Since STD screenings are done with different methods, the way they work will also vary. If you have a swab test done, the swab will be tested for any bacteria, cells, or the present discharge to determine the cause. If you have blood testing done for an STD or STI test, they are looking for what is known as antigens present in the blood. These antigens only form when the body has contracted something and can be viewed similarly to other testing that involves the use of a swab. A blood test can be used to confirm certain STDs or STIs, like syphilis, and can be done at the same time as swab testing.