6.3 Serology

6.3 Serology

Quick Answer: Serologic testing turns a vague joint or connective-tissue complaint into a specific diagnostic direction. Rheumatoid factor and anti-CCP point toward rheumatoid arthritis, ANA screens broadly for lupus and other connective tissue disease, HLA-B27 supports the seronegative spondyloarthropathies (ankylosing spondylitis, reactive arthritis, psoriatic arthritis, IBD-associated arthritis), ASO titers confirm a recent streptococcal trigger, and two-tier Lyme serology confirms tick-borne disease. None of these tests stands alone — each only makes sense in the context of the clinical picture that prompted ordering it.

Rheumatoid Factor (RF)

RF is an antibody (usually IgM) directed against the Fc portion of IgG. It is present in roughly 70-80% of patients with established rheumatoid arthritis (RA), but its major weakness is poor specificity: RF is also positive in Sjogren's syndrome, chronic infections (hepatitis C, endocarditis), other connective tissue diseases, and in up to 5-10% of healthy older adults with no disease at all. A positive RF in isolation, without symmetric small-joint synovitis and morning stiffness lasting more than an hour, is not diagnostic of RA and should not by itself redirect a mechanical joint complaint toward a systemic diagnosis.

Anti-CCP Antibodies

Anti-cyclic citrullinated peptide (anti-CCP) antibodies are far more specific for RA than RF (specificity above 95%) and, importantly, can appear years before clinical symptoms and before RF turns positive. When a patient presents with early symmetric polyarthritis of the hands and wrists, anti-CCP is the test that most reliably confirms RA and predicts a more erosive disease course, which changes the urgency of referral to rheumatology for early disease-modifying treatment.

Antinuclear Antibody (ANA)

ANA is a screening test, not a diagnostic one. It is highly sensitive (positive in over 95% of systemic lupus erythematosus cases) but has notoriously poor specificity — a low-titer positive ANA can be found in 10-15% of the healthy general population and rises further with age, certain infections, and some medications. A positive ANA justifies further work-up (specific antibodies such as anti-dsDNA and anti-Smith, complement levels, and clinical correlation with malar rash, photosensitivity, oral ulcers, serositis, or renal involvement) rather than an automatic lupus diagnosis. A patient with diffuse joint pain and a positive ANA but none of the systemic features of lupus is far more likely to have a mechanical or fibromyalgia-type presentation than connective tissue disease.

HLA-B27

HLA-B27 is a genetic marker, not an antibody, and it does not measure disease activity — a person is either positive or negative for life. It is strongly associated with the seronegative spondyloarthropathies:

The key teaching point is that HLA-B27 is neither necessary nor sufficient for diagnosis: a patient can have ankylosing spondylitis with a negative HLA-B27, and a large fraction of HLA-B27-positive people never develop any spondyloarthropathy. The test is ordered to support a clinical suspicion already raised by history and exam — inflammatory back pain, sacroiliac tenderness, uveitis, dactylitis, or a preceding infection — not as a stand-alone screening tool for back pain in general.

Antistreptolysin O (ASO) Titer

ASO measures antibodies against streptolysin O, a toxin produced by Group A Streptococcus, and confirms a recent (within the past several weeks) streptococcal infection. It is ordered when a patient presents with migratory polyarthralgia, fever, or other features suggesting acute rheumatic fever or post-streptococcal reactive arthritis following a sore throat that may have gone untreated. A rising titer on serial testing (drawn two to four weeks apart) is more meaningful than a single value, since ASO can remain elevated for months after the acute infection has resolved.

Lyme Disease Serology

Lyme testing uses a standardized two-tier protocol:

1. First tier: an enzyme immunoassay (EIA/ELISA) screens for antibodies against Borrelia burgdorferi.
2. Second tier: if the EIA is positive or equivocal, a Western blot confirms the result by detecting antibodies against specific bacterial proteins.

Serology should be interpreted alongside exposure history and symptoms — a documented tick bite or residence/travel in an endemic region, followed by erythema migrans (the classic expanding bull's-eye rash), migratory joint pain, or facial nerve palsy. Early Lyme disease (within the first few weeks) can produce a false-negative antibody test because the immune response has not yet matured, so a clinically convincing early case should not be dismissed solely because serology is still negative.

When to Order What: A Decision Framework

Why This Matters in Practice

Serology reshapes a diagnosis by converting a purely mechanical hypothesis into a systemic or infectious one, which changes management from adjustment and rehabilitation to comanagement or referral. The recurring exam trap is treating any single positive marker as diagnostic; the correct clinical reasoning always weighs the serology against the pretest probability created by the history and physical exam findings that prompted the test in the first place.