A handful of states still require blood tests for couples planning to marry. Most do not. (For information on which states require them, see Chart: State Marriage License and Blood Test Requirements.
Premarital blood tests check both partners for venereal disease or rubella (measles). The tests may also disclose the presence of genetic disorders such as sickle-cell anemia or Tay-Sachs disease. You will not be tested for HIV, but in some states, the person who tests you will provide you with information about HIV and AIDS. In most states, blood tests can be waived for people over 50 and for other reasons, including pregnancy or sterility.
If either partner tests positive for a venereal disease, what happens depends on the state where you are marrying. Some states may refuse to issue you a marriage license. Other states may allow you to marry as long as you both know that the disease is present.