STREPTOCOCCAL ANTIBODY TESTS

Species of Gram positive bacteria from the genus Streptococcus are capable of causing infections in humans. There are several disease-causing strains of streptococci. These strains have been categorized into groups (A, B, C, D, and G), according to their behavior, chemistry, and appearance.

Each group causes specific types of infections and symptoms. For example, group A streptococci are the most virulent species for humans and are the cause of "strep throat," tonsillitis, wound and skin infections, blood infections (septicemia), scarlet fever, pneumonia, rheumatic fever, Sydenham's chorea (formerly called St. Vitus' dance), and glomerulonephritis.

While the symptoms affected individuals experience may be suggestive of a streptococcal infection, a diagnosis must be confirmed by testing. The most accurate common procedure is to take a sample from the infected area for culture, a means whereby the bacteria of interest can be grown and isolated using various synthetic laboratory growth media. This process can take weeks. A more rapid indication of the presence of streptococci can be obtained through the detection of antibodies that have been produced in response to the infecting bacteria. The antibody-based tests can alert the physician to the potential presence of living infectious streptococci.

The presence of streptococci can be detected using antibody-based assays. Three streptococcal antibody tests that are used most often are known as the antistreptolysin O titer (ASO), the antideoxyribonuclease-B titer (anti-Dnase-B, or ADB), and the streptozyme test.

The antistreptolysin O titer determines whether an infection with the group A Streptococcus has precluded the development of post-infection complications. The term titer refers to the amount of antibody. Thus, this test is quantitative. That is, the amount of specific antibody in the sample can be deduced. In an infection the amount of antibody will rise, as the immune system responds to the invading bacteria. These complications include scarlet fever, rheumatic fever, or a kidney disease termed glomerulonephritis.

The ASO titer is used to demonstrate the body's reaction to an infection caused by group A beta-hemolytic streptococci. The beta-hemolytic designation refers to a reaction produced by the bacteria when grown in the presence of red blood cells. Bacteria of this group are particularly important in suspected cases of acute rheumatic fever (ARF) or acute glomerulonephritis. Group A streptococci produce the enzyme streptolysin O, which can destroy (lyse) red blood cells. Because streptolysin O is antigenic (contains a protein foreign to the body), the body reacts by producing antistreptolysin O (ASO), which is a neutralizing antibody. ASO appears in the blood serum one week to one month after the onset of a strep infection. A high titer (high levels of ASO) is not specific for any type of poststreptococcal disease, but it does indicate if a streptococcal infection is or has been present.

Tests conducted after therapy starts can reveal if an active infection was in progress. This will be evident by a decreasing antibody titer over time, as more and more of the streptococci are killed.

The anti-DNase-B test likewise detects groups A beta-hemolytic Streptococcus. This test is often done at the same time as the ASO titer. This done as the Dnase-based test can produce results that are more variable than those produced by the ASO test. This blanket coverage typically detects some 95% of previous strep infections are detected. If both tests are repeatedly negative, the likelihood is that the patient's symptoms are not caused by a poststreptococcal disease.

The final antibody-based test is a screening test. That is, the test is somewhat broader in scope than the other tests. The streptozyme test is often used as a screening test for antibodies to the streptococcal antigens NADase, DNase, streptokinase, streptolysin O, and hyaluronidase. This test is most useful in evaluating suspected poststreptococcal disease following infection with Streptococcus pyogenes, such as rheumatic fever.

The streptozyme assay has certain advantages over the other two tests. It can detect several antibodies in a single assay, is quick and easy to perform, and is unaffected by factors that can produce false-positives in the ASO test. However, the assay does have some disadvantages. While it detects different antibodies, it does not determine which one has been detected, and it is not as sensitive in children as in adults.