Diagnosis of HIV is often not happening until the infection is at a late stage, a study says.
Campaigns need to be targeted at risk populations
One third of nearly 1,000 patients only found out they had HIV when their CD4 immune cell count was low - making them more susceptible to infection.
The study found 168 patients in the UK and Ireland had been to hospital with HIV symptoms a year before diagnosis, the British Medical Journal reported.
It said late diagnosis meant patients had been missing out on drug therapy.
About 50,000 people in the UK are living with HIV, but about a third of these are unaware of the condition, it is estimated.
Lead researcher Ann Sullivan said people at risk of HIV needed to be encouraged to have testing and healthcare professionals had to be more proactive.
"To improve this situation, the proportion of people diagnosed as having HIV as part of routine screening needs to increase, with people at risk being encouraged to have an HIV test.
"Healthcare professionals' awareness of factors associated with late presentation of HIV infection and conditions likely to be related to HIV also needs to increase."
Researchers from Chelsea and Westminster Healthcare NHS Trust, the British HIV Association, Royal Free and University College Medical School and Royal Free Hampstead NHS Trust surveyed more than 100 HIV centres in the UK and Ireland.
Of 977 patients diagnosed with HIV, 301 had people had CD4 count below the threshold for initiating treatment.
A breakdown of the results showed that black African and the elderly were more likely to have a late diagnosis.
A spokesman for the Terrence Higgins Trust said there was a variety of socio-economic reasons for the differences.
"HIV is well understood by the gay community, but for some of the other groups that is not the case.
"HIV is stigmatised within the black African population, it is not really talked about.
"And when you think about the more disadvantaged groups and asylum seekers that may be having trouble with social care, housing or immigration, HIV may not be such a priority."
The spokesman said it was important that at risk populations were targeted with campaigns, more resources were made available and partnership working improved.
A Department of Health spokeswoman said attempts were being made to improve early diagnosis.
"As well as initiatives to encourage HIV testing, such as working with the voluntary sector to pilot HIV testing in community-based settings, we are also working with professional groups to support them to increase their awareness of STIs including HIV."