HIV Antibody Testing
Positive EIA (or ELISA) with Negative Western Blot
Positive EIA with Indeterminate Western Blot
by Patricia Yeargin, MN, MPH

The EIA (or ELISA) is the first test done when a test for HIV is ordered. If the EIA is positive then a Western Blot is done to confirm it. A positive EIA (or ELISA) and a negative Western Blot is a negative HIV test. You don't need to do additional testing; in fact, some labs report that simply as a negative HIV test, so as not to confuse anyone. The EIA is extremely sensitive, and deliberately has a low cutoff for being positive, which is why the Western Blot is done in the first place.

Much of the confusion around HIV antibody testing centers around the so-called "window period of seronegativity," which is the time that elapses between infection with HIV and development of detectable antibodies on the HIV test. The press, and an occasional health care worker, sometimes confuse this period with the asymptomatic period of HIV infection. This leads to the common misconception that an exposed person must be HIV-tested every year for >10 years after their exposure to HIV! In reality, the window period of seronegativity varies from person to person. The current average with state-of-the art antibody testing is 25 days; and the standard outside limit is 6 months, by which time >95% of people who are actually infected will have seroconverted. There have been very rare documented cases of longer window periods. A recent example was an occupationally-exposed health care worker who became co-infected with Hepatitis C at the same time she was infected with HIV. She took 10.5 months to convert on the HIV antibody test. Of note, she was very ill during the post-exposure period; presumably, the dual assault on the immune system contributed to this delay in seroconversion.

Follow-up for a Positive EIA (or ELISA) with Negative Western Blot is the same as for a Negative HIV test, that is:

  1. If the client has had no unprotected sex or potential exposure to HIV in the past 6 months, and therefore cannot be in the "window period of seronegativity," then a positive EIA (or ELISA) with negative Western Blot is considered a negative HIV test. No follow-up is indicated other than education on how to stay negative. You would do exactly the same thing if the EIA was negative. (If the EIA is negative a Western Blot will not be done.)

  2. If the client has had unprotected sex or a potential exposure to HIV in the past 6 months, a negative HIV test means that there is still a possibility that he or she is in the "window" period, that is, the exposure has been recent enough that antibodies may not have reached a detectable level. It doesn't matter if the negative test is negative by a positive EIA (or ELISA) with negative Western Blot, or a negative EIA, the follow-up is the same, it depends on recency of exposure.

Follow-up for a Positive EIA with Indeterminate Western Blot is another matter, and and is definitely different from the follow-up for a positive EIA (or ELISA) with negative Western Blot. Again, if the person has had a potential HIV exposure in the past 6 months, the concern is that they might be in the process of seroconverting and just haven't developed all the antibody bands necessary to read it as a positive Western Blot. This situation may present as:

  1. Recent exposure, or potential exposure to, HIV in the past 6 months, particularly unprotected sex or needle-sharing, and;

  2. Unexplained symptoms, such as fever, rash, swollen lymph nodes, fatigue; consistent with primary HIV infection, not explained by any other diagnosis; which are resolving or have recently resolved spontaneously (except for the nodes, which often are persistently enlarged).

When a patient presents like this, whether they had a positive EIA with indeterminate Western Blot or a negative HIV test, consider a direct HIV antigen test. This kind of test directly detects the virus, not the antibodies to the virus. There are several direct HIV antigen tests that can be used, such as the qualitative PCR, the P-24 antigen, or the commercially available viral load test (Roche's quantitive PCR test, also known as the "HIV-1 Amplicor".) If such a test detected virus, the clinician should talk with the person about whether they were willing to go on three antiretrovirals. The client must understand what s/he is getting into, be motivated to take the medications, and be willing and able to commit to long-term therapy (Information on Adherence and Drug Resistance is available at If you're not experienced in this, do a rapid referral to someone who treats HIV infection, because treatment, if the client is willing, can improve their overall outcome. Treatment guidelines for adults and adolescents are available at

