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.
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.
Antistreptolysin O titer (ASO)
The ASO titer is used to detect the body's reaction to an infection caused by group A beta-hemolytic streptococci. Group A streptococci (also Group C and G) produce the enzyme streptolysin O, that can destroy (lyse) red blood cells. Because streptolysin O is antigenic (contains a proteinforeign to the body), the body reacts by producing anti-streptolysin O antibody (ASO), a neutralizing antibody. ASO appears in the blood one week to one month after the onset of a strep infection. A high titer (high levels of ASO antibody) is indicative that a streptococcal infection is present or may have happened in the recent past.
ASO testing can be done as a screening test by a rapid slide agglutination method. If positive, the antibody concentration is determined by the classical tube test. In this test, a standardized solution of streptolysin O is added to tubes containing serial dilutions of the patient's serum. After incubating, human group O red blood cells are added. If antibodies are present, they neutralize the streptolysin O and no hemolysis is seen. The antibody concentratioin (titre) is the highest dilution of the serum that shows no evidence of hemolysis.
Several, sequential tests for ASO are often performed over time (serial testing) to determine if the blood sample is acute or convalescent. The diagnosis of a previous strep infection is confirmed when serial titers of ASO rise over a period of weeks, then fall slowly. A fourfold or greater rise in titre from the acute to convalescent stage is considered diagnostic. 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 still exhibit abnormal titers.
Anti-deoxyribonuclease 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 post streptococcal 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 great that the patient's symptoms are not caused by a post streptococcal disease. The ADB test is performed by measuring the ability of the serum to block the breakdown by streptococcal DNase B of calf thymus DNA bound to a dye. If the DNA is split by the enzyme, the color changes from blue to pink. If antibodies to DNase B are present in the serum, they neutralize the enzyme and the color remains blue.
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 more 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.
Antihyaluronidase (AH)
Group A streptococci produce an enzyme called hyaluronidase. If a patient with a recent infection with this organism produces antihyaluronidase, the level of antibodies in the blood against this enzyme will rise at about the second week of infection and decline for three to five weeks afterward. The patient's serum is diluted and streptococcal hyaluronidase added to each dilution. After incubation, potassium hyaluronate is added. If hyaluronidase is present, it will breakdown the hyaluronate by hydrolysis. Consequently, a clot will not form when acetic acid is added. If antihyaluronidase is present, it will neutralize the streptococcal hyaluronidase. The potassium hyaluronate added subsequently will not be hydrolyzed by the enzyme. The addition of acetic acid cross links the hyaluronate forming a mucin clot. The highest dilution of serum that forms a clot is the titer. This test is advantageous when used along with the ASO test because it increases clinical sensitivity. However, it should not be used as a singular test for streptococcal antibodies. False positive results may occur from hyperlipoproteinemia.
Streptozyme
The Streptozyme test is often used as a screening test for antibodies to the streptococcal antigens NADase, DNase, streptokinase, streptolysin O, and hyaluronidase. Streptozyme has two advantages over ASO and ADB. It can detect several antibodies in a single assay, and it is technically quick and easy to perform. However, the Streptozyme test is less sensitive and specific than the ASO test. 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. A dilution of serum is mixed with sheep red cells that are coated with streptococcal antigens. A positive test is denoted by red blood cell clumping (agglutination).
Preparation
These tests are performed on blood specimens drawn from the patient's vein. The patient does not need to fast before these tests.
Aftercare
The patient may feel discomfort when blood is drawn from a vein. Bruising may occur at the puncture site, or the person may feel dizzy or faint. Pressure should be applied to the puncture site until the bleeding stops to reduce bruising. Warm packs can also be placed over the puncture site to relieve discomfort. Patients should be informed if serial testing requires return visits.
Complications
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).
Child: 6 months–2 years: up to 50 Todd units; 2–4 years: up to 160 Todd units; 5-12 years: 170-330 Todd units.
Newborn: similar to the mother's value.
ribonuclease-B titer:
Adult: up to 85 units.
Child (preschool): up to 60 units.
Child (school age): up to 170 units.
ronidase (AH):
Titer less than 1:512.
Streptozyme: less than 100 streptozyme units.
Abnormal results
Antistreptolysin O titer: Increased levels are seen after the second week of an untreated acute streptococcal infection, and are also increased with acute rheumatic fever, acute glomerulonephritis, scarlet fever, and other complications of streptococcal infection.
Antideoxyribonuclease-B titer: Increased levels are seen after the first week of an untreated acute streptococcal infection, and are also increased with acute rheumatic fever, acute glomerulonephritis, scarlet fever, and other complications of streptococcal infection.
Titer greater than 1:512. A rise in the titer between and acute patient sample of greater than fourfold is indicative of infection.
Streptozyme: As this is a screening test for antibodies to streptococcal antigens, increased levels require more definitive tests to confirm diagnosis.
Health care team roles
Streptococcal antibody tests are ordered by a physician. The nurse or phlebotomist collects the specimen and conveys it to the lab. The clinical laboratory scientist/medical technologist or clinical laboratory technician/medical technician performs the test. Results are interpreted by the physician.
KEY TERMS
Antibody—A protein manufactured by a type of white blood cell, the lymphocyte, 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 severe enough to cause kidney failure.
Rheumatic fever—A disease that causes inflammation in various body tissues. It is 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. It is rare in the United States today, but a common disorder in developing countries. It usually resolves in two to three months with no long-term adverse effects.
BOOKS
Chernecky, C. and B. Berger, editors. Laboratory Tests and Diagnostic Procedures, 3rd ed., Philadelphia, PA: W.C. Saunders Company, 2001.
McKenna, R., and Keffer, J., editors. The Handbook of Clinical Pathology, 2nd ed. Chicago, IL: American Society of Clinical Pathologists, 2000.
Pagana, K. Deska, and T. Pagana. Mosby's Diagnostic and Laboratory Test Reference. 4th ed. St. Louis, MO: Mosby, 1999.
Sacher, R. and R. McPherson. Widmann's Clinical Interpretation of Laboratory Tests, 11th ed. Philadelphia, PA: F.A. Davis Company, 2000.
