I have an undiagnosed neurological disorder. Since I don't know what I have, I've imagined every possible worst-case scenario. Now I'm scared of being HIV-positive. I've had many blood tests in the past six months or so, for thyroid and syphilis and Lyme and just about everything else. Is it true that all blood samples, regardless of what they're drawn for, are automatically tested for HIV, and that since I've had so many blood tests, I must not be HIV-positive? I know I should have an HIV test done, but I had human immunoglobulin transfusions, and I heard that since these transfusions include human antibodies, an HIV test will most likely be positive regardless.

P.R.

I am surprised that you could have such a lengthy undiagnosed illness without one of your physicians ordering a test for HIV (the virus that causes AIDS). I am by no means suggesting that you have an HIV-related disorder, but most physicians routinely get an HIV test when they have trouble diagnosing almost any medical condition that could be caused by an infection.

The standard HIV test (called the ELISA test) looks for antibodies (disease-fighting proteins) that target a single part of the virus. If this test is negative, it is very unlikely that the person has HIV. If it is positive, it is repeated and then confirmed with a more accurate test called the Western blot. If a person tests positive for HIV on this highly accurate test, it is extremely likely that he or she is infected with HIV. The risk of a false positive reading -- meaning that the test falsely indicates that an uninfected person has HIV -- is less than 1 in 100,000 with the Western blot test, and is most likely to occur in those who have received experimental vaccines for HIV. Receiving immunoglobulin infusions (which are derived from donated blood) can result in false positive reading for some antibody tests, but would be unlikely to result in a false positive HIV test. Remember that anyone having antibody to HIV would be excluded from donating the immunoglobulin you received.

You were probably not tested for HIV, because not all blood samples taken during medical care are tested for the virus. What you may be thinking of is the fact that all blood donated for use by others is routinely tested for HIV and some other viruses. So, if you donate blood frequently, and the Red Cross or other blood banking organization has not notified you that your HIV test is positive, then you are probably negative.

Although it is possible that your blood was tested for HIV without your knowledge, most states in the United States have laws requiring written informed consent before such a test can be performed. Before the blood is drawn, you would have to sign a paper stating that you agree to the test and that you understand the implications of both a positive and a negative result. Even in states where it is not required by law, most labs still have patients sign a consent form anyway. If you did not sign such a paper, it is unlikely that you had an HIV test.

As I have said before in my column, if you are worried that you may have HIV, the only thing to do is get tested. I am not sure why you are afraid to do so, but if there is a chance that you have HIV, I would think you would want to know, for many reasons:

• First and foremost, an HIV diagnosis is not the immediate death sentence that it may have been in the past because there now are effective treatments against the virus, even at the late stages of the infection.
  
• I have no idea, of course, what your neurologic condition could be, but damage to the peripheral nerves, which can result in pain, tingling numbness, or other sensations in the hands and feet, is often treated with immunoglobulin infusions. Since HIV can cause a similar condition called peripheral neuropathy, ignoring HIV as a possible cause of your neurologic disorder is one more reason to be tested.
  

• You may be receiving potentially toxic medications for your undiagnosed condition that you would not need if the true diagnosis were HIV infection.
  
• If you are HIV-positive and are unaware of it, you may be needlessly infecting a sexual partner.
  
• Finally, if you are not HIV-positive, a negative test result can put your mind at ease.