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, but normal blood tests do not completely exclude an inherited muscle disorder (such as a muscular dystrophy).
• Laboratory testing is integral to classifying rheumatic disease, monitoring (disease activity over time and immunosuppressive medications), clinical studies and clinical trials.
• Laboratory tests can be useful to differentiate rheumatological diseases from other differential diagnoses. 
• 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 (and positive in only 5%). 
• Anti-nuclear antibodies (ANAs) are not specific for autoimmune diseases and results must be interpreted in the context of clinical findings.
• Serum uric acid is not useful in the investigation of arthritis (children rarely get gout). Hyperuricaemia may be observed in children with sickle cell disease.
• High creatine kinase (CK) levels should lead to a referral; note that test values > 500 IU/L are unlikely due to “physical activity” or a fall).
• Dark urine (“coca-cola colour”) can be a sign of acute muscle damage, an underlying inherited muscle disorder and acute renal damage (e.g., post streptococcal glomerulonephritis).
• Do not delay referral whilst waiting for results if there is clinical concern.
• Blood tests are not always needed for a referral.