Rheumatology Lab Testing

Rheumatology Lab Testing – Ben Boone

  • 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

 

 

Cryoglobulin Evaluation

  • Clinical context drives interpretation of the test:
    • The presence of detectable cryoglobulins cryoglobulinemic vasculitis
  • Reported as qualitative (positive or negative) and quantitative (percentage = “cryocrit”)
    • This test is highly prone to collection error
    • To perform this test, you need 6 full red-top tubes, kept between 37-39°C from the moment they are collected. This is accomplished using a thermos with a thermometer aka the “Cryo Kit”. At VUMC, this kit can be found in the immunopathology lab on the 4th floor. Place water at 39°C in the kit, then collect the blood at bedside with a nurse, placing each labeled tube directly into the water. Once all 6 tubes are collected, take the Cryo Kit back to the lab. If the temperature is less than 37°C, the lab won’t accept it
  • Essential Cryoglobulinemia (rare ~ 10%): cryoglobulinemia w/o associated disease
  • Cryoglobulinemic vasculitis: small vessel vasculitis with a highly variable presentation
    • Arthralgias, peripheral neuropathy and renal disease (usually MPGN) are common
    • Skin involvement (purpura and erythematous macules, usually in colder areas of the body like the lower extremities)
  • Type I Cryoglobulins
    • Monoclonal immunoglobulins (Igs); usually IgG or IgM; rarely IgA or light chains
    • Classically associated with digital ischemia, skin necrosis, and livedo reticularis
    • Associated with Waldenström macroglobulinemia, multiple myeloma, MGUS, and CLL
  • Mixed Cryoglobulins (these do not produce a single monoclonal antibody like Type I)
    • Type II
      • A mixture of a monoclonal Ig (IgG, IgM, or IgA) combined with polyclonal Igs
      • Associated with chronic viral infections HCV (most common), HBV, and HIV
      • Less commonly seen in autoimmune diseases like SLE and Sjogren’s
    • Type III
      • A mixture of polyclonal IgG and polyclonal IgM
      • Associated with the reverse profile of Type II; most commonly with autoimmune diseases; less commonly with infections

 

Extended Myositis Panel

  • Can be sent when suspecting various forms of myositis such as dermatomyositis, polymyositis, anti-synthetase syndrome, etc.
  • Ordered as “Myositis extended Pnl-ARUP
    • Includes 19 separate antibodies with a summary interpretation provided at the end
  • P155 (TIF1-gamma) and P140 (NXP-2) antibodies are particularly associated with malignancy in pts > 30 y/o (ensure age-appropriate cancer screening)
    • PET/CT and pelvic U/S in female patients should be pursued to search for malignancy