What is the role of lab tests in the workup of acute rheumatic fever (ARF)?

Updated: Dec 10, 2020
  • Author: Robert J Meador, Jr, MD; Chief Editor: Herbert S Diamond, MD  more...
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Answer

According to the AHA, any one of the following can serve as evidence of preceding group A streptococcal infection [25] :

  • Elevated or rising  streptococcal antibody titer; a rise in titer is better evidence than a single titer result
  • A positive throat culture for group A β-hemolytic streptococci
  • A positive rapid group A streptococcal carbohydrate antigen test in a child whose clinical presentation suggests a high pretest probability of streptococcal pharyngitis

Rapid streptococcal tests are principally useful for prevention of ARF. Currently available tests have 90-100% specificity, permitting immediate initiation of antibiotic therapy in patients with positive results. In contrast, the sensitivity of these tests is 70-90%, lessening the value of a negative result.

Antistreptococcal antibodies usually reach a peak titer (in Todd units) at the time of onset of ARF and are more useful for diagnosis. Specific antibodies to streptococcal antigens also indicate true infection rather than mere carriage of the organism. These antibodies target extracellular products produced by streptococci.

Antistreptococcal antibodies include the following:

  • Antistreptolysin O (ASO)

  • Antideoxyribonuclease B (anti-DNAse B)

  • Antistreptokinase

  • Antihyaluronidase

  • Anti-DNAase (anti-DNPase)

Although age, geographic location, and season affect the titers, an elevated titer of at least one of these antibodies indicates streptococcal infection in 95% of patients. ASO titers are found in 80-85% of patients with ARF.

However, note that an isolated positive ASO titer is not necessarily an indication of ARF, as it may also be found in patients with certain related diseases such as rheumatoid arthritis and Takayasu arteritis. Therefore, rising ASO titers should be combined with a careful clinical evaluation and the discovery of other antistreptococcal antibodies to support the diagnosis of ARF.

The sensitivity of throat culture as evidence of recent streptococcal infection is 25-40%. For comparison, the sensitivity of ASO titer (adults with >240 Todd U and children with >320 Todd U) is 80%. The sensitivity of an elevated ASO titer in addition to anti-DNAse B or antihyaluronidase is 90%.

Acute-phase reactants such as C-reactive protein and ESR are usually elevated and helpful in monitoring disease activity.

Other laboratory tests may be helpful but not for definitive diagnosis. Synovial fluid analysis reveals a sterile inflammatory reaction, usually with fewer than 20,000 cells/μL (mainly polymorphonuclear) without crystals.


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