Streptococcal Antibody Tests Health Article

Definition

If left untreated, upper respiratory or skin infections caused by Group A streptococci (Streptococcus pyrogenes) can develop complications, called sequelae. These include scarlet and rheumatic fever and a kidney disease called post streptococcal glomerulonephritis (a sequalea to respiratory but not skin infections). Streptococcal antibody tests detect antibodies to various antigens secreted by Group A streptococci. These include tests for antibodies to streptolysin O, DNase-B, and hyaluronidase that contribute to the virulence of Group A streptococcal infections. Four major streptococcal antibody tests are the antistreptolysin O titer (ASO), antideoxyribonuclease-B titer (anti-DNase-B, or ADB), anti-hyaluronidase (AH), and Streptozyme test. Other tests to determine past streptococcal infections have been developed including the anti-CHO test (possible use for rheumatic fever) and the anti-preabsorption antigen test (for glomerulonephritis).

Purpose

Tests for streptococcal antibodies are preformed in order to document a recent infection with Group A streptococcus (Streptococcus pyrogenes). The antistreptolysin O titer, or ASO, is ordered primarily to determine whether a previous Group A streptococcus infection has caused a post streptococcal disease, such as scarlet fever, rheumatic fever, or glomerulonephritis. The ASO test also detects Groups C and G streptococci, which also produce streptolysin O. Antibodies to streptolysin O are produced in approximately 75-80% of Group A streptococcus infections, but are usually not seen in skin infections caused by this organism. The antibodies are usually detected within one to two weeks following acute pharyngitis.

The anti-DNase-B (ADB) test is also performed to determine a previous infection with Group A streptococci. Antibodies to DNase-B appear almost exclusively in Group A streptoccal infections, appear somewhat earlier than those to streptolysin O, and are produced by 85-90% of persons with group A streptococcal skin infections. When used with the ASO test, the ADB test adds clincial sensitivity.

The antihyaluronidase (AH) test is used as an adjunct to ASO testing (increases clinical sensitivity when used along with the ASO test). This test is not as sensitive as the ASO test, but antibodies to hyaluronidase are produced by some patients with either skin or respiratory infections with Group A streptococci.

Streptozyme is a screening test used to detect antibodies to several streptococcal antigens. An antigen is a substance that can trigger an immune response, resulting in production of an antibody as part of the body's defense against infection and disease. The test is not as sensitive or specific as the ASO test, but can be performed within minutes, providing presumptive results that can be confirmed by use of the ASO or other more specific streptococcal antibody tests.

Precautions

Streptococcal antibody tests are performed on a blood (serum) sample collected by venipuncture. The nurse or phlebotomist performing the procedure should observe universal precautions for prevention of transmission of bloodborne pathogens. Hemolyzed blood samples are unsuitable for these tests. Increased levels of fats (beta lipoproteins) in the blood can cause false-positive test results. Antibiotic therapy can reduce the number of streptococci and decrease levels during these tests, giving a false negative. Steroids may also give false negative results. Group A streptococcal infections of the skin may not produce an ASO response. False negatives in the ASO test may arise from antibody deficiency syndromes, and false positives from hypercholesterolemia, hyperglobulinemia, and liver disorders. A false negative result in the ADB test may occur during hemorrhagic pancreatitis. The streptozyme test is more sensitive for adult patient samples than those obtained from children.

Description

Streptococcal infections are caused by bacteria known as Streptococcus. There are several disease-causing strains of streptococci (groups A, B, C, D, and G), which are identified by their clinical effects, biochemical characteristics, growth requirements, appearance on culture media, cell wall composition, and antigen production. Each group causes specific types of infections and symptoms. These antibody tests are useful for detecting a recent respiratory or skin infection caused primarily by group A streptococci.

Group A streptococci are the most virulent species for humans and are the cause of pharyngitis (strep throat), tonsillitis, wound and skin infections, blood infections (septicemia), scarlet fever, pneumonia, rheumatic fever, Sydenham's chorea (formerly called St. Vitus' dance), and post streptococcal glomerulonephritis.

Although symptoms may suggest a streptococcal infection, the diagnosis must be confirmed by tests. The best procedure, and one that is used for an acute infection, is to take a sample from the infected area for culture, a means of growing bacteria artificially in the laboratory. However, cultures will be negative for growth approximately two to three weeks after the initial infection. Consequently, the streptococcal antibody tests are used to determine if a streptococcal infection was present.