About HIV testing

Window period

Window period is the period between the moment you get infected with HIV and that when the antibody/antigen test shows a positive result. 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/ antigen test remains negative.

One of the testing technologies commonly used nowadays - enzyme-linked immunosorbent assays (ELISAs) – can detect HIV antibody in a sample. 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.

In the past few years, HIV testing technology has gone through multiple stages of development with newer generations offering higher sensitivity, higher specificity and shorter window periods. For example, the fourth generation of HIV test kit can detect both HIV-1 and HIV-2 antibody, as well as p24 antigen. The window period can reduce to 2 to 3 weeks. Therefore, combining the ELISAs and p24 antigen detection assays , as in the 4th generation of HIV test kits, can shorten the window period significantly.

Nowadays, 2^nd,3^rd and 4^th generation test kits are commonly used by professionals or AIDS organizations that provide testing service. However, for self test kits, only 2nd and 3rd generations are available on the Hong Kong market as at December 2016. (Please see section on HIV self-testing). A shorter window period only means that the infection can be picked up earlier, 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.

Advances in HIV treatment have led to a greatly improved survival of infected patients, especially when the diagnosis is made early. There is evidence that HIV treatment contributes significantly to the prevention of onward transmission via sexual contact and other transmission routes.

Antibodies are made in your blood when your body is exposed to an infection. They are considered one of the body's response to infections. The antibodies against HIV, however, cannot get rid of the virus completely. An HIV antibody test looks for these antibodies in blood, oral fluid (not saliva) or urine. It may be conducted in a laboratory by a conventional test (venous blood) or as a rapid test (finger prick). A conventional HIV test (blood taking from vein) takes around 1 week for the result, whereas a rapid test (finger prick) only takes 20-30 minutes and requires confirmation testing for reactive results.

If HIV antibodies are present, the test is positive. It means that the person is infected with HIV.

If you receive a negative result after the window period (i.e. the period after exposure but before the test can find antibodies), it means that you have not been infected.

HIV antigen is a viral protein called p24. High levels of p24 can be present in the blood serum of newly infected individuals during the short period between infection and seroconversion (production of antibodies), making p24 antigen assays possible in diagnosing early HIV infection. However, p24 antigen assays are not always reliable for diagnosing HIV infection after its early stage.

• The rapid HIV test is only a screening test. It uses blood or oral fluid to look for antibodies to HIV. It produces a result in 20-30 minutes.
• Any positive rapid test result should be followed by a confirmatory test of venous blood conducted by a laboratory.
• Currently, at least four rapid HIV tests are approved by the US FDA, some of which are being used in Hong Kong. For all rapid tests, strict adherence to the manufacturers' instructions is imperative to assure accurate results. The performance of rapid tests is generally higher when it is used by trained personnel.
• Agencies providing HIV antibody testing services in Hong Kong
  

  Text Description

  • Performing HIV test using a rapid test kit
  • The testing procedure may vary with different test kits
  • Take out the test strip from the package and place it on a flat surface
  • Clean the fingertip with alcohol pad and allow to air dry thoroughly
  • Puncture the fingertip with a sterile lancet
  • Wipe away the first drop of blood with sterile gauze
  • Collect blood sample by using the EDTA capillary tube (until the sample reach the area between 2 black markers)
  • Apply a sterile gauze to the puncture site until the bleeding stops
  • Place the tube onto the test strip and transfer all the blood to the sample pad
  • Apply one drop of Chase Buffer to the sample pad
  • Wait for 20 minutes and read the preliminary result
  • Results will be displayed on the device window (Result will be invalid without control line)
  • Rapid test is only a screening test. Any positive rapid test result should be followed by confirmatory test of venous blood conducted by a laboratory
  • Gay Men HIV Testing Hotline 2117 1069 - The Department of Health provides free and anonymous HIV testing. All information is kept confidential
  • In Hong Kong, the Department of Health and non-governmental organisations operate HIV testing service. Please visit www.21171069.gov.hk for details
  • Last update: 2019 | All Rights Reserved

Self-testing vs. Conventional test and Rapid test

• Conventional test refers to taking blood from a vein and sending the specimen to a laboratory where trained technicians perform the test, followed by confirmatory tests if necessary.
• Rapid test refers to obtaining a blood drop from a finger-prick (or oral fluid by swabbing the gums), performing the test on-site and reading the result in about 20 minutes by a healthcare worker or community worker.
• The procedure of HIV self-testing and rapid test is the same. The only difference is that self-testing requires the users to read and interpret the results by themselves.
• HIV self-testing comes with both advantages and disadvantages compared to these two methods: