iDaleo,
If you are specifically talking about a PCR DNA (Qualitative
analysis), I would say you are right. A PCR DNA would be very indicative on 28th day post day exposure. However, there is another type of NAT test which is quantitative analysis in nature, PCR RNA. This test is hypersenstitive and can detect in dire conditions ike 5 copies / 100 ml of blood. This test has been lately approved by the FDA as a diognostic test. However, it is not a stand alone test.