Could it be possible if a patient had the ELISA test for HIV to have negative results but still have HIV?

Why or why not?

The ELISA test is like a general screening - it tests for HIV antibodies, but it doesn't want to miss anything, so it looks for anything that even resembles an HIV antibody (some things in the body are similar in structure).

If the ELISA comes back negative, that means it didn't find any HIV antibodies, or even anything that kind of looks like an HIV antibody.

If the ELISA comes back positive, a second test is done called a Western Blot to determine if it is an HIV antibody that was found, or if it was just something that is similar to HIV (but harmless, like certain amino acids).

So, in general, if the ELISA comes back negative that would mean the patient is negative.

HOWEVER, the ELISA tests for antibodies, and it takes the body anywhere from one to six months to make enough antibodies to show up on an HIV test. This one to six month period is called the "Window Period." So, if the patient was tested in the window period, he/she might test negative even when positive.

Also, though this isn't common, if someone were in the very late stages of HIV infection, they may not have enough antibodies left to show up on an HIV test (HIV attacks antibodies). However, if this were the case they would likely have other complications due to the HIV infection.

I believe HIV can lay dormant and undetectable for up to 6 months.

yes for a number of reasons... first and foremost, the test is not 100% accurate... so there can always be (although its very uncommon) false negatives and positives. Second if a person were infected and took the test before the body had a chance to make sufficient anti-bodies to be detected, they would test negative but would definately be positive.

I agree with rvermieux. Right on the money.
One thing to add:
99% of people will test positive by 13 weeks. The 6 month "catch all" is really for the ~1% of people with already compromised immune systems (chemo/radiation, transplant recipient/anyone on large dose steroids.) who take longer to make antibodies to HIV infection because of their pre-existing condition.

95% of people will test positive by 8 weeks
75% of people will test positive by 17-20 days.

And HIV is NEVER "dormant", it is a very active infection and untreated it makes more than 10 Billion copies of itself every day (with no symptoms).

Rve and BJC are both right.

There is one thing I want to add...NEVER should a person...regardless of the results...rely on ONE HIV test.

A second and third test should ALWAYS be done for confirmation of a negative or positive.

There is another HIV test that is done...called NAT (nucleoic acid test). This test is used mostly by the American Red Cross. It's expensive and generally not done in doctor's offices and hospitals. The safety of our blood supply is paramount and the use of the NAT test helps keep it that way.

EMT

Unlikely although sometimes you can get a false negative result if the test is taken right after being infected. It takes a while for the antibodies to show up.