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Subject: Streptozyme, Antistreptococcal Antibodies, Antistreptolysin O [ASO], Anti–Dnase-B [ADB]
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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. Group A streptococci are the most virulent species for humans and are the cause of “strep” throat, tonsillitis, wound and skin infections, blood infections, scarlet fever, pneumonia, RF, Sydenham chorea (formerly called St. Vitus dance), and GN. 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. However, cultures are useless about 2–3 weeks after initial infection, so the ASO, streptozyme, and ADB screen tests are used to determine if a streptococcal infection is present.
High titers of these antibodies have been associated with PANDAS (Pediatric Autoimmune Neuropsychiatric Disorder Associated with Streptococcal Infections) and with autism, Tourette syndrome, tic disorder, Parkinson disease, and OCD.
Streptococcal infections are probably a significant environmental trigger for narcolepsy.
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 S. 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, although 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.
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.
ASO appears in the blood serum 1 week to 1 month after the onset of a strep infection. A high titer is not specific for any type of poststreptococcal disease, but it does indicate if a streptococcal infection is or has been present. Serial ASO testing is often performed to determine the difference between an acute or convalescent blood samples. The diagnosis of a previous strep infection is confirmed when serial titers of ASO rise over a period of weeks and then fall slowly. ASO titers peak during the 3rd week after the onset of acute symptoms of a streptococcal disease; at 6 months after onset, approximately 30% of patients exhibit abnormal titers.
Elevated titers are seen in 80–85% patients with acute RF and 95% in acute GN.
Anti–DNase B or ADB
This test also detects antigens produced by group A streptococcus and is elevated in most patients with RF and poststreptococcal GN.
This test is often done concurrently with the ASO titer. When ASO and ADB are performed concurrently, 95% of previous strep infections are detected.
Normal values may vary with season of the year, age, and geographic location of the patient. Expected values for normal adults as reported in the literature are typically <100 IU/mL. The ULN ASO titer for pediatrics is <100 IU/mL; in school-age children or young adults, it is between 166 and 250 IU/mL. A twofold increase in the ASO value, using serial analysis, within 1–2 weeks of the initial result is supportive of a prior streptococcal infection. In the absence of complications or reinfection, the ASO level usually falls to preinfection activity within 6–12 months.
Normal range: ULN, 116 IU/mL.
Direct diagnostic value in scarlet fever, erysipelas, and streptococcal pharyngitis and tonsillitis. Indirect diagnostic value in RF, GN, detection of subclinical streptococcal infection, and differential diagnosis of joint pains of RF and RA.
Increased in pyoderma, postimpetigo nephritis caused by GAS, RF, and pharyngitis.
When evaluating patients with acute RF, 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, although ASO is the recommended test, when ASO and ADB are done together, the combination is better than either ASO or ADB alone.
With the ASO test, false-positive results are observed with increased levels of serum beta lipoproteins produced in liver disease and contamination of serum with Bacillus cereus or Pseudomonas species. In addition, these titers are not formed as a result of streptococcal pyoderma. Technically, false-positive results occur due to the oxidation of reagents.
A single ASO analysis may not be meaningful due to the variability of ASO values within the normal population. Both clinical and laboratory findings should be considered in reaching a diagnosis.
Streptococcal infections already treated with antibiotics may not produce increased results.