A more thorough explanation:

A blood test is a medical examination of blood to determine various characteristics, including the presence of antibodies and antigens that may indicate disease. It is also used to establish paternity or to test for sexually transmitted diseases in marriage-license applicants.

A serological test is a type of blood test that specifically detects the presence of antibodies and antigens. This type of test is often used to diagnose infectious diseases, such as HIV, hepatitis, and syphilis. In some states, serological tests are required for couples applying for a marriage license to determine the presence of venereal disease.

John went to the doctor for a blood test to check for high cholesterol levels. The doctor drew a sample of John's blood and sent it to the lab for analysis. The results of the blood test showed that John's cholesterol levels were high, and the doctor recommended changes to his diet and lifestyle to lower his cholesterol.

Another example is a couple applying for a marriage license in a state that requires a serological test. They would need to have a blood test to check for sexually transmitted diseases before they can get married.

These examples illustrate how blood tests can be used to diagnose and monitor medical conditions, as well as to ensure public health and safety.