Direct HIV Antigen Tests
Viral Load: The viral load test is sensitive, widely available and it is useful in the next phase of treatment if it is positive. Depending on the type used, it will show as low as 50-400 viral copies per ml; you will need to check with the lab you use about its lower limit of detection. If the viral load test is positive, it will show an actual number of viral copies, which will be useful in the next phase of treatment. During primary HIV infection, viral copies may peak as high as 20,000,000 copies per ml., but detection of any amount of virus would be considered a positive test insofar as whether or not the person is infected with HIV. In addition to the FDA-approved HIV-1 Amplicor™ another widely available test is the Branched-Chain DNA (B-DNA) test for HIV, made by Chiron. It measures viral load in a different manner than the Amplicor, which uses a quantitative polymerase chain reaction (Q-PCR) method. The B-DNA method generally results in lower viral readings than the Q-PCR method. When tracking a patient's viral loads over time, it is best to compare test results obtained by the same method.
P-24 Antigen: The P-24 antigen test is not very sensitive, and will only show up as positive if there is a huge amount of viral replication going on. The test is used for screening donated blood, and the literature suggests that it will become positive 6 or 7 days before the HIV antibody test becomes positive. Because such cases may take 30 days or moreafter onset of symptoms for the HIV antibody test to become positive, this suggests that one could have a negative P-24 antigen up to 21 days after symptom onset. The virus load test, in contrast, should be showing actual viral counts well before then.
Qualitative PCR: The Qualitative PCR test shows only as positive or negative, and will not be useful in guiding antiretroviral therapy even if it's positive, which necessitates a second test and further delay in treatment.

There are 3 possibilities when you re-check their HIV test (Western Blot) three months later (which you would, regardless of other tests or actions taken in the interim, if they had a positive EIA and an Indeterminate Western Blot):

  1. It will be positive, in which case, you treat them as a new seroconverter, check virus load, CD4 panel, and other baseline tests you would do on clients newly diagnosed as HIV positive, and consider triple antiretroviral therapy right away. If it has not already been started, if the patient understands the regimen, if s/he can incorporate it into his or her life, and is willing and able to commit to the therapy. (Treatment guidelines for HIV-infected adults are at and other websites.)

  2. It will become negative, in which case, they are considered uninfected;

  3. It will remain indeterminate, in which case, that individual may be a "stable indeterminate;" it can still be verified later, but you would expect the person to be actually uninfected. If in doubt, refer to an infectious disease specialist who has experience with other tests to be certain that the individual is uninfected.

If the person is in the process of seroconverting, their Western Blot will be generally be positive when you re-check it 3 months later (see #1 above). If the person just has a persistently stable positive EIA with indeterminate Western Blot, s/he is not considered positive. There are other reasons besides HIV, such as autoimmune diseases and multiparity, which could produce this picture on the HIV antibody test.

Primary or acute HIV infection refers to the early, often symptomatic phase of HIV infection. People recently infected with HIV (in one series, between 5 and 30 days after infection) are likely to have these symptoms before their HIV antibody test becomes positive. More than 70% of patients have symptoms, which may include unexplained fever (39º to 40.5º C; 102.2º to 104.9º F), fatigue, weight loss (1.4 to 10 kg; 3.1 to 22 lbs), lymphadenopathy, pharyngitis, and flat, non-pruritic maculopapular rash. Less commonly, patients may have arthalgias, myalgias, malaise, nausea, vomiting, or diarrhea, urticaria , neuropathy, aseptic meningitis, and mucosal ulcerations. This syndrome appears to be under-recognized in primary care and emergency room settings, with only about 25% of patients in one series who sought medical care for such symptoms eventually being suspected of having primary HIV infection.

Further reading on Primary HIV Infection:

  • Schacker T, Collier AC, Hughes J, et al. Clinical and epidemiologic features of primary HIV infection. Annals of Internal Medicine, 1996; 125 (4) pp. 257-264.
  • Carr A, Cooper DA. Primary HIV Infection. In Sande MA, Volberding PA (Eds) The Medical Management of AIDS, Fifth Ed. 1997; Philadelphia: WB Saunders, 89-106.

Other Resources:


Updated November 16, 1998


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