Stool Antigen Test Indicates Active, Not Past, H. pylori Infection
No, a positive stool antigen test (SAT) for H. pylori indicates current active infection, not a past or resolved infection. This is a critical distinction from serology testing.
Key Distinction Between Test Types
The stool antigen test detects active bacterial presence by identifying H. pylori antigens currently being shed in the stool. This fundamentally differs from serology, which detects antibodies that persist for months to years after the bacteria have been eradicated 1.
Why This Matters Clinically:
- Stool antigen tests become negative after successful eradication - Studies demonstrate that SAT accurately confirms eradication 4-6 weeks post-treatment with specificity of 92-97% 2, 3, 4
- Serology remains positive indefinitely - Antibodies against H. pylori (especially CagA) stay elevated for months or even years after bacterial clearance, making serology useless for confirming eradication 1
- SAT detects present but not previous infection - Research explicitly states this distinction, noting that stool antigen testing identifies current infection status 5
Clinical Application
For diagnosis of active infection: SAT has excellent accuracy with sensitivity of 92.4% and specificity of 91.9% in untreated patients 4. The more recent meta-analysis shows conventional SAT performs even better than rapid SAT (sensitivity 92.19% vs 85.79%) 6.
For confirming eradication: The 2024 ACG guidelines recommend universal post-treatment test-of-cure, and SAT is an appropriate non-invasive option for this purpose 7. Testing should occur at least 4 weeks after completing therapy and after stopping PPIs for 2 weeks 1.
Important Caveats:
- PPI interference: Stop PPIs for 2 weeks before testing, as they reduce bacterial load and cause false-negative results in 10-40% of cases 1
- Timing matters: A positive SAT as early as 7 days post-treatment predicts eradication failure with 100% positive predictive value 2
- Antibiotics affect accuracy: Recent antibiotic exposure can influence test performance 8
Bottom line: If your stool antigen test is positive, you have active H. pylori infection right now that requires treatment. If it's negative after treatment, the bacteria are gone. This is the opposite of serology, which tells you about past exposure but cannot distinguish active from cleared infection.