AIDS/HIV (Human Immunodeficiency Virus)

What is AIDS?

AIDS is a disease caused by HIV (Human Immunodeficiency Virus). It affects a person’s immune system and reduces its ability to fight infection.

What is the difference between HIV and AIDS?

HIV is the virus and AIDS is the end point of an HIV infection. AIDS is defined by a number of illnesses that affect a person who has the HIV virus. These illnesses usually happen after a person has been infected with HIV for a number of years.

How does someone become infected with HIV?

People can be infected after having unprotected sexual intercourse or sharing injecting equipment with someone who has the virus. HIV is contained in body fluids such as blood, semen, and vaginal secretions.  It is generally believed that HIV is not transmitted by oral sex.

Risk of HIV infection  
Vaginal Intercourse 1/1000
Anal Intercourse 1/250 - 1/500
Needle stick Injury 1/300
Sharing of Needle 1/100 - 1/150

HIV can be transmitted by:  

  • Vaginal or anal intercourse without a condom.
  • Sharing drug injecting equipment. 
  • During pregnancy, childbirth, or breast feeding if the mother is HIV positive. 

HIV is rarely transmitted by:  
Vaginal or anal sex with a correctly used condom (using only water-based lube).
Oral sex without a condom (ejaculation increases the risk, as may gum disease).
Needle stick injuries.
 HIV has not been reported as being transmitted by: 

  • Kissing or cuddling. 
  • Shaking hands/social contacts. 
  • Sharing knives and forks, cups or glasses. 
  • Sharing toilet seats or mosquito bites.

Testing for HIV Infection 

There are a number of methods to test for HIV using blood, saliva or urine samples. However, the most accurate and the most common way is by testing a person’s blood.  There are three biological markers that can be tested to diagnose an HIV infection:

  1. Testing for HIV antibodies (the body’s immune response to an HIV infection); 
  2. Testing for the p24 Antigen (a protein that is produced during the early phase of an HIV infection); 
  3. Testing for the HIV virus itself (that causes AIDS)

1.      HIV antibody test

This is the most common and standard test to detect an HIV infection.  It can be done with blood, saliva or urine.  However, the techniques used for testing affect the window period significantly.  With the modern ELISA technique, HIV antibodies can be detected as soon as 3-4 weeks after infection.  While a rapid test for HIV is available (results in 10 minutes) it may take up to 3 months after the infection for the antibody level to be detected in the blood.  Therefore many health services may still quote three months as the window period for an HIV test.   

2.      HIV p24 Antigen test

The p24 antigen test measures the p24 protein that is produced during the early phase of an HIV infection. This is detectable earlier than HIV antibodies due to the initial burst of virus replication and is associated with high levels of the virus during which the individual is highly infectious.  P24 is usually detectable 2 weeks after infection and a positive result is highly specific for an early infection with specificity in the range of 99.7 – 99.9%.  However, the level of p24 may decline after 4 weeks of infection due to the production of antibodies. 

3.      HIV P24 Antigen and Antibody test (Combo Test)

The 4th generation HIV test detects both p24 antigens and antibodies thus reducing the window period for an HIV test to 2 weeks after exposure.  This improves the accuracy of screening and reduces the anxiety of a 3 month wait.  The results of “combo” test can be available within 24 hours from the time a blood sample is taken.  There is information on the web regarding the various testing times for HIV. 

The “combo” test has been used in Europe for over a decade and was approved by the United States Federal Drug Administration (FDA) in 2010.  It is also being used by Blood Banks to screen donor blood.   Due to the different types of test kits being used most international guidelines (eg the British HIV Association (BHIVA), suggest 4th generation HIV tests be undertaken 1 month after exposure.  Some international guidelines such as the WHO European STD Board still suggest a repeat test 3 months after exposure.
At Hong Kong Sexual Health Centre, our in-house laboratory (Neo-Lab) is equipped with two HIV combo testing systems that are FDA and CE certified. Beckman Coulter Access 2 Analyzer using Bio-Rad GS HIV Combo Ag/Ab EIA kit is highly sensitive in detecting HIV P24 and antibody in both chronic and primary infection. The bioMerieux Vidas HIV panel can further confirm and differentiate P24 antigen and antibody independently.

4.      HIV RNA Test

The HIV RNA test is sometimes called the HIV PCR test and can detect an infection within 1 week of exposure. However it requiresspecialised equipment and personnel to perform the test and may take about 2-3 days for the results to become available. During actual infection, within 2-3 weeks, the viral load of HIV virus can reach over 10 million copies/ml of blood. HIV RNA test can detect HIV virus less than 100 copies/ml of blood. So, it is highly sensitive in detection of early infection. 

