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Blood and Urine Tests

  • The diagnosis of chronic rheumatic diseases of childhood can be complex. 
  • Laboratory tests are used to support the diagnosis of the majority of rheumatic illnesses, although no one test (or combination of tests) can absolutely confirm a diagnosis.
  • Laboratory tests can flag an underlying muscle disease, such as a muscular dystrophy, but normal blood tests do not completely exclude an inherited muscle disorder.
  • Laboratory testing is integral to monitoring disease activity over time, monitoring immunosuppressive medications and to clinical studies and clinical trials.
  • Laboratory tests can be useful to differentiate rheumatological diseases from other differential diagnoses and mimics of rheumatic disease. 
  • The roles of the most commonly requested laboratory investigations are discussed.  
  • The role of investigations before starting immunosuppressive treatment is discussed. 

Key points to remember 

  • Blood tests can be normal despite pathology. 
  • Rheumatoid factor is usually negative in children with arthritis. 
  • Serum uric acid is not useful in the investigation of arthritis (children rarely get gout !) 
  • High creatine kinase (CK) levels should lead to a referral (note test values > 500 IU/L can not be due to “physical activity” or a fall).
  • Dark urine (“coca-cola like”) can be a sign of acute muscle damage and can indicate an underlying inherited muscle disorder.
  • Do not delay referral waiting for results if there is clinical concern.
  • Blood tests are not always needed for a referral.

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