Medical Questions » Pathology Questions » Question No. 780
| Question: | How can blood tests determine if you had a particular infection years ago? I am pregnant and my doctor said after doing tests that I had German measles in the past, but I cannot remember this. |
| Answer: | If you get an infection, or a vaccination against an infection, your blood produces antibodies to fight it. These antibodies remain even after the infection has cleared, and usually prevent most viral infections from developing again, in other words you are immune to it. Rubella (German measles) and chickenpox are examples of infections which, once suffered, will not usually recur.
A doctor may order a blood test to find out if a patient is at risk from a particular disease. For example, the doctor will want to know if a woman has had rubella (which can harm the foetus if the mother develops it during early pregnancy) or whether she should be immunised against it—although immunisation during pregnancy is not advisable.
Sometimes tests are carried out to detect the presence of antigens themselves (antigens are substances that the body regards as foreign and to which it will develop antibodies). The presence of particular antigens in the blood indicates that the organism is still active and that, even though the symptoms have subsided, the person may be a carrier of a disease such as hepatitis B.
The progress of certain diseases can be assessed by testing the blood for specific immunoglobulins (antibodies) which differ according to whether the disease is current or past. Many infectious diseases such as glandular fever, AIDS, various forms of hepatitis, Ross River fever and measles can be diagnosed and followed by this type of test.
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