Understanding the ANA Blood Test (Antinuclear Antibody Test)

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In order to understand the ANA blood test (antinuclear antibody test), it is important to first understand different types of antibodies.

  • Antibodies are proteins, produced by white blood cells, which normally circulate in the blood to defend against foreign invaders, such as bacteria, viruses, and toxins.
  • Autoantibodies, instead of acting against foreign invaders, attack the body's own cells. This is abnormal.
  • Antinuclear antibodies are a unique group of autoantibodies which have the ability to attack structures in the nucleus of cells. The nucleus of a cell contains genetic material, known as DNA (deoxyribonucleic acid).

An ANA blood test (antinuclear antibody test) is usually performed on a blood sample as part of the diagnostic process for certain autoimmune diseases.

How the Test Is Performed

To perform the ANA blood test, sometimes called FANA (fluorescent antinuclear antibody test), a blood sample is drawn from the patient and sent to the lab for testing. Serum from the blood specimen is added to microscope slides which have commercially prepared cells on the slide surface. If the patient's serum contains antinuclear antibodies, they bind to the cells (specifically the nuclei of the cells) on the slide.

A second antibody, commercially tagged with a fluorescent dye, is added to the mix of patient's serum and commercially prepared cells on the slide.

The second (fluorescent) antibody attaches to the serum antibodies and cells which have bound together. When the slide is viewed under an ultraviolet microscope, antinuclear antibodies appear as fluorescent cells.

  • If fluorescent cells are observed, the ANA blood test result is positive.
  • If fluorescent cells are not observed, the ANA blood test result is negative.

    ANA Blood Test Report

    An ANA blood test report has three parts:

    • Positive or negative for ANA
    • If positive, a titer is determined and reported
    • The pattern of fluorescence is reported

    ANA Titer

    A titer is determined by repeating the positive test with serial dilutions until the test yields a negative result. The last dilution which yields a positive result (i.e., fluorescence observed under the microscope) is the titer that is reported. Here is an example:

    Serial Dilutions:
    1:10 positive
    1:20 positive
    1:40 positive
    1:80 positive
    1:160 positive (titer reported as 1:160)
    1:320 negative

    Significance of ANA Pattern

    ANA titers and patterns can vary between laboratory testing sites due to variation in the methodology used. The commonly recognized patterns include:

    • Homogeneous - total nuclear fluorescence due to an antibody directed against DNA. Common in systemic lupus erythematosus.
    • Peripheral - fluorescence occurs at edges of the nucleus in a shaggy appearance. Anti-DNA and anti-nuclear envelope antibodies cause this pattern. Also seen in systemic lupus erythematosus.
    • Speckled - speckled fluorescense due to an antibody directed against different nuclear antigens.
    • Nucleolar - from antibodies directed against specific proteins involved in the maturation of RNA. Seen in people with scleroderma (systemic sclerosis).

      Positive ANA Blood Test Result - What Does It Mean?

      Antinuclear antibodies are found in people with various autoimmune diseases, but not exclusively. Antinuclear antibodies can also be found in people with infections, cancer, lung diseases, gastrointestinal diseases, hormonal diseases, blood diseases, skin diseases, in elderly people, or people with a family history of rheumatic disease. Antinuclear antibodies are actually found in about 5 percent of the healthy general population, too.

      ANA results are just one factor considered when a diagnosis is being formulated. A patient's clinical symptoms and other diagnostic tests must also be considered by the doctor.

      The medical history is also significant because some prescription drugs can cause "drug-induced antinuclear antibodies".

      Incidence of ANA in Various Diseases

      Statistically-speaking, the incidence of positive ANA test results (in percent per condition) is:

      Subsets of the ANA blood tests are sometimes used to determine the specific autoimmune disease. For this purpose, a doctor may order anti-dsDNA, anti-Sm, Sjogren's syndrome antigens (SSA, SSB), Scl-70 antibodies, anti-centromere, anti-histone, and anti-RNP.

      The Bottom Line

      The ANA blood test is complicated. That said, the results—positive or negative, titer, pattern, and subset test results—can provide doctors with valuable clues to help diagnose autoimmune rheumatic diseases.

      Sources:

      Peng and Craft. Textbook of Rheumatology. Ninth edition. Elsevier. Chapter 55 - Antinuclear Antibodies. 

      Clinical Diagnosis, Todd-Sanford

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