How to Interpret Typhidot Test Results
Do not use Typhidot or any serologic tests to diagnose typhoid fever—blood culture is the gold standard and serologic tests should be avoided due to poor performance characteristics. 1, 2
Why Serologic Tests Should Not Be Used
- The Infectious Diseases Society of America explicitly states that serologic tests including Widal and rapid antibody tests should NOT be used to diagnose enteric fever due to poor performance characteristics 2
- The CDC requires isolation of Salmonella typhi from clinical specimens for confirmation and states that serologic evidence alone is not sufficient for diagnosis 1, 2
- Blood culture remains the only recommended diagnostic test, with 2-3 cultures of 20 mL each collected before antibiotic administration 1
Understanding Typhidot Performance (If Already Ordered)
Despite guideline recommendations against serologic testing, if a Typhidot test has already been performed, here's how to interpret it—but do not rely on it for clinical decision-making:
Test Characteristics
- Typhidot detects IgM (suggesting acute/recent infection) and IgG (suggesting past or current infection) antibodies against S. typhi antigens 3
- Research shows highly variable sensitivity ranging from 58-100% and specificity from 50-99%, making it unreliable 4, 3, 5
- The most recent outbreak data from Thailand (2014) demonstrated poor performance with IgM sensitivity of only 58.3% and specificity of 74.1%, while IgG was even worse at 25.6% sensitivity and 50.5% specificity 5
Interpretation Patterns
- IgM positive alone: May suggest early acute infection, but false positives are common 3, 5
- IgG positive alone: May indicate past infection or late acute infection, but has very poor specificity 3, 5
- Both IgM and IgG positive: May suggest current or recent infection, but does not confirm active disease 3
- Both negative: Does not rule out typhoid fever given the low sensitivity 5
The Correct Diagnostic Approach
Primary Testing
- Obtain blood cultures immediately before starting antibiotics—this is the only test recommended by major guidelines 1, 2
- Collect 2-3 blood cultures of 20 mL each in adults to maximize detection since bacteremia levels are low (median 0.3 CFU/mL) 1
- Blood culture sensitivity is 40-80%, highest in the first week of symptoms 6
When Blood Cultures Are Negative
- Consider bone marrow culture if clinical suspicion remains high, especially if antibiotics were already given—this has higher sensitivity (35-65%) than blood culture 6
- Stool, duodenal fluid, and urine cultures may provide additional diagnostic yield 2, 6
Critical Pitfall to Avoid
- Never diagnose or exclude typhoid fever based on Typhidot or any serologic test results 1, 2
- Always obtain blood cultures even if serologic tests are positive or negative 1, 2
- The high false-positive and false-negative rates of serologic tests make them clinically unreliable for guiding treatment decisions 5