When does Typhidot IgG (Immunoglobulin G) become positive in typhoid fever?

Timing of Typhidot IgG Positivity in Typhoid Fever

Typhidot IgG antibodies typically become positive during the second week of typhoid fever infection, though they may persist for longer periods and are therefore less useful for diagnosing acute infection compared to IgM antibodies. 1

Serological Response Timeline in Typhoid Fever

• Blood cultures have the highest diagnostic yield within the first week of symptom onset (sensitivity 40-80%), while serological tests including Typhidot become more useful after this initial period 1
• IgM antibodies appear earlier in the course of typhoid fever and are more indicative of acute infection 2, 3
• IgG antibodies typically develop during the second week of illness and may persist for longer periods, making them more useful for determining past infection rather than current acute infection 2, 4
• Both IgM and IgG antibodies show significant detection rates in the second and third weeks of illness 3

Diagnostic Value of Typhidot Testing

• Newer rapid serological tests like Typhidot (detecting IgM and IgG antibodies against specific 50kD Outer Membrane Protein antigen) have shown mixed results in clinical studies 1
• The sensitivity of Typhidot varies considerably between studies:
  • Some studies report high sensitivity (92-97%) and specificity (89-98%) compared to blood culture 4, 5
  • Other studies report much lower sensitivity (26.7%) and specificity (61.5%) 6
• Combined IgM and IgG testing provides better diagnostic accuracy than either antibody alone 3

Clinical Implications and Pitfalls

• Blood culture remains the gold standard for diagnosis of typhoid fever despite its limitations in sensitivity 1, 7
• The Widal test (traditional serological test) lacks sensitivity and specificity and is not recommended as a standalone diagnostic tool 1
• Important pitfall: Relying solely on Typhidot IgG for diagnosis of acute typhoid fever may lead to misdiagnosis, as IgG can indicate past infection or previous vaccination 6, 3
• IgM detection is more useful for diagnosing current infection, while IgG is better for identifying past exposure or vaccination 2, 4

Practical Recommendations

• For acute diagnosis of typhoid fever, blood cultures should be obtained within the first week of symptoms 1, 7
• If using Typhidot, test for both IgM and IgG, with IgM being more indicative of current infection 3, 4
• Interpret Typhidot results in conjunction with clinical presentation and other laboratory findings, never as a standalone diagnostic tool 6, 3
• Consider that vaccination history may affect interpretation of serological tests including Typhidot IgG 7