Anti-Streptolysin O (ASO)

Bacterial/ Viral

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Details

ASO antibodies are produced by the immune system in response to an infection caused by Group A Streptococcus (Streptococcus pyogenes). The bacteria secrete a toxin called streptolysin O, which is harmful to red blood cells. The ASO test detects antibodies against this toxin

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Anti-Streptolysin O (ASO) Test - Comprehensive Medical Guide

Why is it done?
- Test Purpose: The ASO test measures antibodies produced by the immune system in response to Group A Streptococcal (Group A Strep) infection. It detects previous or current streptococcal exposure by identifying antistreptolysin O antibodies in the blood.
- Primary Indications: Diagnosis of acute rheumatic fever (ARF) or post-streptococcal glomerulonephritis (PSGN); evaluation of patients with suspected rheumatic heart disease; confirmation of recent Group A Streptococcal infection when clinical diagnosis is uncertain
- Clinical Circumstances: Ordered 2-3 weeks after Group A Streptococcal pharyngitis (throat infection); when patients present with joint pain, fever, and cardiac symptoms; during evaluation of new-onset heart murmur; in cases of recurrent streptococcal infections
Normal Range
- Reference Values: Less than 200 IU/mL (International Units per milliliter) or less than 1:200 titer (when reported as titer); specific cutoff values may vary by laboratory and testing method
- Normal Result: Negative result indicates no recent Group A Streptococcal infection or remote/past infection with waning antibodies
- Elevated Result: Greater than 200 IU/mL or 1:200 titer suggests recent or acute Group A Streptococcal infection
- Borderline Values: Values between 200-300 IU/mL may be considered borderline; serial testing (repeat ASO titer in 2 weeks) is recommended to detect rising antibody titers, which is more significant than a single elevated value
- Units of Measurement: IU/mL (International Units per milliliter) or reported as titer ratios (1:200, 1:400, etc.)
Interpretation
- Negative Result (<200 IU/mL): Suggests no recent Group A Streptococcal infection; makes acute rheumatic fever or acute glomerulonephritis less likely (though does not completely exclude recent infection in early phase); may indicate past infection with declining antibody levels
- Single Elevated Result (>200 IU/mL): Indicates past or recent streptococcal infection; cannot definitively distinguish between recent and remote infection; a single elevated titer is less specific than rising titers
- Rising Titer (Fourfold Increase): Comparing two samples 2-3 weeks apart showing a fourfold rise is highly significant and indicates recent or acute Group A Streptococcal infection; strongly suggestive of acute rheumatic fever or post-streptococcal sequelae
- Falling Titer: Suggests infection occurred in the distant past with declining immune response; indicates resolution of acute phase
- Factors Affecting Results: Timing of test (elevated levels peak 3-6 weeks after infection); previous streptococcal infections; prophylactic penicillin use (may suppress antibody response); immunosuppressive conditions; chronic streptococcal infection; rheumatoid factor or other autoimmune conditions; laboratory methodology variations
- Clinical Significance: Most useful when combined with clinical presentation and other diagnostic tests (throat culture, rapid strep test); serial testing more valuable than single result; approximately 80% of patients with acute rheumatic fever have elevated ASO titers; 90% of patients with post-streptococcal glomerulonephritis show elevated levels
Associated Organs
- Primary Organ Systems: Cardiovascular system (heart and valves); nervous system; musculoskeletal system (joints); kidneys; respiratory tract (throat/pharynx)
- Associated Diseases - Acute Rheumatic Fever (ARF): Inflammatory disease following Group A Streptococcal pharyngitis; affects heart (myocarditis, endocarditis, pericarditis); causes joint inflammation (polyarthritis); may involve nervous system (Sydenham's chorea); skin manifestations (erythema marginatum, subcutaneous nodules)
- Associated Diseases - Post-Streptococcal Glomerulonephritis (PSGN): Immune-mediated kidney inflammation; follows Group A Streptococcal pharyngitis; presents with hematuria, proteinuria, and hypertension; can lead to renal failure if severe
- Associated Diseases - Rheumatic Heart Disease (RHD): Chronic sequela of acute rheumatic fever; causes permanent heart valve damage; leads to valvular stenosis or regurgitation; can result in heart failure and atrial fibrillation
- Potential Complications: Permanent cardiac damage and valve dysfunction; stroke (from cardioemboli or atrial fibrillation); heart failure; renal failure; recurrent streptococcal infections if not properly managed; endocarditis risk (especially with prosthetic valves)
Follow-up Tests
- Repeat ASO Titer: Recommended 2-3 weeks after initial test to assess for rising titer (fourfold or greater increase indicates recent infection); more diagnostically significant than single elevated value
- Anti-DNase B Titer: Complementary test for streptococcal antibodies; useful when ASO is negative but clinical suspicion remains high; can detect streptococcal infection in cases where ASO testing is inconclusive
- Erythrocyte Sedimentation Rate (ESR): Measures inflammation; elevated in acute rheumatic fever; used to assess disease activity and monitor treatment response
- C-Reactive Protein (CRP): Acute phase reactant; elevated in acute rheumatic fever and active infection; helps monitor inflammatory status and response to anti-inflammatory therapy
- Echocardiography: Ultrasound of the heart; essential for detecting cardiac involvement in acute rheumatic fever; evaluates valve function, chamber size, and ventricular function; used for baseline assessment and monitoring
- Electrocardiogram (ECG): Records heart electrical activity; may show conduction abnormalities, arrhythmias, or signs of inflammation in acute rheumatic fever
- Renal Function Tests: Blood urea nitrogen (BUN), creatinine; assess kidney function if post-streptococcal glomerulonephritis is suspected; monitor for complications
- Urinalysis: Detects hematuria and proteinuria; useful when glomerulonephritis is suspected following streptococcal infection
- Throat Culture or Rapid Strep Test: Confirms Group A Streptococcal throat infection; most useful when performed early in illness before antibodies develop; provides direct microbiological evidence
- Monitoring Frequency: For acute rheumatic fever: baseline and follow-up testing to confirm diagnosis; for established rheumatic heart disease: periodic echocardiography (annually or as clinically indicated); for secondary prophylaxis monitoring: as directed by cardiologist; ASO levels typically do not require routine monitoring once diagnosis is established
Fasting Required?
- Fasting Requirement: No - Fasting is NOT required for the ASO test. Blood can be drawn at any time of day regardless of food or fluid intake.
- Fluid Intake: No restrictions; normal water intake is acceptable and may be beneficial for adequate hydration before blood draw
- Medications: Continue all regular medications as prescribed; no medications need to be withheld before ASO testing; inform healthcare provider of any immunosuppressive medications as these may affect antibody levels
- Patient Preparation: Wear comfortable, loose-fitting clothing with easily accessible sleeves for venipuncture; inform phlebotomist of any needle anxiety or previous difficult blood draws; remain calm and relax arm muscles during blood collection; no special preparation is needed
- Timing Considerations: Test should be performed 2-3 weeks after suspected streptococcal infection for optimal antibody detection; initial specimen can be collected at any time; for serial testing, second sample ideally collected 2-3 weeks after the first specimen
- Special Instructions: No special instructions necessary; standard blood draw procedures apply; serum or plasma may be collected depending on laboratory preference; samples are typically stable and can be transported according to standard laboratory protocols

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