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Streptococcal Antibody Tests

Streptococcal Antibody Tests

Definition

Streptococcal infections are caused by a microorganism called Streptococcus. Three streptococcal antibody tests are available: the antistreptolysin O titer (ASO), the antideoxyribonuclease-B titer (anti-Dnase-B, or ADB), and the streptozyme test.

Purpose

The antistreptolysin O titer, or ASO, is ordered primarily to determine whether a previous group A Streptococcus infection has caused a poststreptococcal disease, such as scarlet fever, rheumatic fever, or a kidney disease called glomerulonephritis.

The anti-DNase-B (ADB) test is performed to determine a previous infection of a specific type of Streptococcus, group A beta-hemolytic Streptococcus. Identification of infections of this type are particularly important in suspected cases of acute rheumatic fever (ARF) or acute glomerulonephritis.

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.

Precautions

For the ASO test, increased levels of fats, called beta lipoproteins, in the blood can neutralize streptolysin O and cause a false-positive ASO titer. Antibiotics, which reduce the number of streptococci and thereby suppress ASO production, may decrease ASO levels. Steroids, which suppress the immune system, consequently may also suppress ASO production. Also Group A streptococcal infections of the skin may not produce an ASO response. Antibiotics also may decrease anti-DNase-B (ADB) levels.

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 behavior, chemistry, and appearance. Each group causes specific types of infections and symptoms. These antibody tests are useful for group A streptococci. 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.

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 are useless about two to three weeks after initial infection, so the ASO, anti-DNase-B, and streptozyme tests are used to determine if a streptococcal infection is present.

Antistreptolysin O titer (ASO)

The ASO titer is used to demonstrate the body's reaction to an infection caused by group A beta-hemolytic streptococci. 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.

Serial (several given in a row) ASO testing is often performed to determine the difference between an acute or convalescent blood sample. The diagnosis of a previous strep infection is confirmed when serial titers of ASO rise over a period of weeks, then fall slowly. ASO titers peak during the third week after the onset of acute symptoms of a streptococcal disease; at six months after onset, approximately 30% of patients exhibit abnormal titers.

Antideoxyribonuclease-B titer (anti-DNase B, or ADB)

Anti-DNase-B, or ADB, also detects antigens produced by group A strep, and is elevated in most patients with rheumatic fever and poststreptococcal glomerulonephritis. This test is often done concurrently with the ASO titer, and subsequent testing is usually performed to detect differences in the acute and convalescent blood samples. When ASO and ADB are performed concurrently, 95% of previous strep infections are detected. If both are repeatedly negative, the likelihood is that the patient's symptoms are not caused by a poststreptococcal disease.

When evaluating patients with acute rheumatic fever, the American Heart Association recommends the ASO titer rather than ADB. Even though the ADB is more sensitive than ASO, its results are too variable. It also should be noted that, while ASO is the recommended test, when ASO and ADB are done together, the combination is better than either ASO or ADB alone.

Streptozyme

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 Streptococcus pyogenes infection, such as rheumatic fever.

Streptozyme has certain advantages over ASO and ADB. It can detect several antibodies in a single assay, it is technically quick and easy, and it is unaffected by factors that can produce false-positives in the ASO test. The disadvantages are that, 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. In fact, borderline antibody elevations, which could be significant in children, may not be detected at all. As with the ASO and ADB, a serially rising titer is more significant than a single determination.

Preparation

These tests are performed on blood specimens drawn from the patient's vein. The patient does not need to fast before these tests.

Risks

The risks associated with these tests are minimal, but may include slight bleeding from the blood-drawing site, fainting or feeling lightheaded after the blood is drawn, or blood accumulating under the puncture site (hematoma).

Normal results

Antistreptolysin O titer:

  • adult: 160 Todd units/ml
  • child: six months to two years: 50 Todd units/ml; two to four years: 160 Todd units/ml; five to 12 years: 170-330 Todd units/ml
  • newborn: similar to the mother's value

Antideoxyribonuclease-B titer:

  • adult: 85 units
  • child (preschool): 60 units
  • child (school age): 170 units

Streptozyme: less than 100 streptozyme units.

KEY TERMS

Antibody A protein manufactured by a type of white blood cells called lymphocytes, in response to the presence of an antigen, or foreign protein, in the body. Because bacteria, viruses, and other organisms commonly contain many antigens, antibodies are formed against these foreign proteins to neutralize or destroy the invaders.

Antigen A substance that can trigger a defensive response in the body, resulting in production of an antibody as part of the body's defense against infection and disease. Many antigens are foreign proteins not found naturally in the body, and include bacteria, viruses, toxins, and tissues from another person used in organ transplantation.

Glomerulonephritis An inflammation of the glomeruli, the filtering units of the kidney. Damage to these structures hampers removal of waste products, salt, and water from the bloodstream, which may cause serious complications. This disorder can be mild and cause no symptoms, or so severe enough to cause kidney failure.

Rheumatic fever A disease that causes inflammation in various body tissues. Rare in most developed countries, but reported to be on the increase again in parts of the United States. Joint inflammation occurs, but more serious is the frequency with which the disease permanently damages the heart. The nervous system may also be affected, causing Sydenham's chorea.

Sydenham's chorea A childhood disorder of the central nervous system. Once called St. Vitus' dance, the condition is characterized by involuntary, jerky movements that usually follow an attack of rheumatic fever. Rare in the United States today, but a common disorder in developing countries. Usually resolves in two to three months with no long-term adverse effects.

Abnormal results

Antistreptolysin O titer: Increased levels are seen after the second week of an untreated infection in acute streptococcal infection, and are also increased with acute rheumatic fever, acute glomerulonephritis (66% of patients will not have high ASO titers), and scarlet fever.

Antideoxyribonuclease-B titer: Increased titers are seen in cases of acute rheumatic fever and post-streptococcal glomerulonephritis.

Streptozyme: As this is a screening test for antibodies to streptococcal antigens, increased levels require more definitive tests to confirm diagnosis.

Resources

BOOKS

Pagana, Kathleen Deska. Mosby's Manual of Diagnostic and Laboratory Tests. St. Louis: Mosby, Inc., 1998.

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Streptococcal Antibody Tests

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.

See also Antibody and antigen; Antibody formation and kinetics; Bacteria and bacterial infection

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"Streptococcal Antibody Tests." World of Microbiology and Immunology. . Retrieved August 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/streptococcal-antibody-tests