Screening HIV tests: when they can be done with accuracy
The table below summarizes the most common HIV used for screening in the United States.
The table explains
- what each screening test is checking for,
- how soon after exposure it can be done, and
- when each test is most accurate.
*Pay attention to the fact that Ab tests (IgG, IgM), which check for your body’s response to infection, have to be done longer after exposure because the response takes longer to be detected than do different parts of the HIV virus (e.g., RNA, P24).
| Test name | What it detects | Earliest time the test can be done | Ideal time for testing |
|---|
| HIV third-generation test | IgG, IgM | - 22 days post exposure
- 50% accuracy
| 50 days post exposure is 99% accurate |
| HIV fourth-generation | IgG, IgM + p24 | - 13 days post exposure
- 50% accuracy
| 44 days post exposure is 99% accurate |
| HIV RNA PCR | HIV virus itself | - 10–12 days post exposure
- 90% accuracy
| 42 days after exposure is 99% accuracy |
| HIV DNA PCR | HIV virus itself | - 9–10 days post exposure
- 97% accuracy
| 42 days after exposure 99% accuracy |
| HIV P24 | P24 antigen | - 2 weeks after exposure
- 95% accuracy
| 36 days after exposure 99% accuracy |
Generations of HIV antibody tests: when they can be done and their accuracy
This table summarizes four generations of HIV Ab tests that currently can be performed. They all confirm seroconversion—the body’s response to the virus via production of antibodies.
*Please note that with each generation, the test has become more accurate sooner.
| HIV Ab tests | What it detects | Earliest time the test can be done | Ideal time for testing |
|---|
| First generation | IgG | 35–45-day post exposure 50% accuracy | 95% after 6 weeks > 99% after 6 months |
| Second generation | IgG and recombinan antigens | 25–35 days post exposure 50% accuracy | 57 days post exposure is 99% accurate |
| Third generation | IgG and IgM | 22–35 days post exposure 50% accuracy | 50 days post exposure is 99% accurate |
| Fourth generation | IgG, IgM and P24 | 15–20 days post exposure 50% accuracy | 44 days post exposure is 99% accurate |
HIV viral load tests: when they can be done and their accuracy
Viral load tests check for the actual HIV virus, not the body’s response to that virus. For that reason, they can detect HIV much earlier and are used for early HIV detection
| Viral load tests | What they detect | Earliest time the test can be done | Ideal time for testing |
|---|
| HIV RNA | Cutoff: 50 copies | 10–15 days 90% accurate | 42 days post exposure 99% accurate |
| Ultrasensitive | Cutoff: 1–5 copies | 5 days post exposure 90% accurate | 42 days post exposure 99% accurate |
*These tests are not FDA approved for HIV diagnosis but nevertheless are widely used to detect HIV early on. If such a test is positive, the diagnosis needs to be confirmed with another test.
Two common HIV screening approaches
Here we discuss two different testing approaches for HIV screening.
Screening is done to find the infection although the patient is not symptomatic.
Both approaches are widely used. Which one is used depends on test availability and the timing of sexual contact. Let’s talk about each approach in more detail.
New testing approach
The fourth-generation HIV test for Ag and Ab is the preferred test for HIV screening and is currently widely used.
What it detects
This test can detect the antibody and antigen (P24) at the same time.
When it is preferred
The P 24 antigen can be seen a bit earlier in the disease process, which is why the accuracy of this test is better for early HIV detection than that of an Ab test.
Test interpretation
- If the test is negative
- If the fourth-generation test is negative and done after enough time has passed since exposure, no other testing is necessary.
- If the test is positive
- If the fourth-generation test is positive, an HIV 1/2 Ab test to confirm positivity is necessary.
- A confirmatory test will also tell us what type of HIV virus has been detected, which helps with treatment planning.
Old testing approach
This test is more widely available and costs less.
What it detects
The test check for HIV Ab 1 and 2
When it is preferred
This test is excellent for detecting an existing STD but not for detecting early HIV detection since early on, production of antibodies has not begun, and the test might have been performed during a time when it is not accurate.
