Frequently Asked Questions about Serology

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General Serology





General Serology FAQs

What is an antigen?

An antigen is usually a protein or carbohydrate and is part of an infectious agent such as a virus or bacterium.

What is an antibody?

Antibodies are proteins that are produced by a human or animal host in response to a foreign substance known as an antigen. The antibody attaches to the antigen and aids the host in removing the infectious agent.

What does IgG or IgM mean?

IgG stands for immune globulin G and IgM stands for immune globulin M.  Both are types of antibodies. IgM is generally produced the first time a host is exposed to an antigen. IgM will eventually decline, and then the host produces IgG, which lasts much longer. Detection of IgM indicates acute or primary infection, IgG indicates past infection or immunity.

What are paired sera, and when are they needed?

Serum (sera is plural) is the liquid portion left after the cells are removed from whole blood. Serum contains antibodies. Paired sera are required when the only test available to detect a certain disease can only detect specific IgG or total antibody. Sera drawn from a patient 14-21 days apart are tested simultaneously. If there is a significant rise in titer (amount of antibody), significant decrease in titer, or seroconversion, the patient is considered to have a current infection. If a test is available to test specifically for IgM, then only a single serum is required.

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What does reactive or nonreactive mean?

Generally, reactive means positive for the presence of the analyte being detected in a test system and nonreactive means negative for the presence of the analyte being detected in the test system.

What does tissue reactive mean?

When a patient's serum is tested for antibodies to certain viral infections part of the testing procedure is to include an uninfected tissue control whose purpose is to detect nonspecific reactions between the patient's serum and the uninfected tissue control.  If such reactions occur, results are invalid and are reported as "Tissue Reactive."

What does anti-complementary mean?

When a serum specimen nonspecifically binds or destroys complement in the complement fixation test, results are invalid and reported as "Anti-complementary." Possible causes include intrinsic factors in certain sera, contamination of specimen or medications taken by the patient.

What does ELISA or EIA mean?

ELISAs or EIAs are types of serologic tests. See the EIA page for details on how these tests work.

What does Equivocal mean?

An equivocal result interpretation indicates that the analyte being detected in the test system falls within the range between negative and positive. Usually this is within 10% of the positivity cut off value for that assay. Generally a second specimen should be drawn and submitted for testing.

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FAQ's for HIV/STD Lab

Is testing available for HIV-2 or HTLV-1?

Yes, these tests can be performed at the Centers for Disease Control (CDC) in Atlanta, GA. The results take from two to six weeks to be returned.

Is viral load testing for HIV available?

We are currently looking into the feasibility of offering this testing on a limited basis. It should be available soon.

Is a name required on HIV specimens?

Individuals may be tested using a pseudonym, such as John Doe, or may be tested using a numerical designation, but the numbers must be spelled out, such as Jane Nineteen, rather than Jane 19.

What is the turnaround time for HIV specimens?

For HIV specimens, the results of the screen can usually be obtained by telephone the day after we receive the specimen. Results from the confirmatory test (Western Blot) take 3-4 days. For Syphilis specimens, the screen results are available the day after we receive the specimens, and the confirmatory test results may take 2-5 days depending on the testing required.

How do I obtain information on OraSure testing?

We have prepared a sheet with names and telephone numbers required for setting up an account for Oral Fluid testing. This can be obtained by calling (512) 776-7657 (if you call and the voice mail answers, leave a FAX number, and it will be sent to you).

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What does Nonreactive on my HIV-1 result report mean?

Nonreactive indicates that the serum tested does not have detectable antibodies to HIV-1. There are several reasons for this:

  • Person is uninfected with HIV-1.
  • Person is in the window period immediately after infection when antibodies are not yet being produced.
  • Person is in the final disease stages when the immune system is no longer capable of producing detectable antibody.

How long is the window period in HIV infection?

A safe estimate from various studies is that the time interval between actual infection and the first positive antibody test is one to three months. The difficulty in being more definitive is pinpointing exactly when a person became infected. The longest documented time from infection to a positive EIA was eight months in a nurse who was exposed in a needle-stick accident.

What can cause a false positive HIV-1 EIA result?

  • The presence of antibodies to cellular components in which the viral antigen was grown.
  • This is common in multiparous women and in individuals who have had a blood transfusion.
  • The presence of autoantibodies in individuals with autoimmune disease.
  • The presence of antibodies to some parasitic agents such as malaria.
  • Infection with another human retrovirus.
  • In 2-5% of individuals who have recently received an influenza vaccination, there may be a temporary false positive EIA.

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What is an Indeterminate HIV-1 Western Blot?

A western blot where the band pattern does not meet the criteria published by the Centers for Disease control for Reactive.

What can cause an Indeterminate HIV-1 Western Blot?

  • The presence of antibodies to cellular components in which the viral antigen was grown.
  • This is common in multiparous women and in individuals who have had a blood transfusion.
  • The presence of autoantibodies in individuals with autoimmune disease.
  • The presence of antibodies to some parasitic agents such as malaria.
  • Infection with another human retrovirus.
  • An individual who is infected with HIV-1 and is in the process of seroconversion.

How many different types of HIV are there?

There are two major HIV viruses; HIV-1 and HIV-2. HIV-1 is most common here in the United States. HIV-2 is most common in Africa. HIV-1 has a 10 different subtypes, or clades, designated by letters A through J. Subtype B is most common here, accounting for the majority of cases identified in the US.

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What can cause a false positive RPR?

  • A history of drug abuse
  • After a person has a smallpox vaccination
  • Lupus erythematosis
  • Mononucleosis
  • Leprosy
  • Viral Pneumonia
  • Active case of Malaria

What does an inconclusive TP-PA result mean?

Inconclusive TP-PA - If Reactive results are seen with both sensitized and unsensitized particles, nonspecific agglutination is indicated. The serum may or may not be reactive for T. palladium. In the TDSHS laboratory, specimens that show (> ±) reactivity with the unsensitized particles are reported as Inconclusive due to nonspecific agglutination and are then tested by the FTA-ABS.

What does and equivocal TP-PA result mean?

Equivocal TP-PA - Specimens that show low reactivity, defined as either ( 1+) or (±) reactivity with the sensitized particles and (-) reactivity with the unsensitized particles, are repeated on the TP-PA. If the results of the repeat are the same, it is reported as Equivocal and are then tested by the FTA-ABS.

What are reagin tests?

Reagin tests: These are nontreponemal tests that measure IgG and IgM antibodies to lipoidal material released from damaged host cells and to lipids and lipoproteins produced by the treponems. Rapid Plasma Reagin Card and Venereal Disease Research Laboratory tests are two common examples of reagin tests. The antilipoidal antibodies can also be produced in response to nontreponemal diseases of an acute and chronic nature in which tissue damage occurs. Therefore, without some other evidence for diagnosis of syphilis, a reactive nontreponemal test does not confirm T. pallidum infection.

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Last updated November 30, 2015