Rheumatology Lab Testing

• Rheumatologic labs must be interpreted in the context of the clinical situation
• It is challenging to interpret positive/abnormal labs with low pretest probability
• Sensitivities and specificities referenced below refer to scenarios with moderate-to-high probability clinical situations

Rheumatoid Factor (RF)

• A group of antibodies targeted against the Fc region of IgG
• When positive in RA patients, called “seropositive RA” (typically more symptomatic)
• A higher RF level is more specific for RA
• 70 - 80% sensitive for RA, but not very specific
  • Present in 5-15% of healthy people
  • Commonly positive in Sjogren’s, Mixed Connective Tissue Disease (MCTD), Mixed Cryoglobulinemia, SLE, and rarely inflammatory myopathies
  • Can be positive some infectious diseases (i.e. malaria, hep B/C, rubella)
  • Often positive in RA patients before Anti-CCP and even before symptoms/diagnosis
  • Can be used as a surrogate when mixed cryoglobulinemia is suspected, since cryoglobulins can take up to a week to result and RF results in <48 hour
     

Anti-cyclic Citrullinated Peptides (Anti-CCP)

• Very specific for RA (>95%); only 70% sensitive for RA
• When positive in RA patients, called “seropositive RA” (typically more symptomatic)
• A high titer is associated with more aggressive disease in RA

Anti-nuclear Antibodies (ANA)

• Don't order for nonspecific symptoms – High Value Rheumatology Care Recommendation
• Should always be ordered with the reflex survey (ANA w/ Rfx ENA/DNA)
• Reported as both a titer and a pattern
  • Titer
    • At VUMC, 1:80 is considered “positive”; however, a higher titer is more specific (albeit less sensitive) for ANA-associated rheumatologic disease
    • ~30% of the general population has a “positive” ANA at 1:40, whereas only ~1% of people have a true ANA-associated rheumatologic disease
  • Pattern (three clinically significant ones)
    • Smooth/homogenous – associated with Anti-dsDNA and Anti-histone antibodies
    • Speckled – associated with Anti-RNP, Anti-Smith, Anti-SSA/Ro, Anti-SSB/La
    • Nucleolar – associated with Anti-Scl-70
• If ANA is ≥ 1:80 the following studies will be sent:
  • Anti-dsDNA (Quantitative and Qualitative)
    • Classical teaching is anti-dsDNA antibodies are specific for SLE. However, this depends on lab methodology; VUMC uses ELISA assay (less specific)
    • In some pts, dsDNA varies with disease activity
    • If dsDNA is positive, there is an increased risk of renal lupus
  • Positive if > 25 IU/mL ~70% sensitive and specific for SLE

• • Anti-SSA/Ro (Qualitative)
    • Non-specific; ~70% sensitive for Sjogren’s; present in ~30% of SLE and MCTD pts
    • Present in ~20% of RA and idiopathic inflammatory myopathy pts
    • Maternal positivity for SSA is associated with congenital heart block in infants
  • Anti-SSB/La (Qualitative)

• Similar profile to Anti-SSA/Ro but less common
• ~50% of Sjogren’s and ~20% of SLE/MCTD pts
• Very rare to have Anti-SSB w/o Anti-SSA in a pt with true Sjogren’s disease
  • Anti-Scl-70 (Qualitative)
    • Very specific for systemic sclerosis (>99%); 20-60% sensitive for systemic sclerosis
    • ~70% sensitive for diffuse cutaneous systemic sclerosis
    • Only ~10% sensitive for CREST syndrome
  • Anti-Smith (Qualitative)
• Very specific >99% for SLE; but only ~20% sensitive for SLE
  • Anti-RNP (Qualitative)
• Required for MCTD diagnosis; present in ~40% of SLE; rarely in systemic sclerosis

Other notable ANAs

• Anti-histone: Associated w/drug-induced lupus (VUMC order: Histone IgG-ARUP)
• Anti-centromere: Seen in limited scleroderma (CREST syndrome)
• Anti-neutrophil Cytoplasmic Antibodies (ANCA)
  • Qualitative: p-ANCA, c-ANCA, negative, or indeterminate
  • Quantitative titers: anti-proteinase 3 (PR3), anti-myeloperoxidase (MPO) IgG antibodies

C3 and C4

• Complement levels should be checked in any pt in whom you suspect active SLE
• Complement may also be low in diseases that decreases the liver’s synthetic function
• ↓ C3/C4 in diseases that form immune complexes, activating the classic complement pathway: mixed cryoglobulinemia, Sjogren’s, MPGN, and antiphospholipid syndrome

C-reactive Protein (CRP) and Erythrocyte Sedimentation Rate (ESR)

• • Both tests are non-specific markers of inflammation
  • Should be ordered alongside other rheumatology labs to corroborate disease activity
• CRP measures a specific protein made by the liver that activates the complement pathway
  • CRP is IL-6 dependent
  • ↓ IL-6 (pts on tocilizumab or tofacitinib) results in ↓CRP regardless of disease activity
• ESR is the rate that RBCS settle in a standardized tube in one hour
  • Fibrinogen is the primary driver of the ESR
  • The positive fibrinogen binds to the negatively-charged RBCs, allowing them to settle faster in the tube as RBCs no longer repel each other (both negatively charged)
  • Both Low fibrinogen (DIC or HLH) and anemia falsely lower the ESR

Creatinine Kinase (CK)

• CK can be elevated by exercise, rhabdomyolysis, endocrinopathy, cardiac disease, renal disease, malignancy, medication effect, neuromuscular disease, connective tissue disease
  • Vigorous exercise can elevate CK up to 30-times the upper limit of normal
• CK can be elevated in asymptomatic pts (should be rechecked in 1 wk to ensure resolution)
• Elevated CK in pts with objective proximal muscle weakness can direct the differential diagnosis to inflammatory myopathies (dermatomyositis, inclusion body myositis, polymyositis, and immune-mediated necrotizing myopathy)
  • Notably, CK is normal in polymyalgia rheumatica