Typhidot Test: Principle and Interpretation
Direct Answer
Serologic tests including Typhidot should NOT be used to diagnose typhoid fever due to poor performance characteristics, and blood culture remains the gold standard diagnostic test. 1, 2
Test Principle
The Typhidot test is a dot enzyme immunoassay that detects IgM and IgG antibodies against a specific 50kD outer membrane protein (OMP) antigen of Salmonella typhi. 3 The test works by:
- Detecting antibody response to S. typhi OMP antigen rather than the organism itself 3, 4
- Differentiating acute from past infection through separate detection of IgM (acute infection) and IgG (past infection or later stage) antibodies 4
- Providing rapid results within 1 hour compared to 48 hours for blood culture 3
Why This Test Should Be Avoided
The Infectious Diseases Society of America explicitly states that serologic tests should NOT be used to diagnose enteric fever. 2 This strong recommendation is based on:
Poor Performance Characteristics
- Extremely low sensitivity of 26.7% in one study, meaning 73.3% of confirmed typhoid cases tested negative 5
- Poor specificity of 61.5%, with 38.5% false positive results in patients without typhoid 5
- Positive predictive value of only 7.4%, making positive results unreliable 5
- Statistically insignificant association with actual disease (p=0.067) 5
Contradictory Research Evidence
While some older studies reported better performance (sensitivity 92.3%, specificity 98.8% 6; sensitivity 100%, specificity 80% 4), the most recent guideline evidence from IDSA (2017) and CDC (2025) explicitly recommends against using serologic tests. 1, 2
The variability in reported performance (sensitivity ranging from 26.7% to 100% across studies 5, 6, 4) demonstrates unreliability and lack of standardization. 5, 6, 3, 4
Interpretation (If Test Is Performed Despite Recommendations)
If Typhidot results are available, interpret with extreme caution:
- IgM positive alone: Suggests possible acute infection, but requires blood culture confirmation 3, 4
- IgG positive alone: May indicate past infection or vaccination, not acute disease 4
- Both IgM and IgG positive: Could represent acute infection in endemic areas or recent past infection 3, 4
- Both negative: Does NOT rule out typhoid fever given poor sensitivity 5
Recommended Diagnostic Approach Instead
Blood culture is the gold standard and should be performed in all patients with clinical suspicion of typhoid fever. 2, 7
Optimal Diagnostic Strategy:
- Collect 2-3 blood cultures of 20 mL each before antibiotic administration 7
- Highest yield in the first week of symptoms 8, 7
- Bone marrow culture if antibiotics already given or blood cultures negative despite high clinical suspicion (higher sensitivity than blood) 2, 7
- Adjunctive cultures from stool, duodenal fluid, and urine may be beneficial 1, 2
Critical Clinical Pitfall
Using Typhidot as the sole diagnostic tool causes misdiagnosis and inappropriate treatment. 5 General practitioners and consultants have relied on this test despite its poor performance, leading to:
- Missing true typhoid cases (low sensitivity) 5
- Treating non-typhoid illnesses as typhoid (low specificity) 5
- Delaying appropriate blood culture collection 7
The CDC requires isolation of Salmonella typhi from clinical specimens for confirmation and states that serologic evidence alone is not sufficient. 2