Test interpretation
- If the test is negative
- If the HIV 1/2 Ab test is negative and done 90 days post exposure, no further testing is necessary.
- If the test is positive
- If the HIV 1/2 Ab test is positive, the next step is to do a western blot. This is a confirmatory test that is performed automatically on the same blood sample.
- If the HIV 1/2 Ab test is positive and the confirmatory western blot is positive, the diagnosis of HIV is established
- If the HIV 1/2 Ab test is positive but the western blot test is negative, the test is considered indeterminate and further testing with HIV RNA need to be done.
Please note that with any approach, a positive test needs to be confirmed with a different HIV test.
| What it detects | What is it good for? | The earliest it can be done | When the test is most reliable | If the test is positive |
|---|
| New approach | HIV Ab and P24 | Early HIV | 2 weeks post exposure 95% accurate | 42 days post exposure 99% accurate | Confirmatory HIV Ab test is necessary |
| Old approach | HIV Ab (third generation): IgG and IgM | Existing HIV | 22 days post exposure 50% accurate | 50 days post exposure 99% accurate | Confirmatory western blot test is |
Early detection HIV testing
Here we’ll go over HIV testing at the earliest possible time. We’ll address the accuracy of the tests, timing, and test interpretation.
The best tests for early detection of HIV are the HIV Ag/Ab or HIV viral load (RNA PCR).
| Viral load tests | What it detects | Earliest time the test can be done | Ideal time for testing |
|---|
| Fourth generation HIV | IgG, IgM and P24 | 13 days post exposure 50% accurate | 44 days post exposure 99% accuracy |
| HIV RNA | Can detect as few as 50 virus copies | 10–15 days 90% accurate | 42 days post exposure 99% accurate |
| HIV RNA (ultrasensitive) | Can detect as few as 10 virus copies | 5 days post exposure 90% accurate | 42 days post exposure 99% accurate |
It is ideal if both early detection HIV tests, fourth-generation HIV and HIV RNA, are done at the same time.
Person considered negative
Both tests are done at the ideal time after exposure (see above), and they are both negative
Repeat test is necessary
If both tests are negative, but clinical suspicion is high or it was a high-risk contact, repeat the test in 2 weeks
Person considered positive
- If both the HIV Ag/Ab test and RNA PCR (viral load) test are positive, this confirms an HIV infection of unknown duration.
- HIV Ab test is necessary to start the treatment
- If HIV Ag/Ab is negative but RNA PCR (viral load) is positive, this is highly suggestive of early HIV infection. A repeat HIV Ag/Ab test to confirm is necessary in a few weeks.
Summary of early HIV test interpretation
| Fourth-generation HIV test | HIV RNA | Interpretation |
|---|
| + | + | A person is considered HIV-positive. HIV ½ Ab test is needed to start the treatment. |
| - | - | Two possibilities
1. No HIV infection if the test was done 6 weeks after exposure
2. If clinical suspicion is high and the test was done before the ideal time, repeat the test in 2 weeks |
| - | + | Early HIV infection. Repeat the fourth- generation test in a few weeks to confirm. |
This table summarizes positive and negative results for available HIV tests.
It should help you understand the following:
- what a positive test means for each test and if another confirmatory test is necessary
- what a negative test means, its accuracy, and if another test is necessary
*Please note that for any positive test, a confirmatory test, usually of a different type, is necessary.
**A negative test is usually conclusive unless it is high-risk person and the test was done during the period when that particular test is not accurate.
| If test is positive | If test is negative |
|---|
| HIV Ab | Might have HIV
Confirmatory tests that are necessary and can be done:
- western blot
- HIV RNA
- HIV p24
| or
- Repeat test might be necessary if the exposure is recent (less than 7 weeks ago) and was high risk
|
| Fourth generation (Ag and Ab) | Might have HIV
Confirmatory tests that are necessary and can be done:
| or
- Repeat test might be necessary if the exposure was recent (less than 6 weeks ago) and was high risk
|
| HIV RNA | Might have an HIV.
