From the Guidelines
The most appropriate screening labs for autoimmune disorders include antinuclear antibody (ANA) with a cut-off point of 1:160, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), complete blood count (CBC), comprehensive metabolic panel (CMP), and rheumatoid factor (RF), with additional tests ordered based on specific clinical symptoms and suspicion of particular autoimmune conditions, as recommended by recent guidelines 1. When ordering these tests, it is essential to consider the clinical context and pre-test probability, as false positives can occur in healthy individuals.
- The ANA test should be used as an initial screening tool, with a cut-off point of 1:160, which has been shown to have a high positive likelihood ratio and diagnostic performance in unselected populations 1.
- Additional tests, such as anti-dsDNA, anti-ENA, anti-CCP, anti-SSA/SSB, and anti-TPO, should be ordered based on specific clinical symptoms and suspicion of particular autoimmune conditions.
- Interpretation of test results requires clinical correlation, as positive results do not always indicate disease, and negative results do not rule out autoimmune conditions.
- Testing should be guided by specific symptoms like joint pain, skin rashes, fatigue, or organ dysfunction, and follow-up with a rheumatologist is recommended for abnormal results, as they can determine if additional specialized testing is needed. The choice of ANA detection method is also crucial, with the indirect immunofluorescent assay (IFA) being the reference method, and solid-phase ANA screening immunoassays (SPAs) being acceptable if they have equivalent performance 1.
- The laboratory report should always detail which method was used, and when several methods are used, the results obtained with each method should be reported 1.
From the Research
Screening Labs for Autoimmune Disorders
The following labs are used to screen for autoimmune disorders:
- Complete blood count with differential
- Comprehensive metabolic panel
- Measurement of inflammatory markers such as C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR)
- Autoantibody tests such as rheumatoid factor (RF) and anti-cyclic citrullinated peptide (CCP) antibody
- Flow cytometry 2
Specific Labs for Rheumatoid Arthritis
For rheumatoid arthritis, the following labs are used:
- Rheumatoid factor (RF)
- Anti-cyclic citrullinated peptide (CCP) antibody
- C-reactive protein (CRP)
- Erythrocyte sedimentation rate (ESR) These labs have been shown to have high sensitivity and specificity for diagnosing rheumatoid arthritis, with anti-CCP antibody being more specific than the other parameters 3
Interpretation of Lab Results
When interpreting lab results, it is important to consider the possibility of discordance between different tests, such as CRP and ESR. This discordance can occur in approximately 1 in 8 patients and can be caused by various factors, including infections, connective tissue diseases, and cardiovascular events 4
- High CRP/low ESR discordance is often associated with infections, particularly in the urinary tract, gastrointestinal tract, lungs, and bloodstream
- High ESR/low CRP discordance is often associated with connective tissue diseases, such as systemic lupus erythematosus, and ischemic strokes or transient ischemic attacks 4