Unfortunately, no test can detect HIV immediately after infection.
If you think you might be at risk after an encounter in the last 72 hours, contact your healthcare provider for Post-Exposure Prophylaxis (PEP) immediately.
Window period is the time between the exposure and when a test shows positive for HIV. This varies from person to person and is also dependent on the type of test used.
- Nucleic acid tests (HIV viral load) can tell whether a person has been infected 10 – 33 days after exposure. However, it is very expensive and not routinely used for screening due to low level false positives.
- Antigen / antibody tests can detect HIV infection 18- 45 days after exposure.
Antigens are foreign substances that activate your body’s immune system. The p24 antigen typically appears 5 days before antibodies against HIV develop. Hence the Alere 4th generation HIV 1/2 Ag/Ab Combo kit is a more sensitive test and can detect infection earlier.
If you get tested after a potential HIV exposure and the result is negative, get tested again after the window period is over. Keep in mind that you will need yet another repeat test after that if there was a 2nd exposure while waiting for the 1st window period to be over.
What happens when you go for a test?
HIV testing involves withdrawing a small sample of blood from your vein. If the Alere 4th generation HIV rapid test is being utilised, results will be ready in 15 minutes. Other types of screening / confirmatory tests will be sent to a laboratory and it may take between a few days and a few weeks for you to receive a final result.
If a person gets a laboratory-based antigen/antibody test on blood plasma less than 45 days after a possible HIV exposure and the result is negative, follow-up testing is recommended 45 days after the exposure. For all other tests, Centers for Disease Control and Prevention (CDC) recommends testing again at least 90 days after exposure to be certain it is negative.
Who should get screened?
According to CDC guidelines, all individuals between the ages of 18 and 64 should get tested for HIV at least once in their lifetime. However, if you were tested more than a year ago (with a negative result) repeat testing is recommended as soon as possible if you:
- Are a man who has had sex with another man.
- Have had anal or vaginal sex with a partner who has HIV.
- Have had more than one sex partner since your last HIV test.
- Have injected drugs and shared needles, syringes, or other drug injection equipment with others.
- Have exchanged sex for drugs or money.
- Have been diagnosed with or treated for another sexually transmitted infection.
- Have been diagnosed with or treated for hepatitis or tuberculosis (TB).
- Have had sex with someone who could answer yes to any of the above questions or someone whose sexual history you are unsure of.
You should be tested at least once a year if you fall into any of the above categories. Sexually active gay and bisexual men may benefit from more frequent testing (for example, every 3 to 6 months).
HIV Risk Behaviours
The risk of contracting HIV varies widely depending on the type of exposure or behaviour (such as sharing needles or having sex without a condom). Some exposures to HIV carry a much higher risk of transmission than others. Certain behaviours pose a biological risk, however it is too low and not difficult to quantify. But risks do add up over time. Even relatively small risks can gradually add up and lead to a high lifetime risk of getting HIV.
In other words, there may be a relatively small chance of acquiring HIV after a single risky exposure with an infected partner but, if repeated many times, the overall likelihood of becoming infected is actually much higher.
The table below lists the risk of transmission per 10,000 exposures for various types of exposures. (Patel P, Borkowf CB, Brooks JT. Et al. Estimating per-act HIV transmission risk: a systematic review. AIDS. 2014)
Estimated Per-Act Probability of Acquiring HIV from an Infected Source, by Exposure Act
| Type of Exposure | Risk per 10,000 Exposures |
| Parenteral | |
| Blood Transfusion | 9,250 |
| Needle-Sharing During Injection Drug Use | 63 |
| Percutaneous (Needle-Stick) | 23 |
| Sexual | |
| Receptive Anal Intercourse | 138 |
| Insertive Anal Intercourse | 11 |
| Receptive Penile-Vaginal Intercourse | 8 |
| Insertive Penile-Vaginal Intercourse | 4 |
| Receptive Oral Intercourse | Low |
| Insertive Oral Intercourse | Low |
| Other | |
| Biting | Negligible |
| Spitting | Negligible |
| Throwing Body Fluids (Including Semen or Saliva) | Negligible |
| Sharing Sex Toys | Negligible |
HIV transmission through these exposure routes is technically possible but unlikely and not well documented.
Factors that may increase the risk of HIV transmission include sexually transmitted infections, acute and late-stage HIV infection, and high viral load. Factors that may decrease the risk include condom use, male circumcision, antiretroviral treatment, and pre-exposure prophylaxis. None of these factors are accounted for in the estimates presented in the table.
