Blood tests for rheumatology conditions

Scope

General Pitfalls

  • CRP increases with BMI e.g. CRP 20 might be normal in a patient with BMI 40.
  • CRP is usually normal in myeloma.
  • Please perform CRP and Plasma Viscosity (PV) in suspected rheumatological disease – a discrepancy between PV / CRP is useful clinical information.
  • Rheumatoid factor has about a 25% false +ve and false –ve rate and is not very useful for diagnostic purposes.
  • CCP antibody is a more sensitive and specific test used in the diagnosis of rheumatoid arthritis but it can only be requested in secondary care.
  • Normal inflammatory markers can be seen in inflammatory arthritis, if clinical suspicion remains high despite normal blood tests then the patient should be referred. This is especially the case in psoriatic arthritis where inflammatory markers are frequently normal.
  • Urate is used to guide treatment rather than diagnose gout.

When to check Anti-Nuclear Antibody (ANA)?

  • An ANA should only be checked when an autoimmune disease is suspected;
    • Symptoms of autoimmune conditions include inflammatory arthritis and myalgias, rashes, mouth ulcers, raynauds, recurrent serositis (pleurisy or pericarditis) and sicca symptoms (dry eyes and mouth)
    • Many patients are referred with a positive ANA with no convincing features of an autoimmune condition. This often causes unnecessary anxiety for the patient.
  • What does a positive ANA reading mean?
    • A positive ANA reading alone does not indicate an autoimmune disease.
    • The prevalence of ANAs in healthy individuals is about 3-15%. The production of these autoantibodies is strongly age-dependent, and increases to 10-37% in healthy persons over the age of 65.
    • The design of the ANA test means that many normal individuals will have a positive test at low titres (e.g. 1:80, 1:160).
    • Even when detected at a high titre, a positive ANA result, by itself does not indicate that the patient has, or will develop, an autoimmune disease.
    • A positive ANA can occur in other conditions such as viral infections, cancers and in patients who have relatives with autoimmune conditions.

In summary:

  • Do not check ANA unless you suspect the patient has an autoimmune disease.
  • Low titre ANA is unlikely to be significant and the patient does not need to be referred.

Supporting Information

Pathway Group

This guideline has been signed off by the Western Locality on behalf of NEW Devon CCG.

Publication date: February 2017

 

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