Confirmatory tests that are necessary and can be done:
or
| or
- Repeat test might be necessary if the exposure was recent (less than 6 weeks ago) and was high risk
|
| HIV P24 | Might have HIV
Confirmatory tests that are necessary and can be done
or
| or
- Repeat test might be necessary if the exposure was recent (less than 6 weeks ago) and was high risk
|
| Western blot | Might have HIV
Confirmatory tests that are necessary and can be done:
or
| Negative test means:
or
- Repeat test might be necessary if the exposure was recent (less than 6 weeks ago) and was high risk
|
Frequently Asked Questions
In this section, our expert doctor will answer the most frequently asked questions about HIV testing. We have reviewed real-life patient questions and grouped them into topics so the doctor can go over them. Dr. Fuzayloff has been a practicing physician at a busy STD center in Midtown Manhattan, NYC, for over two decades.
The best HIV screening test is the fourth-generation test combined with the Ag/Ab test.
- If this test is positive, it should be followed by HIV 1 and 2 Ab test and HIV RNA PCR test.
- This test is the best at catching chronic HIV and early HIV.
How often should an HIV screening test(https://stdcenterny.com/hiv-testing-nyc.html) be considered? 
A screening test is one that is done when the person does not have symptoms. Beginning in 2006, the CDC started recommending routine HIV testing for everyone as part of routine medical care. Here we summarize who needs to consider HIV screening, and how often.
Once in a lifetime
- Everybody (including people with low or no risk) from ages 13 to 64 should be screened at least once for HIV.
Once a year
- Bisexuals
- Injected drug users
- Prostitutes and their clients
- People with multiple sex partners
- People who have had unprotected sex
- Prisoners
- Transgender people
More than once a year
When do doctors advise that HIV testing be done?
Doctors should consider HIV testing and urge that it be done when a person has early HIV symptoms or is at high risk.
Symptoms
Since early HIV can present with a lot of different nonspecific symptoms, the threshold for suspicion should be very low:
- Fever with other nonspecific symptoms
- Skin rash and fever
- Mouth ulcers
- Enlarged lymph nodes
- Diarrhea
- Loss of appetite and weight loss
High-risk persons
People who had high-risk contact should be screened and follow-up testing should be done 4–6 months later depending on the initial test results.
- MSMs
- Injected drug users
- Sex workers or their clients
- People with multiple sex partners
- People practicing unprotected sex
- Prisoners
- Transgender people
How soon after high-risk contact should an HIV test be performed?
The early-detection ultrasensitive HIV RNA PCR test is sensitive 5 days after exposure.
Tell me about the third-generation HIV antibody or ELISA test—how accurate is it and how is it interpreted?
What does it detect?
It is a third-generation antibody test that checks for IgG and IgM for both the HIV 1 and 2 viruses.
When is it used?
- It is used as one of the available screening methods.
- Very accurate for chronic HIV: 100% accuracy
- Not very sensitive for early HIV detection
Test interpretation
- If the test is positive
- If the test is positive, it needs to be reconfirmed with a western blot test.
- If the test is negative when done for screening
- If the test is negative 50 days after exposure (99% accurate), no further testing is necessary.
- If the test is negative after high-risk contact
- Fourth-generation test or HIV RNA test is necessary
What about the fourth-generation HIV test: how accurate is it and how is it interpreted?
What does it detect?
The fourth-generation HIV test, unlike the regular antibody test, detects both Ag (P24) and Ab.
When is it used?
- Very accurate with chronic HIV detection: 100% accuracy
- Accurate for early HIV detection
- 80% more accurate than Ab test alone for early HIV infection
Test interpretation
- If the test is positive
- If the test is positive, it needs to be confirmed with a western blot test.
- If the test is negative when done for screening
- If the test is negative 44 days after exposure, no further testing is necessary.
- If the test is negative after high-risk contact
- HIV RNA test is more accurate for early detection
- or
- Repeat fourth-generation test in a few weeks
- or
- Do HIV 1/2 Ab test 3 months later
Please tell me about the HIV confirmatory test (aka western blot)
The HIV 1/2 confirmatory test is also known as the western blot test.
What does it detect?
