When does Typhidot IgM (Immunoglobulin M) become positive in typhoid fever?

When Does Typhidot IgM Become Positive in Typhoid Fever?

Typhidot IgM antibodies typically become positive 7-15 days after the onset of symptoms in typhoid fever, with 90% of patients seroconverting by the third week of illness. 1

Timing of Antibody Response in Typhoid Fever

• IgM antibodies to specific antigens (detected by tests like Typhidot) develop in the second week of acute illness 1
• Seroconversion typically occurs 7-15 days after symptoms appear 1
• By the third week of illness, 90% of patients will have detectable antibodies 1
• Blood cultures have the highest yield within the first week of symptoms, making them more useful for early diagnosis 1

Sensitivity of Typhidot IgM at Different Stages

• In the first week of illness, Typhidot M shows sensitivity of approximately 86.2% 2
• The sensitivity increases with longer duration of illness 3
• Overall sensitivity of Typhidot M across all stages is reported as 89.1-92.3% 2, 4
• Specificity ranges from 90-100% in various studies 2, 4, 5

Factors Affecting Antibody Detection

• Early antibiotic treatment may prevent or delay the development of antibodies 1
• Immunosuppression can affect antibody production and detection 6
• Previous typhoid vaccination may cause false positive results 6
• Prior typhoid infection may lead to persistent antibodies 6

Diagnostic Algorithm for Typhoid Fever

• First week of illness: Blood culture is the gold standard (40-80% sensitive) but Typhidot IgM may already be positive in many cases 1, 2
• Second week onwards: Typhidot IgM sensitivity increases significantly 3
• For definitive diagnosis: A four-fold rise in antibody titers between paired sera taken 3-6 weeks apart is most reliable 1
• Single positive Typhidot IgM result in a patient with compatible clinical features is suggestive but not definitive 6

Limitations of Typhidot IgM Testing

• False negatives are common in the very early stage of disease 3
• Cross-reactivity may occur with other Salmonella species 6
• Background antibody levels in endemic areas may affect interpretation 6
• The Widal test (an older serological test) lacks sensitivity and specificity compared to Typhidot 2, 5

Clinical Implications

• Typhidot IgM is superior to the Widal test for early diagnosis 2, 5
• Blood culture remains the gold standard but has lower sensitivity (27.3-40%) 2, 4
• Combining diagnostic methods may improve overall detection rates 3
• Treatment should not be delayed while waiting for seroconversion in clinically suspected cases 7