Laboratory Testing for Post-Streptococcal Infection with Swelling
For a patient presenting with swelling after a suspected streptococcal infection, order ASO (antistreptolysin O) first, and if negative or equivocal, add anti-DNase B antibody testing to confirm prior streptococcal infection when evaluating for post-streptococcal complications such as acute rheumatic fever or acute glomerulonephritis. 1
Critical Distinction: Acute vs Post-Streptococcal Disease
The timing and type of testing depends entirely on whether you're diagnosing an acute infection or confirming a prior infection that led to complications:
For Acute Streptococcal Infection (NOT Applicable Here)
- Rapid antigen detection test (RADT) or throat culture are the only appropriate tests for diagnosing active streptococcal pharyngitis 2
- Antibody tests (ASO, anti-DNase B) have no value in diagnosing acute pharyngitis because they reflect past, not present, immunologic events 3
For Post-Streptococcal Complications (Your Clinical Scenario)
- ASO and anti-DNase B are specifically indicated to confirm preceding streptococcal infection in patients with suspected post-streptococcal sequelae 3, 1
- These tests are valuable for confirming prior infections in patients suspected of having acute rheumatic fever or acute glomerulonephritis 3, 2
Recommended Testing Algorithm
Step 1: Order ASO as the first-line test 1
- ASO begins rising approximately 1 week after infection and peaks at 3-6 weeks 2
- Upper limit of normal (ULN) in children ages 2-12 is 240 IU/mL 4
- Recent Italian data suggests ULN may be as high as 515 IU/mL in healthy children, varying by region 5
Step 2: If ASO is negative or equivocal but clinical suspicion remains high, order anti-DNase B 1
- Anti-DNase B rises 1-2 weeks after infection and peaks at 6-8 weeks 2, 1
- Anti-DNase B may remain elevated when ASO has normalized 1
- ULN for anti-DNase B in children ages 2-12 is 640 units 4
Step 3: Combined testing maximizes diagnostic yield
- ASO and anti-DNase B together detect streptococcal infection in up to 98% of proven cases 2, 1
- Anti-DNase B is particularly valuable because it remains elevated in streptococcal skin infections where ASO may be normal 6
Additional Laboratory Tests for Post-Streptococcal Complications
Consider inflammatory markers if evaluating severity:
- ESR (erythrocyte sedimentation rate) and C-reactive protein may be indicated for suspected post-streptococcal adverse outcomes 3
- However, C-reactive protein levels are not helpful for diagnosing acute group A streptococcal pharyngitis itself 3
Critical Interpretation Caveats
Age-appropriate reference ranges are essential:
- School-age children have considerably higher antibody titers than adults 7, 4
- Normal values vary by age group, region, and population 5, 4
- Using adult reference ranges in children leads to false-positive interpretations 1
Timing limitations:
- Both ASO and anti-DNase B can remain elevated for several months after uncomplicated infections 2, 1
- An elevated titer does not distinguish active infection from carrier state 1
- Serial testing is required for optimal interpretation 1
Common Pitfalls to Avoid
Never use antibody tests to diagnose acute pharyngitis 2
- A patient with chronic tonsillitis and elevated ASO may simply have had a prior infection, not an active one 2
- Up to 20% of asymptomatic school-age children may be GAS carriers with elevated antibodies 2
Do not use antibody tests alone to guide antibiotic treatment or surgical decisions 2
- Antibody tests cannot distinguish between active infection and carrier state with intercurrent viral illness 2
- Microbiological confirmation (throat culture or RADT) is required for acute infection diagnosis 2
Remember that anti-DNase B testing requires serial measurements 1