|MadSci Network: Virology|
The two test outcomes, "not detected" and "negative," probably relate more to how the test was done than to the disease. HIV (human immunodeficiency virus) is obviously a virus, which sometimes causes AIDS (acquired immunodeficiency syndrome), whereas syphilis is caused by a bacterium, Treponema pallidum. What they have in common is that both are often sexually transmitted. Tests for either may be directed to the causative agent or to the body's response to infection.
HIV is much smaller than bacteria and comprises only ribonucleic acid (RNA) coated with protein. Because HIV cannot be cultivated in the laboratory, detection of HIV is based on either its nucleic acid or its coat protein. The test for RNA uses a reverse transcription-polymerase chain reaction (RT-PCR). HIV coat protein is typically detected by one of several tests using "antibody" (a protein produced in an animal or human that reacts specifically with the HIV coat protein). In either case, the test result will be positive only if the sample was taken from blood at a time when the HIV level is above the detection limit. The RT-PCR test tends to be more sensitive but more expensive and exacting, so it may give a positive result earlier in the course of infection when virus levels are still low.
Because T. pallidum is a bacterium, it can be cultured (grown) on medium in the laboratory. Samples for this purpose may be taken from blood or from lesions that look representative of syphilis. In any case, the bacterium grows rather slowly, so it may take days to get a result. It is highly specific.
When the body is infected, it often responds by producing antibodies (proteins that combine specifically with proteins of the infecting agent). The response is complex, and may take time, so most antibody tests are said to become positive within three months of the beginning of infection. This means that samples taken earlier may yield a false-negative result; but the antibody response is usually strong, so that a positive result is easy to detect. People's antibody response cures some infections, but unfortunately not HIV or syphilis. Screening tests for antibody against HIV or the bacterium of syphilis are relatively inexpensive and can be run in large numbers. Obviously, if one needs results sooner than three months after exposure, it is necessary to test for the agent and accept a risk of false-negative results if done too soon. On the other hand, antibodies persist after the infection has passed; even with treatment, HIV infections don't go away, but people can be cured of syphilis and yet carry antibody against the agent for the rest of their lives.
Typically, then, a "negative" test result means that antibody was not detected, whereas "not detected" means that the causative in agent was not detected in the sample at the level of sensitivity of the test. A positive antibody test indicates current or previous infection; whereas detection of the causative agent (HIV or T. pallidum) indicates current infection, but a "not detected" may indicate that the sample was taken too soon or that the test was not sensitive enough, rather than that there was no infection.
Dean O. Cliver
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