In which cases is a Typhidot test positive?

Typhidot Test Positivity

Typhidot is positive in cases of typhoid fever caused by Salmonella typhi, detecting IgM and/or IgG antibodies against the organism, though its clinical utility is severely limited by poor sensitivity and specificity.

What the Test Detects

Typhidot detects antibodies (IgM and IgG) against Salmonella typhi antigens in patients with typhoid fever. 1, 2

• IgM antibodies indicate acute or recent infection 2
• IgG antibodies suggest past infection or later stages of current infection 2
• The test can show positivity for both IgM and IgG simultaneously, IgM alone, or IgG alone 2

Critical Limitations You Must Understand

The CDC explicitly states that serologic evidence alone is not sufficient for diagnosis of typhoid fever, and isolation of S. typhi from blood, stool, or other clinical specimens is required for confirmation. 3, 4

Performance Varies Dramatically Between Studies

The evidence shows contradictory performance characteristics:

• Poor performance study: Sensitivity of only 26.7% and specificity of 61.5%, with 73.3% of culture-confirmed typhoid cases showing negative Typhidot results 5
• Better performance studies: Sensitivity ranging from 86-92% and specificity from 80-100% in other evaluations 1, 2, 6

This wide variation makes Typhidot unreliable as a standalone diagnostic tool. 5

False Positive Results Occur In:

• Previous typhoid vaccination 4
• Prior typhoid infection 4
• Other febrile illnesses 4
• Cross-reactions with other Salmonella species 4
• Endemic areas where background antibody levels are elevated 4
• 38.5% of patients with diseases other than typhoid showed positive Typhidot in one study 5

False Negative Results Occur In:

• Prior antibiotic treatment 4
• Immunosuppression 4
• Early in disease course (though some studies suggest reasonable sensitivity in first week) 6

The Correct Diagnostic Approach

Blood culture remains the gold standard for diagnosis, with highest yield within the first week of symptoms. 7, 4, 8

Diagnostic Algorithm:

1. Obtain blood cultures first in any patient with sustained fever, headache, malaise, anorexia, relative bradycardia, constipation or diarrhea, and nonproductive cough 3, 7

2. Blood cultures have 40-80% sensitivity and are essential for confirmation 8

3. Do not rely on Typhidot alone - if used at all, it should only complement blood culture, never replace it 4, 2

4. Consider bone marrow culture if blood culture is negative but clinical suspicion remains high, as it has higher sensitivity (35-65% vs blood culture) 8

Clinical Bottom Line

Given the poor and inconsistent performance of Typhidot, especially its low sensitivity (26.7%) and high false-negative rate (73.3% of confirmed cases) in the most recent quality study, its use should be discouraged. 5 The test's association with actual disease was statistically insignificant (p=0.067) 5, making it unreliable for clinical decision-making despite being rapid, easy, and affordable.

Always obtain blood cultures and base treatment decisions on clinical presentation and culture results, not serologic testing alone. 3, 4