Dr Narong Sahametapat, permanent secretary for public health, said only 250,000 of an estimated 500,000 HIV-infected people had had a blood test, knew the result and entered treatment.
He urged people at risk to regularly have their blood tested for HIV. Once aware of the test results, those without HIV could protect themselves in future and those with the infection could prevent its transmission to others and seek quick treatment.
Dr Narong said the Public Health Ministry, through its nationwide hospitals, offered two free-of-charge HIV tests to each person a year and provided comprehensive services relating to HIV and Aids, including counselling, examination and treatment with antiretroviral drugs.
The ministry and its partners announce July 1 as Voluntary Counselling and Testing Day (VCT Day) to encourage people to have a test for the sake of disease control.
Public hospitals are helping with the campaign, which targets people with past and present risky behaviour, including those injecting drugs and failing to use condoms.
Dr Somsak Akksilp, deputy director-general of the Department of Disease Control, said HIV tests came with same-day results.