All the above tests vary in terms of cost, turnaround time and most importantly, the window period, ie the time from infection to the time when it can be detected.  The following table summarises the differences: 

  Antibody test P24+Antibody HIV RNA
Window Period  Up to 3 months 2 weeks 3-7 days
Reporting time 10 minutes  24 hours 2-3 days
Cost  $ $$ $$$

How accurate are the tests?

Modern HIV tests are very accurate. Our health care professionals will determine which test is the most appropriate for you depending on when you were exposed.  As the tests are designed for screening purposes, they are highly sensitive in detecting the infection.  However, a positive result does not always indicate infection.  False positives can happen in 1-3 cases for every 1000 tests performed.  To confirm a positive result, further tests will be performed on the blood sample for verification.

The window periods specified apply in most standard situations, however, it can be affected by the use of antiviral medications, immune-suppressants or immunological conditions of individuals.


Many people are confused by the different information found on the web. So stop searching! It will only add to your confusion and anxiety! Follow these simple steps: 
Make an appointment to see our doctor who can assess your risk and provide the best plan of action.
If you are super anxious, consider PEP (see below) within 3 days.
You may consider having a HIV RNA (PCR) test between days 7 through 10 after exposure.
If you are concerned and want an early result have a HIV Combo 4th generation test 14 days after exposure.
If you are not so concerned but just want to make sure things are fine, have just 1 HIV Combo test after 1 month.
You can repeat the tests if you are still concerned. HIV phobia is not uncommon and should be handled with care and counselling. Our in-house counselors can assist you.
If you choose option 2, have a HIV rapid test first to exclude any pre-existing infections. Then have an HIV RNA (PCR) test straight after PEP, followed by a HIV Combo test 1 month later. A repeat test at 3 months is optional. 

If you choose option 3 or 4 you can repeat the HIV combo test at 1 month and/or 3 months if you are still concerned. 

Symptoms of HIV 

The symptoms immediately after contracting HIV can be very similar to other viral infections, such as influenza.  Therefore if you have any of these symptoms, it does not mean you have HIV.  Approximately 50% of people newly infected with HIV do not develop any symptoms. 
Symptoms of an acute HIV infection may include:  
fever, fatigue, rash are primary symptoms (Must have symptoms)
Sore throat, diarrhea, swollen tonsils and/or other lymph nodes, joint or muscle ache, headache, nausea, night sweats, vomiting are secondary symptoms which if present without primary symptoms, are unlikely related to HIV
If you have any of these symptoms and had a high risk exposure, do not hesitate to get tested for HIV infection. 

HIV Emergency Help - PEP (Post Exposure Prophylaxis) 

PEP is an antiviral treatment given to individuals to reduce the chance of seroconverting due to a potential HIV infection.  PEP has been shown to be highly effective if given early after a high risk exposure, ie unprotected intercourse or needle stick injury.  PEP should begin as soon as possible after an exposure as its protective effect is reduced if begun after 72 hours. The usual course of treatment is 28 days. Because PEP can delay viral replication, individuals who have taken PEP should have an HIV test, 1, 3 and 6 months after the initial exposure.
As PEP is expensive and has a number of side effects you should consult a specialised medical professional before starting the treatment.  If you require PEP urgently please email as we are one of the few centres in Hong Kong that can provide you with this medication.

Other things to consider before having HIV test:
Having an HIV test is usually stressful. Ongoing support is provided by our centre. 

  • Everyone who is intending to have an HIV test is encouraged to discuss their concerns with a counsellor, nurse or doctor.   
  • All medical information and any discussions at the centre are confidential. 
  • Testing for HIV can be coded (we do not need to use your real name). 
  • The Department of Health receives statistics on all new cases but no names are provided. 
  • In many countries, there is a legal requirement for HIV infected people to inform present and future sexual partners, if unprotected intercourse is anticipated. 
  • Some people with HIV experience discrimination in their personal relationships and employment. 
  • There may be restrictions on obtaining life insurance and visas in some countries. 

The information contained in the Website is not intended nor implied to be a substitute for professional medical advice. Readers are advised to seek professional opinions of your physician or other qualified health provider prior to starting any new treatment or if you have or suspect you have a medical problem or with any query you may have regarding any medical condition. Nothing contained in the website is intended to be for making medical diagnosis or promotion of any treatment or medication.

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