Antistreptolysin-O Titer

Test Description

Streptolysin-O is an enzyme produced by Group A beta-hemolytic streptococcal
bacteria. When confronted by this foreign enzyme, the body produces antibodies
against it. The antibodies appear 7 to 10 days after the acute streptococcal infection
and continue to rise for 2 to 4 weeks. The ASO level will typically fall to preinfection
levels with 6 to 12 months. The antistreptolysin-O (ASO) test is designed to
detect these antibodies. If the antibodies are present, the person has had a streptococcal
infection. Over 80% of patient with acute rheumatic fever and 95% of
those with acute streptococcal glomerulonephritis have elevated ASO levels. ASO
levels do not typically rise with cutaneous infections.
This test is considered less sensitive than the anti-DNase B test. When both
tests are consistently performed on blood samples, 95% of the streptococcal infections
can be identified. The test is particularly useful in determining whether such
conditions as joint pain or glomerulonephritis are the result of a streptococcal
infection.

Normal Values

                     Adult: <160 Todd units/mL
                     Ages 5–12: <170-33- Todd units/mL
                     Ages 2–5: <160 Todd units/mL
                     Ages 0–2: <50 Todd units/mL

Possible Meanings of Abnormal Values

Increased

Acute rheumatic fever
Poststreptococcal endocarditis
Poststreptococcal glomerulonephritis
Scarlet fever

Contributing Factors to Abnormal Values

• Hemolysis of the blood sample may alter test results
• Drugs that may decrease ASO titers: antibiotics, corticosteroids.
• False-positive results may occur when blood sample has high lipid content.

Interventions/Implications

Pretest

• Explain to the patient the purpose of the test and the need for a blood sample to be drawn.
• No fasting is required before the test.

Procedure

• A 7-mL blood sample is drawn in a red-top collection tube.
• Gloves are worn throughout the procedure.

Posttest

• Apply pressure at venipuncture site. Apply dressing, periodically assessing for continued
  bleeding.
• Label the specimen and transport it to the laboratory.
• Report abnormal findings to the primary care provider.

Clinical alert                     

• For the most reliable results, the Anti-DNase-B test should be performed in conjunction
  with the ASO test.
• ASO titers are usually repeated in 10–14 days for comparison with initial results
   to determine if the antibody level is rising.
• The ASO test does not predict if complications will occur following a streptococcal
   infection, nor does it predict the severity of the disease.