Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1426 page)

   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.

   
ASO titer

   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.

   Use