IMMUNOSEROLOGIC TESTING - LABORATORY ASSAYS IMPORTANT IN CLINICAL RHEUMATOLOGY

OBJECTIVE:

To understand both the usefulness and limitations of currently available immunoserologic testing.

1. Autoantibodies that are worth knowing something about
   1. Antinuclear antibodies (ANA) – a SCREENING TEST only.
   2. Problems with ANA interpretation can be serious for the patient and can lead to over treatment with morbid drugs an loss of insurance coverage.
   3. Mitigating factors:
      1. Age - low titers are epi-phenomenon that occur during aging
      2. Gender - females more likely to have non-pathogenic low titer ANA's
      3. Drugs - especially pronestyl, hydralazine and INH
      4. Recent viral infection.
   4. Significant ANA found in the Extractable Nuclear (ENA)antibody panel
      1. Native (double stranded) DNA - very specific but only moderate sensitivity for true SLE.
      2. Sm (Smith) - high specificity but low sensitivity for true SLE.
      3. RNP (ribonuclear protein ) - essentially a non-specific autoantibody; however, high titers are associated with a syndrome called mixed connective tissue disease (MCTD).
      4. SS-A/SS-B - fairly specific for Sjogren syndrome.
      5. Centromere antibodies –codes for CREST
      6. SCL-70 (anti-topoisomerase ). These antibodies can be useful in assessing patients with scleroderma syndromes and can lead to high suspicion of occult pulmonary hypertension and GI abnormalities which influence prognosis
      7. Anti-Jo - high specificity but very low sensitivity for polymyositis. This may be complemented with the Myositis panel which further defines prognosis in some myopathies.
      8. SCL-70 - high specificity for scleroderma but very low sensitivity for scleroderma.
         
         
2. Antineutrophil cytoplasmic antibodies (ANCA) - large family of antibodies associated with some forms of vasculitis, periarteritis nodosa and Wegener's Granulomatosis.
   
   
3. Anti-cardiolipin/anti-phospholipid (APL) family - very important to differentiate APL as a epi-phenomenon from the pathogenic APL associated with hyper coagulable states.• beta-2-glycoprotein antibodies
   
   
4. Complement assays . These assays are also subject to over interpretation and can lead to inappropriate therapy.
   1. CH50 - used for screening for complement deficiencies only .
   2. C3 & C4 - may be associated with complement activation but not reliable because C3 also acts as an acute phase reactant and a significant number of the normal and SLI population have heterozygous C4 deficiencies.
   3. Only selected patients should be followed with C3 and C4 assays as activity indicators.

Never treat a patient based solely on ANA and complement assays.

5. Miscellaneous Assays .
   1. Erythrocyte sedimentation rate (ESR) - more helpful when within the normal range, also minor rises with age, menses and disretic renal disease.
   2. CRP - a sophisticated, high cost erythrocyte sedimentation rate.
   3. Rheumatoid factor - usually not very helpful except in very unusual presentations of rheumatoid arthritis or mixed cryoglobulinemia.
      
      
6. Lymphocyte Phenotyping.
   1. These assays are done by FACS analysis and monoclonal probes expensive and usually not helpful except in:
      1. Classification of immunodeficiency
      2. Classification and prognostic indicator in some leukemias and lymphomas.
   2. HLA typing - Their use is self evident in certain forms of transplantation, however rarely used outside of this discipline except in perhaps genetic/pedigree studies.
   3. B-27 panel - is usually not indicated in males with ankylosing spondylitis, however, determination of this HLA antigen can be helpful in unusual presentations of ankylosing spondylitis especially in females and patients with inflammatory enthesiopathies related to inflammatory bowel disease, Reiter's Syndrome, etc.
      
      
7. Synovial fluid analysis - even small amounts of synovial fluid can be helpful. Critical assessment of synovial fluid (in order of priority) includes:
   1. gram stain and culture
   2. crystal search
   3. total white counts
      
      
8. Assessment of Serum Ig levels.
   1. General: Normal levels have wide ranges and are directly related to the age of the patient.
   2. Increased levels
      • Polyclonal
      • Monoclonal
   3. Decreased levels
      • Immunodeficiency states
      • Protein wasting/losing states
        
        
9. Measurement of T-cell function :
   1. In vitro measurements - almost all current tests are not helpful in most clinical situations.
   2. In vitro tests: Anergy panels can be useful if interpreted in light of concomitant drugs and nutritional state.

References

Thomas C, Robinson JA. The Positive Antinuclear Antibody Test: When is a Positive Result Clinically Relevant?  Postgrad Med, 1993; 55-58.

Kavanaugh AF, Solomon DH. Guidelines for immunologic laboratory testing in the rheumatic diseases: anti-DNA antibody tests. Arthritis Rheum 2002;47:546–55

Autoantibodies in vasculitis. Arthritis Research & Therapy. 5(3):147-52, 2003.

Rational use of ANCA in the diagnosis of vasculitis. Rheumatology. 41(5):481-3, 2002 May.