As shown by a recent study comparing different HIV tests, the fourth-generation laboratory tests used in clinics detect almost 100% of infections and have short window periods. However, the OraQuick test that has been licensed for home testing in the United States is less accurate when used with a sample of oral fluid – it misses around one-in-ten infections and has a longer window period (up to twelve weeks).
The modelling compared the outcomes if people switched from testing in a clinic to testing at home with OraQuick. The same results would not necessarily apply if a more sensitive home test were available. Nor would they apply if home sampling was used in combination with a more sensitive test – as is the case with the Terrence Higgins Trust postal test scheme, using a test with a window period of four weeks. Another UK home sampling project, Dean Street at Home is offering a fingerprick blood test with a window period of four weeks or an oral fluid test with a window period of 14 weeks.
The modelling study found that OraQuick’s longer window period could mean that people with recent HIV infection remain undiagnosed.
This would be particularly likely to occur if a person who already tests every six months switched from testing in a clinic to using a home test. If this person had HIV, it is highly likely that he or she would have a recent infection – one which the home test might miss.
In contrast, if home testing was mostly used by people who tested less frequently (for example, every few years) or previously did not test at all, it is more likely that they would have had HIV for longer. Self tests are able to detect most of these infections.
Also, if home testing leads to people testing more often or to avoiding unprotected sex with individuals of a different HIV status, it could help lower prevalence.
While home sampling projects are already provided in the UK, home testing will become legal in April. This study highlights the importance of the technical specifications of the tests used and also of the frequency with which people test. At the moment we cannot know who will use home tests and whether their use will supplement or replace other HIV and STI screening practices.
While it can be hoped that home testing will help reduce the number of people with undiagnosed HIV and lead to fewer infections, this model shows that in certain circumstances it could have a negative effect.