Performance and limitations
Accuracy
Sensitivity: refer to the ability of a test to show positive results in infected people, e.g., if a test is done on 10 infected people and show positive results for 9 of them, then the test is said to have a sensitivity of 90%. Tests with low sensitivity will result in more false-negatives, which means missed diagnosis for infected people causing false sense of security.
Specificity: refer to the ability of a test to show negative results in non-infected people, e.g., if a test is done on 10 non-infected people and show negative results for 9 of them, then the test is said to have a specificity of 90%. Tests with low specificity will result in more false-positives, causing unnecessary anxiety for non-infected people.
Window period
-
Window period is the period between the moment a person get infected with HIV and that when the HIV test shows a positive result. The window period for HIV antibody testing is 3 months. During this period, the virus replicates rapidly in the infected person who becomes highly infectious and can transmit the virus to other people even though his HIV antibody test remains negative.
-
After initial exposure, HIV antibody takes some time to develop in human body before reaching a detectable level, this period varies in different individual and can be as long as 3 months.
-
HIV testing technology has gone through multiple stages of development with newer generations offering higher sensitivity, higher specificity and shorter window periods. A shorter window period means that the infection can be picked up earlier, yet a negative result still cannot exclude infection if the user has recent exposure in the previous 3 months. Users must take note of this and repeat the test or consult a doctor whenever in doubt.
Generation of HIV test kits
Window period
1st
At the earliest 5 weeks
2nd
Down to at the
earliest 4 weeks
earliest 4 weeks
3rd
Down to at the
earliest 3 weeks
earliest 3 weeks
4th
Down to at the
earliest 2 weeks
earliest 2 weeks
Effect of biotin
-
Biotin, also named vitamin H, vitamin B7 or coenzyme R, is commonly used in dietary supplement products that improve hair, skin and nail conditions. Taking high dose (10 mg or more per day) of biotin may affect many laboratory test results, including false negative result of HIV antibody and antigen tests. If you are taking dietary supplements containing biotin, please stop taking it 48 hours before the test. In addition, biotin amount in ordinary food products do not affect the test results.
-
For details, please refer to the Important Safety Alerts of the Medical Device Division of the Department of Health