HIV (Human Immunodeficiency Virus) is a virus that causes AIDS (Acquired Immunodeficiency Syndrome) in which the person’s immune system progressively fails leading to various opportunistic infections that would seldom occur on a healthy person.

HIV is transmitted mainly through body fluids such as blood, vaginal fluid, semen, and breast milk.  Transmission occur mainly through:

1)      High risk unprotected sexual contact (vaginal, anal and oral)

2)      Sharing of needles between intravenous drug users

3)      Vertical transmission from mother with HIV to the unborn child

When the body is infected with HIV, the virus attacks the CD4 white cell and replicates in it.  The infected CD4 cells eventually dies and new virus are then released and are able to infect other CD4 cells.  As the disease progresses, more and more CD4 cells are killed leading to a decrease in one arm of the immune system called cell-mediated immunity.  A decrease of CD4 count below a critical level will lead to manifestations of AIDS which is characterized by susceptibility to various opportunistic infections and cancers that are normally rare in an immunocompetent person.

Clinical symptoms of HIV infection

HIV infection can be divided into 3 phases:

1)      Acute infection (lasting up to 2 weeks)

During the initial phase, symptoms of HIV infection are no different from other acute viral illnesses.  Symptoms include:

a)      Fever and chills

b)      Muscle and joint aches

c)       Enlarged lymph glands

d)      Sore throat

Patient subsequently recovers from the symptoms and will become asymptomatic.  However, HIV virus actively replicates in the body and CD4 cells are progressively being infected and destroyed.

2)      Asymptomatic infection (lasting up to 10 years)

This stage of infection can last for years where the HIV carrier remains well and asymptomatic but the HIV actively replicates while slowly reducing the CD4 counts.  Although asymptomatic, these carriers are highly infectious!  The immune system progressively weakens and once the CD4 count goes below 200, the patient enters the late stage of infection (also known as AIDS).

3)      Late stage infection (AIDS)

AIDS is characterized by multiple opportunistic infections and rare cancers that rarely occur in a healthy or immunocompetent person.  These conditions include:

a)      Kaposi Sarcoma

b)      TB reactivation

c)       Pneumocystis Carinii pneumonia

d)      Disseminated Cytomegalovirus infection

e)      Mycobacterium Avium Complex infection

And many more

HIV patient will eventually succumb to one of these opportunistic infections.

Testing of HIV infection

 P24 chart

Screening for HIV test can be done via detection of

–          Antibody test (anti p24 antibody)

–          Antigen test (p24 antigen)

–          Viral RNA test (PCR)

As seen from the graph, sensitivity of each test depends on the time it is tested.

a)      Antibody Test

Most common test done by clinics.  This can be done either through

  • Regular blood drawing test where sample is sent to the lab
  • Rapid test via finger prick (blood) or oral swab (saliva)

Although antibody test is sensitive, there can be occasions of false positive.  If the test result is reactive, there is a need for confirmatory Western Blot test (only done in Singapore General Hospital) to confirm if it is indeed a true positive.

One disadvantage of antibody test is the presence of window period.  As seen from the above graph, HIV antibodies only become significant from 3rd month onwards hence testing done during the window period will produce a false negative result.  If a person is considered high risk, there is a need to repeat a second test 3 months after the first.

b)      Antigen Test

As antigen level rises faster than antibody level, antigen test has a higher sensitivity than antibody test during the traditional ‘window period’.  Antigen test can be reactive within 1 month of infection (hence a person can be tested after 1 month of point of high risk contact instead of the usual 3 months for antibody test).  This can be done via blood test (sent to the lab or rapid test).

c)       Viral RNA test

This is the most sensitive test as it detects the presence of the HIV RNA (genetic material).  It can be done as early as 2 weeks after the point of high risk contact but it is also the most costly.  This is done via blood test (sent to the lab).

Summary of timeframe for HIV testing


Time from contact Test Available
0 – 2 weeks None available

If high risk contact to consider prophylactic anti-viral therapy   (best done within 72 hours of exposure)


After 2 weeks Viral RNA PCR


After 4 weeks / 1 month P24 antigen test – Rapid test kit (p24 antigen + antibody) – result within 20 minutesRegular blood test (p24 antigen + antibody) –   sent to lab


After 12 weeks / 3 months P24 antibody test – Rapid test kit (p24 antibody) – result within 20 minutesRegular blood test (p24 antibody) – sent to lab





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