In order to understand the ANA test (antinuclear antibody test), it is first important to 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 an abnormality.
- Antinuclear antibodies are a unique group of autoantibodies that have the ability to attack structures in the nucleus of cells. The nucleus of a cell contains genetic material referred to as DNA (deoxyribonucleic acid).
An ANA test (antinuclear antibody test) can be performed on a patient's blood sample as part of the diagnostic process for certain autoimmune diseases.
How the Test Is Performed
To perform the ANA 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 patient's 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 test is considered positive.
- If fluorescent cells are not observed, the ANA test is considered negative.
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 (fluorescence observed under the microscope) is the titer which gets reported. Here is an example:
1:160 positive (reported titer)
Parts of an ANA Report
An ANA report has three parts:
- Positive or negative
- If positive, a titer is determined and reported
- The pattern of fluorescence is reported
Significance of ANA Pattern
ANA titers and patterns can vary between laboratory testing sites, perhaps because of variation in methodology used. These are the commonly recognized patterns:
- Homogeneous - total nuclear fluorescence due to antibody directed against DNA. Common in systemic lupus erthematosus (SLE).
- Peripheral - fluorescence occurs at edges of nucleus in a shaggy appearance. Anti-DNA and anti-nuclear envelope antibodies cause this pattern. Also seen in SLE.
- Speckled - results from antibody directed against different nuclear antigens.
- Nucleolar - results from antibodies directed against specific proteins involved in maturation of RNA. Seen in patients with systemic sclerosis (scleroderma).
Positive ANA Test Result Explained
Antinuclear antibodies are found in patients who have various autoimmune diseases, but not only in autoimmune diseases. Antinuclear antibodies can be found also in patients with infections, cancer, lung diseases, gastrointestinal diseases, hormonal diseases, blood diseases, skin diseases, and in elderly people or people with a family history of rheumatic disease. Antinuclear antibodies are actually found in about 5% of the normal population, too.
ANA test 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. 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:
- Systemic lupus erythematosus (lupus or SLE) - over 95%
- Progressive systemic sclerosis (scleroderma) - 60-90%
- Rheumatoid arthritis - 25-30%
- Sjogren's syndrome - 40-70%
- Felty's syndrome - over 75%
- Juvenile arthritis - 15-30%
Subsets of the ANA 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 ANA test is complex, but the results (positive or negative, titer, pattern) and possible subset test results can give physicians valuable diagnostic information.
Clinical Diagnosis, Todd-Sanford