Types of Antibodies
In order to understand the ANA (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 as normal antibodies do, attack the body's own cells.
- 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).
There is an ANA (antinuclear antibody) test which can be performed on a patient's blood sample as part of the diagnostic process to detect certain autoimmune diseases.
ANA (Antinuclear Antibody) Test Explained
To perform the ANA (antinuclear antibody) 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 commerically prepared cells on the slide surface. If the patient's serum contains antinuclear antibodies (ANA), 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 viewed under an ultraviolet microscope, antinuclear antibodies appear as fluorescent cells.
- If fluorescent cells are observed, the ANA (antinuclear antibody) test is considered positive.
- If fluorescent cells are not observed, the ANA (antinuclear antibody) 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) is the titer which gets reported. For example, if a titer performed for a positive ANA test is:
The reported titer in our example is 1:160.
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 nucleoprotein. Common in SLE (lupus).
- Peripheral - fluorescence occurs at edges of nucleus in a shaggy appearance. Anti-DNA antibodies cause this pattern. Also common in SLE (lupus).
- Speckled - results from antibody directed against different nuclear antigens.
- Nucleolar - results from antibody directed against a specific RNA configuration of the nucleolus or antibody specific for proteins necessary for maturation of nucleolar RNA. Seen in patients with systemic sclerosis.
Positive ANA Result Explained
ANAs are found in patients who have various autoimmune diseases, but not only autoimmune diseases. ANAs 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. ANAs are actually found in about 5% of the normal population.
The ANA results are just one factor in diagnosing, and must be considered together with the patient's clinical symptoms and other diagnostic tests. Medical history also plays a role because some prescription drugs can cause "drug-induced ANAs".
Incidence of ANA in Various Diseases
Statistically speaking the incidence of positive ANA (in percent) per conditon 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 - 100%
- Juvenile arthritis - 15-30%
Subsets of the ANA (antinuclear antibody) test 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-RN.
The ANA (antinuclear antibody) 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