By its nature, information on undiagnosed infection is challenging to produce accurately. Public Health England produces estimates based on anonymised blood samples which have been taken for other purposes (e.g. for syphilis testing in GUM clinics; from newborn infants, providing information on pregnant women).

The data are adjusted in the light of other information, such as CD4 cell counts when people are diagnosed and estimates of the size of sub-populations (e.g. the number of African adults living in the country). Known biases are corrected for (e.g. people who attend GUM clinics tend to have higher levels of infection than other people).

For 2012, it was estimated that 22% of people living with HIV were unaware of their infection. There are inequalities between different demographic groups – heterosexual men have the highest rates of undiagnosed infection. The routine offer of HIV testing in antenatal settings probably contributes to a smaller proportion of heterosexual women having undiagnosed infection, in comparison with heterosexual men.

Around 4,300 African women and 3,000 African men are living with undiagnosed HIV. An African community study, conducted a decade ago, found that of those individuals with undiagnosed HIV, just over half had never tested and a third said their last HIV test result was negative.

Around 7,300 gay and other men who have sex with men have undiagnosed HIV. Although the proportion of infections that are undiagnosed is relatively low in this group, the number of individuals concerned is large. Research conducted a decade ago in gay commercial venues found that 62% of the men with undiagnosed infection believed themselves to be HIV negative.