How is a test for antinuclear antibody performed?
Because ANAs can be present in various situations, the ANA test is not used to make a diagnosis, but simply to suggest to the doctor the possibility of autoimmune disease.
To perform an ANA test, a blood sample is taken from a vein and sent to a laboratory for testing.
One form of ANA test is called the ELISA. In this test, the person's blood sample is mixed with antigens (portions of proteins that bind antibodies). If the antibody for that antigen is in the blood, the test can find it.
Laboratories often use ELISA as a first screening, but will follow up with another test called the Fluorescent Antinuclear Antibody (FANA) test to confirm the results. FANA results are reported in titers and the pattern of autoantibodies made (homogeneous, speckled, centromere, etc.).
Titer reading is determined by adding saline (salt water) to a sample of plasma (the protein-rich liquid part of blood) a certain number of times. For example, one part of plasma mixed with 40 parts of saline is a 1:40 mixture. This mixture is called a "titer." The mixture is then taken through a series of additional steps of dilution (watering down), creating tubes of 1:80, 1:160, 1:320, and 1:640 mixture, respectively. If an ANA is found at higher titer, it indicates a higher amount of ANAs.
How is an antinuclear antibody test used?
An ANA test is usually not performed as part of a routine physician visit. Instead, the test should be ordered only when there is a strong chance that a child's symptoms are caused by an ANA-positive rheumatic disease such as lupus, Sjogren’s syndrome, or mixed connective tissue disease.
An ANA test should not be ordered as a screening test for non-specific complaints such as musculoskeletal pain, fever or rash.
The ANA test is also used as a prognostic marker (a way of telling ahead of time) in children with juvenile idiopathic arthritis (JIA). (The ANA test is not used to diagnose JIA.) Those children who have a positive ANA test are at higher risk of having uveitis (inflammation inside the eye); therefore, they require more frequent eye exams by an ophthalmologist (eye doctor) compared to JIA patients with negative ANA status.