Microbiology Cheat Sheet
Enteric Fever: Test by Week
Enteric fever test by week for NEET-PG and FMGE: blood culture week 1, bone marrow culture, Widal O and H antibodies, Typhidot, XLD agar, and TCV vaccine.
MedNext Academy | 3 min read
Enteric Fever: Test by Week
Enteric fever test by week for NEET-PG and FMGE: blood culture week 1, bone marrow culture, Widal O and H antibodies, Typhidot, XLD agar, and TCV vaccine.
This topic covers the pathogens, weekly clinical evolution, and laboratory diagnosis of enteric fever, with emphasis on choosing the right test at the right stage.
High-yield lines
- Enteric fever is caused by human-restricted Salmonella Typhi and Salmonella Paratyphi, which cause systemic bacteraemic illness rather than gastroenteritis.
- The Vi capsular antigen of S. Typhi masks the O antigen, resists complement, and is the basis of the Vi polysaccharide and Vi conjugate vaccines.
- S. Typhi has monophasic flagella (H antigen d), unlike most non-typhoidal Salmonella, which are diphasic.
- Blood culture is the mainstay in week 1 with 60 to 80 percent sensitivity, declining thereafter and with prior antibiotics.
- Bone marrow culture is the single most sensitive test at 80 to 95 percent and remains positive even after several days of antibiotic therapy.
- The Widal test becomes interpretable from week 2, and only a fourfold rise in paired sera is reliable, as single titres are unreliable in endemic areas.
- In the Widal test, O antibodies (IgM) rise and fall early indicating recent infection, while H antibodies (IgG) persist longer and may reflect past infection or vaccination.
- Typhoid follows a weekly progression: step-ladder fever and relative bradycardia in week 1, rose spots in week 2, and intestinal perforation risk in week 3.
- Salmonella produces red colonies with black centres on XLD agar due to H2S production, distinguishing it from Shigella.
- About 1 to 5 percent of typhoid patients become chronic carriers, harbouring organisms in the gallbladder, often associated with gallstones.
- Multidrug-resistant S. Typhi and strains with decreased ciprofloxacin susceptibility are common in South Asia, and current therapy uses azithromycin or ceftriaxone.
- Typbar-TCV, the Vi conjugate typhoid vaccine, produces immunological memory, is effective from 6 months of age, and is now WHO-preferred for endemic countries.
- Typhidot detects IgM and IgG against S. Typhi outer membrane protein and is more useful than Widal for early diagnosis.
- SPS in blood culture bottles and bile broth neutralise the bactericidal activity of serum, improving Salmonella recovery.
Mapped competency codes
- MI3.3
- MI3.4
Continue into the full chapter
This summary maps to MI3-gastrointestinal-and-hepatobiliary-system.
Frequently Asked Questions
Which test is most sensitive for enteric fever?
Bone marrow culture, at 80 to 95 percent sensitivity, which remains positive even after several days of antibiotic therapy.
When is blood culture most useful in enteric fever?
In the first week of illness, with 60 to 80 percent sensitivity, before antibiotics and before the organism moves into the reticuloendothelial system.
How should the Widal test be interpreted?
A fourfold rise in titre between acute and convalescent sera is the only reliable interpretation, as single titres are unreliable in endemic populations.
Which typhoid vaccine is WHO-preferred for endemic countries?
Typbar-TCV, the Vi conjugate vaccine, which produces immunological memory and is effective from 6 months of age.
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