Unit Converter
Antistreptolysin-O (ASLO)
(Antibody to Streptolysin-O - Evidence of Recent Streptococcal Infection)
Synonyms
- ASO
- ASLO
- Antistreptolysin O antibody
- Anti-streptolysin-O
- ASO titre
Units of Measurement
- IU/mL
- U/mL
- kU/L
- kIU/L
(All units are numerically identical:
1 IU/mL = 1 U/mL = 1 kU/L = 1 kIU/L.)
Description
Antistreptolysin-O (ASO) is an antibody produced against streptolysin-O, a hemolytic exotoxin secreted by Group A Streptococcus (Streptococcus pyogenes).
ASO does not diagnose acute infection, but indicates recent exposure (2–8 weeks prior) and helps diagnose post-streptococcal complications:
- Acute Rheumatic Fever (ARF)
- Post-streptococcal glomerulonephritis (PSGN)
- Post-streptococcal reactive arthritis
Biological Background
Streptolysin-O is:
- An oxygen-labile toxin
- Strongly immunogenic
- Causes β-hemolysis on blood agar
Following infection:
- Antibodies typically appear in 1–3 weeks
- Peak at 3–5 weeks
- Decline over 6–12 months
Clinical Significance
Elevated ASO
Indicates recent streptococcal infection (throat or skin).
1. Acute Rheumatic Fever (ARF)
- ASO is one of the supporting investigations in the Jones criteria.
- High or rising titers strongly support diagnosis.
2. Post-streptococcal Glomerulonephritis
- ASO elevated in throat infections
- Anti-DNase-B preferred for skin infections
- Used together for better sensitivity
3. Post-streptococcal Reactive Arthritis
4. Scarlet Fever / Streptococcal Pharyngitis (Retrospective)
Useful when patient presents late.
Low / Normal ASO
Occurs in:
- No recent strep infection
- Streptococcal skin infections (ASO response weak)
- Early infection (before antibodies rise)
- When Anti-DNase B is more sensitive
A negative test does not rule out rheumatic fever or PSGN.
Reference Intervals
Typical Adult & Child Values
| ASO Level | Interpretation |
| < 200 IU/mL | Normal (adults) |
| < 150 IU/mL | Normal (younger children) |
| > 200–300 IU/mL | Positive / Suggests recent streptococcal infection |
| > 400–500 IU/mL | Strong evidence of recent infection |
| > 1000 IU/mL | Very high; often in ARF or severe pharyngitis |
Titer Dynamics
- A rising titer on repeat testing (10–14 days apart) is more diagnostic than a single level.
ASO vs Anti-DNase B
| Feature | ASO | Anti-DNase B |
| Best for | Throat strep infections | Skin strep infections |
| Rise | 1–3 weeks | 2–6 weeks |
| Peak | 3–5 weeks | 6–8 weeks |
| Utility | ARF, PSGN | PSGN, skin infection cases |
Using both tests increases diagnostic sensitivity.
Diagnostic Uses
1. Support Diagnosis of Acute Rheumatic Fever
- ASO helps satisfy evidence of preceding strep infection in Jones criteria.
2. Diagnose PSGN (Post-Strep Glomerulonephritis)
- High ASO indicates recent infection
- Anti-DNase B often combined for accuracy
3. Post-Streptococcal Reactive Arthritis
- Helps confirm prior GAS infection
4. Late presentations of pharyngitis
- When throat culture/RADT no longer positive
Analytical Notes
- Automated immunoassay or turbidimetric inhibition methods used.
- Hemolysis and lipemia may minimally interfere.
- Repeat testing (paired sera) improves accuracy.
- Biotin interference possible with some assays.
Clinical Pearls
- ASO is not a test for active infection - it detects past exposure.
- High or rising ASO supports ARF diagnosis, but normal ASO does not exclude it.
- For skin infections, measure Anti-DNase B, not ASO.
- ASO levels may remain elevated for months after infection.
- Not useful for determining response to antibiotics.
Interesting Fact
The ASO test, developed nearly a century ago, remains a cornerstone for diagnosing post-streptococcal immune complications, even though rapid streptococcal antigen tests and PCR exist today.
References
- Tietz Clinical Chemistry and Molecular Diagnostics, 8th Edition - Autoimmune & Infectious Markers.
- AAP / WHO / Jones Criteria - Acute Rheumatic Fever Guidelines.
- IDSA Guidelines - Group A Streptococcal Disease.
- Mayo Clinic Laboratories - ASO Test Catalog.
- ARUP Consult - Post-Streptococcal Disease Evaluation.
- IFCC Immunoassay Standards.
- MedlinePlus / NIH - ASO Test Overview.