant, you cannot be a little bit infected. Either you are or are not.
Hope this helps to clarify things. EWH