Autoimmune antibody tests are among the most complex results to interpret in routine laboratory medicine. A positive ANA is common in the general population and does not, by itself, indicate autoimmune disease. An ANCA result needs to be read alongside the clinical picture. This article explains the most commonly requested autoimmune antibody tests, how they are performed, and what the results mean.
ANA: Antinuclear Antibody ¶
ANA is performed by indirect immunofluorescence on HEp-2 cells. A positive result means antibodies in the patient's serum bind to components of the cell nucleus. The result is reported as a titre (e.g. 1:80, 1:160, 1:320) and a pattern (e.g. Homogeneous, speckled, nucleolar). A titre of 1:40 is positive in around 25-30% of healthy individuals. A titre of 1:160 or above is more clinically significant. The pattern can guide further testing: a speckled pattern may prompt anti-Ro and anti-La testing; a homogeneous pattern may prompt anti-dsDNA.
ANCA: Anti-Neutrophil Cytoplasmic Antibody ¶
ANCA is associated with a group of vasculitic conditions including granulomatosis with polyangiitis (formerly Wegener's) and microscopic polyangiitis. ANCA is detected by ELISA and reported with specificity for PR3 (proteinase 3) and MPO (myeloperoxidase). PR3-ANCA is more commonly associated with granulomatosis with polyangiitis; MPO-ANCA with microscopic polyangiitis and eosinophilic granulomatosis with polyangiitis. A positive ANCA in the absence of clinical features of vasculitis requires careful interpretation and should not be acted upon in isolation.
Rheumatoid Factor and Anti-CCP ¶
Rheumatoid factor (RF) is an antibody directed against the Fc portion of IgG. It is positive in around 70-80% of patients with rheumatoid arthritis but is also elevated in other inflammatory conditions and in around 5% of healthy individuals. Anti-CCP (anti-cyclic citrullinated peptide) is more specific for rheumatoid arthritis. A patient who is RF-negative but anti-CCP-positive still has a high probability of rheumatoid arthritis. The combination of both positive results, in the right clinical context, is highly suggestive.
How to Use These Results Clinically ¶
Autoimmune antibody tests are screening and supporting tools, not diagnostic tests in isolation. A positive ANA in a patient with no symptoms of autoimmune disease does not require treatment. A negative ANCA does not exclude vasculitis in a patient with active renal disease and haemoptysis. These tests are most useful when ordered with a specific clinical question in mind and interpreted by a clinician familiar with the conditions being considered. The laboratory report from Pulse Path Lab Core includes a structured interpretive comment for all autoimmune panels, written by a consultant pathologist.
The Pulse Path Lab Core autoimmune antibody screen is available as a single panel at $145, with a turnaround of 3-5 business days. For ordering information or to discuss a specific clinical question, contact enquiries@pulsepathlabcore.com.