Laboratory findings consistent with the diagnosis of rheumatic fever include elevated inflammatory markers, presence of streptococcal infection, and elevated or rising antistreptolysin O antibody titer.

Laboratory findings consistent with the diagnosis of rheumatic fever include:

• Leukocytes
• C-reactive protein
• Erythrocyte sedimentation rate (ESR)

Throat culture for group A beta-hemolytic streptococci may be performed. However many patients may have negative culture when rheumatic fever develops.^[1]

Rapid streptococcal antigen test may be performed. However, it has a lower sensitivity (70%) and statistically equal specificity (98%) as throat culture; a negative test results rules out streptococcal infection.^[2]

Elevated or rising antistreptolysin O antibody titer is often noted. The antibodies usually peak approximately during fourth or fifth week after the onset of infection. Patients should be tested at intervals of two weeks to detect raising titers. Antistreptococcal antibodies may also be noted in patients who are streptococcal carriers with asymptomatic pharyngitis.