HEp-2 cells are laboratory-grown human cells used in a specific blood test. This test primarily helps detect antinuclear antibodies (ANA), proteins that mistakenly target the body’s own healthy cells. HEp-2 refers to a cell line, not a type of hepatitis or a virus, despite the similar-sounding name.
The Role of HEp-2 Cells in ANA Testing
HEp-2 cells are human epithelial cells, originally derived from a laryngeal carcinoma, widely used in the indirect immunofluorescence assay (IFA) for antinuclear antibodies. These cells are particularly suited for ANA testing. They are large and flat, making them easy to view under a microscope after staining.
HEp-2 cells contain all human cellular components, including the nucleus, cytoplasm, and nucleolus, displaying hundreds of antigens. When a patient’s blood serum is applied, any autoantibodies present bind to specific targets within the cell, becoming visible under a fluorescent microscope. This visualization is important for identifying ANAs and observing their binding patterns.
Understanding the ANA Test
The Antinuclear Antibody (ANA) test is a screening tool that identifies autoantibodies reacting with cellular components. The indirect immunofluorescence assay (IFA) using HEp-2 cells is considered the gold standard method for this screening. During the test, a patient’s blood sample is diluted and applied to slides containing fixed HEp-2 cells.
A secondary antibody, tagged with a fluorescent dye, is then added, attaching to bound ANAs and causing them to glow under a fluorescent microscope. ANA test results are typically reported in two ways: a titer and a fluorescent pattern. The titer indicates the concentration of autoantibodies in the blood, expressed as a dilution ratio, such as 1:80 or 1:160. A higher titer, like 1:160 or above, generally suggests a greater concentration of autoantibodies and a higher likelihood of an autoimmune condition.
The fluorescent pattern describes how antibodies bind to specific parts of the HEp-2 cells, creating distinct appearances. Common patterns include homogeneous, speckled, nucleolar, and centromere, among others. These patterns offer clues about specific autoantibodies and potential underlying conditions. For instance, a homogeneous pattern often suggests antibodies to DNA or histones, while a speckled pattern can be associated with various conditions.
Clinical Significance of ANA Results
A positive ANA test result indicates the presence of autoantibodies but is not a definitive diagnosis. It primarily serves as a screening tool to suggest potential autoimmune activity, prompting further investigation. A positive ANA test is strongly associated with systemic autoimmune diseases, such as systemic lupus erythematosus (SLE), Sjögren’s syndrome, scleroderma, and mixed connective tissue disease. Nearly all individuals with SLE (95-100%) have a positive ANA test result.
However, a positive ANA can also occur in healthy individuals, with prevalence increasing with age. Up to 15% of healthy individuals may have a positive ANA, typically at lower titers. Certain infections, such as viral hepatitis or HIV, and specific medications can also induce a positive ANA result. Therefore, interpreting ANA results requires careful consideration of a patient’s symptoms, medical history, and other laboratory findings.
Higher titers, generally 1:160 or greater, are more likely to be clinically significant, especially when accompanied by relevant symptoms. If an ANA test is positive, additional, more specific antibody tests are often performed to pinpoint a diagnosis. The ANA test helps guide clinical decisions when combined with a comprehensive patient evaluation, rather than serving as a standalone diagnostic marker.