No single specific laboratory test can confirm the diagnosis of acute rheumatic fever. Laboratory evidence of a previous streptococcal infection is confirmed by a search for the organism itself (i.e., culture) or evidence of an immune response to a group A streptococcal antigen. The throat culture remains the gold standard for confirmation of the presence of group A streptococci, although rapid antigen detection tests are available. All patients suspected of having acute rheumatic fever should have at least one throat culture performed before beginning antibiotic therapy. Rapid antigen detection tests may be used if it is recognized that these tests have reduced sensitivity. For small numbers of group A streptococci, the test may be falsely negative. On the other hand, because the specificity of most of these tests is quite good, a positive result of a rapid antigen detection test provides evidence of group A streptococci. If a rapid antigen detection test is negative, a throat culture should be obtained in patients in whom rheumatic fever is suspected.

 

Streptococcal antibody tests are another method of documenting the presence of a previous group A streptococcal infection. The most commonly used test is the ASO test. Other tests that may be used are the anti-DNase B test and the AH test. A commercially available agglutination screening test is less satisfactory because of its technical difficulties. An elevated streptococcal antibody titer is clear evidence of a previous group A streptococcal infection, but a more reliable way of demonstrating the earlier infection is by showing a rise in titer between acute and convalescent sera. The ASO test reaches its peak 3{endash}–6 wk after infection, whereas the anti-DNase B test reaches its peak slightly later (6{endash}–8 wk). If acute and convalescent sera are tested, they should be tested simultaneously. Values defining an elevated titer may vary with the age of the patient, the interval since the streptococcal infection, and the population.

 

Acute-phase reactants such as the ESR or CRP are usually elevated at the onset of acute rheumatic fever. However, these tests are nonspecific. Determination of rheumatoid factor, tests for the presence of antinuclear antibody, and determination of the complement level are rarely helpful in making a diagnosis of acute rheumatic fever. Occasionally, nonspecific elevations of serum gamma globulin may be seen.

 

The electrocardiogram may indicate a 1st-degree heart block (prolonged PR interval), and on rare occasions, 2nd- or 3rd-degree block may also be present. In first attacks, electrocardiograms are otherwise usually unremarkable. In patients with chronic rheumatic heart disease, electrocardiographic manifestations of resulting cardiac disease, such as left atrial enlargement, may be evident.

 

No specific findings are revealed by the common, chest roentgenogram, but cardiomegaly is common, especially in individuals with significant carditis.
Some individuals with subclinical evidence of valvular disease may show valvular regurgitation on two-dimensional Doppler echocardiography. This observation may explain why many patients without evidence of carditis at the time of the acute attack present in the 4th or 5th decade of life with evidence of mitral valve disease. An echocardiogram is useful in evaluating patients suspected of having rheumatic fever or rheumatic heart disease