It differentiates between HIV 1 and HIV 2 viruses.
When is it used?
It is usually done as a confirmatory test for positive screening HIV tests (fourth-generation, ELISA third-generation, or HIV RNA viral load test ).
Here are the reasons it is done only as a confirmatory test:
- It is very time-consuming test, unlike commonly used screening HIV tests; it can take weeks for the lab to provide the results.
- It can take up to two months after contracting HIV for the test to become accurate.
- It is not ideal for detecting HIV 2 virus (0.2% of all HIV cases). A special western blot test for HIV 2 should be requested.
I think might have been exposed to HIV when I had sex 2 months ago. I’m very uncomfortable being screened for HIV. I’ve had multiple blood tests (for other medical reasons) since but not for HIV.
Can another test give any hints about a person’s HIV status?
An HIV test should be done, but we can look at other things that indirectly can indicate whether HIV is likely:
- Complete blood count (CBC)
- White blood cell count goes down initially
- Number of atypical lymphocytes goes up
- Anemia (low hemoglobin and hematocrit on the test)
- Thrombocytopenia (low platelet counts on the test)
- Heterophile Ab test
- Mononucleosis test can become positive
Why is detecting HIV early so important?
It helps to diagnose the patient with HIV earlier so treatment can be initiated and transmission of the virus to other people can be avoided.
Can I tell how long I have had HIV from the blood test?
It is impossible to say how long you have had HIV based on the blood test unless it was recently contracted.
Here’s why:
- When the antibody detected, it is the end of the first HIV stage (acute HIV) and the beginning of chronic HIV (that lasts for the rest of the person’s life).
- In most HIV-positive people, HIV antibodies can be detected in their blood within 34 days, although in some people it can take up to 3 months.
So, once the antibody is detected (in 1 to 3 months), the disease has gone into its chronic stage and is permanent.
What are the most common reasons for indeterminate HIV tests?
There are three different reason for indeterminate HIV tests. Two of them are related to early HIV infection and the third is not HIV related.
Let’s discuss them briefly here!
Two HIV-related reasons
Early infection
The most common reason for an indeterminate HIV test is early infection.
- Both fourth-generation HIV test (Ag/Ab) or third-generation HIV Ab test can detect antibodies before the western blot confirmatory test meets the criteria to be positive. It takes longer to establish an HIV diagnosis based on the western blot test.
HIV 2 virus
0.2% of HIV infections are caused by the HIV 2 virus.
- Screening tests can pick up HIV 1 and 2, but the western blot confirmatory test checks for the HIV 1 virus—the most common one—only. So, a special western blot confirmatory test is necessary to confirm HIV 2 virus.
Non-HIV related reasons
Some medical conditions other than HIV can cross-react and be falsely sensed as HIV in testing. Here are the most common:
- Cancer
- Autoimmune conditions (lupus, arthritis, etc.)
- Pregnancy
- Flu vaccine
How should I proceed if my test results are indeterminate?
If the initial test (Ab or Ag/Ab) is showing positive results but the confirmatory test (western blot) is negative, the HIV test is considered indeterminate.
What is the next step?
1. The blood test for viral load (HIV RNA) is necessary:
- If the viral load on the HIV RNA test is more than 1,000 copies, the diagnosis is confirmed.
- If the viral load is less, a new test should be done using a different blood specimen. It can be done 1 week later.
2. Another way of confirming indeterminate HIV test results is to repeat the HIV 1/2 Ab test in 3 months.
*Please keep in mind that with an indeterminate HIV test, avoidance of sex is recommended until further testing has been done.
I am positive on a fourth-generation HIV test but negative on a western blot confirmatory test. What are my chances of being positive?
The chance of your being HIV-positive is less than 10%.
What percentage of people have HIV but don’t know it?
Out of 1.5 million people in the United States who have HIV, 13.5% don’t know they have it for one reason or another. Nearly 40% of newly diagnosed HIV cases were transmitted by people who did not know their HIV status.
When is HIV transmission highest?
HIV transmission is usually highest in the early stages of HIV, when the viral load is very high.
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