How is blood tested for HIV?

How is blood tested for HIV?

Testing 'algorithms' are a sequence of specific tests, or assays, which are organised to create a certain HIV testing strategy.³⁷ These must take into consideration the resources, infrastructure and staff expertise available in different countries so the specified algorithms are always followed, to ensure consistency in the testing of blood.³⁸

Screening blood for HIV in Kazakhstan

Initial HIV testing uses antibody tests to detect antibodies to HIV in the blood. As the virus becomes established the body makes increasing amounts of antibodies. However, it can take between 3 weeks and 3 months after initial infection before an individual produces antibodies and HIV is detectable. This gap is known as the window period and blood donations infected with HIV screened with antibody tests at this time may not be detected.³⁹

However, other tests exist to further reduce this window period, such as p24 antigen tests, which screen for proteins attached to the HIV infected cell and nucleic acid testing (NAT), which screen for the genetic material of HIV.⁴⁰ These tests reduce the window period down to about 12 days. It is because these 12 days remain that donor screening and counselling is still important to further reduce the chance of a person infected with HIV giving blood.

The NAT test is particularly important where prevalence is high as the number of window period donations are more likely. However, HIV prevalence is often highest in poorer countries and unfortunately NAT tests are expensive and therefore these countries usually only have antibody tests. The chance that an HIV-infected donation will not be detected is therefore greater in these countries.

In 2009 blood screened for HIV in Greater Accra, Ghana amounted to 33,294 units of blood, of which 3.68 percent was found to be HIV positive. ⁴¹ Ghana tests 100 percent of its blood donations, however this is done using only antibody tests. Therefore the window period remains a significant interval, which suggests some units may continue to pass through screening undetected.

In October 2005, South Africa introduced NAT testing and as a result there were no cases of HIV transmission by blood transfusion reported to the haemovigilance programme, a transfusion surveillance system.^{42 43}

During the testing process a screening policy, good laboratory practice and a quality assurance system should be in place to avoid any HIV positive samples passing undetected. ^{44 45} The WHO asserts this is reliant on the formulation and implementation of a national blood policy by a country's government.⁴⁶