Diagnosis of Typhoid Fever (Enteric Fever)

Typhoid fever is caused by Salmonella enterica serotype Typhi (S. Typhi), with a similar syndrome caused by S. Paratyphi A, B, and C. Because its clinical presentation is relatively non-specific, a systematic diagnostic approach combining clinical assessment, laboratory testing, and culture is essential.

1. Clinical Diagnosis

History & Epidemiology

The diagnosis must be considered in any febrile traveler returning from an endemic region (Indian subcontinent, Southeast Asia, Africa, South America) or anyone with exposure to contaminated food/water. Key risk factors include travel to endemic areas, exposure to carriers, poor sanitation, and consumption of street food.

Clinical Stages

Week	Dominant Features
Week 1	Insidious fever rising stepwise to 39–40°C, headache, malaise, dry cough, relative bradycardia (pulse-temperature dissociation), constipation
Week 2	Sustained high fever ("typhoidal" plateau), splenomegaly, hepatomegaly, abdominal distension, rose spots
Week 3–4	Risk of intestinal bleeding and perforation; delirium ("muttering delirium"), prostration

Classic Signs

Rose spots: 1–4 mm blanching pink macules on the trunk/chest, seen in ~30% of patients; faint and easily missed in dark-skinned individuals
Relative bradycardia: Pulse disproportionately slow relative to the fever height
Splenomegaly: Develops after the first week
Leukopenia: WBC normal or low (unlike sepsis from other causes)

2. Laboratory Investigations

Routine Blood Tests

CBC: Leukopenia or normal WBC (not leukocytosis); anemia, thrombocytopenia possible
LFTs: Mildly elevated transaminases
Electrolytes/renal function: To detect complications

Definitive Diagnosis: Culture

Culture is the gold standard. Sensitivity varies by site and timing:

Culture Site	Sensitivity	Timing	Notes
Blood culture	40–60%	Weeks 1–2 (highest yield)	Low yield in early/treated cases; small volumes reduce sensitivity
Bone marrow culture	~80%	Any stage	Most sensitive single test; still positive after up to 5 days of antibiotics
Stool culture	30–40% (early); improves wk 3	Week 2–3 onward	Negative in 60–70% in week 1
Urine culture	Lower yield	Week 2 onward	Adjunct test
Intestinal secretions (duodenal string test)	High	Any stage	Can be positive even with negative bone marrow
Combined (blood + bone marrow + intestinal secretions)	>90%	—	Best combined strategy

Key principle: Blood cultures should be taken before antibiotics. If blood cultures are negative but clinical suspicion remains high, bone marrow culture is the next step — it is not reduced by prior antibiotic therapy.

3. Serological Testing (Widal Test & Rapid Diagnostics)

Widal Test

Detects agglutinating antibodies against S. Typhi O and H antigens
Significant limitation: Low specificity due to cross-reactions with other Salmonella serotypes, prior vaccination, and endemic background titers; also, a single acute-phase titer is unreliable
A fourfold rise in paired (acute + convalescent) titers is more meaningful
Not recommended as the sole diagnostic test

Rapid Immunodiagnostic Tests (Tubex, Typhidot)

Detect IgM (and IgG) antibodies to O and H antigens
Sensitivity: 70–80%; Specificity: 80–90% (2017 systematic review)
Useful at point of care in resource-limited settings (cheap, simple, no lab equipment)
Not sufficiently accurate to replace blood culture as the primary diagnostic approach
— Harrison's Principles of Internal Medicine 22E, p. 1360

4. Molecular Diagnosis (PCR)

PCR targeting S. Typhi-specific genes from blood samples
Sensitivity: 40–100% depending on the gene target and platform
Historically limited in low-resource settings, but increasing availability post-COVID-19 pandemic due to expanded molecular diagnostic infrastructure
Offers faster results than culture; useful in antibiotic-pretreated patients

5. Differential Diagnosis

Other conditions to consider in a febrile traveler include:

Malaria (must be excluded first)
Viral hepatitis (A, B, E)
Dengue fever
Rickettsial infections (typhus)
Leptospirosis
Amebic liver abscess
Brucellosis
Acute HIV infection
Bacterial enteritis

Tintinalli's Emergency Medicine notes: "Once malaria is excluded, typhoid fever is commonly the cause of a febrile illness lasting >10 days."

6. Diagnostic Algorithm Summary

Febrile patient + travel history / exposure
        ↓
Exclude MALARIA first
        ↓
Clinical features: stepwise fever, relative bradycardia, rose spots, splenomegaly, leukopenia
        ↓
Blood culture × 3 (before antibiotics) ──→ Gold standard (40–60% sensitive)
        ↓ (if negative, high suspicion persists)
Bone marrow culture ──────────────────→ Most sensitive (~80%), works after antibiotics
        ↓
Rapid tests (Typhidot/Tubex) ─────────→ Point-of-care support in endemic areas
Widal test (paired titers) ───────────→ Adjunct only; not stand-alone
PCR ──────────────────────────────────→ Emerging, fast, useful in treated patients
Stool/urine cultures ─────────────────→ Week 2 onward

7. Key Points

Feature	Detail
Causative organism	Salmonella Typhi / Paratyphi
Definitive test	Culture (blood → bone marrow → stool/urine)
Best sensitivity (single test)	Bone marrow culture (~80%)
Combined culture yield	>90% (blood + bone marrow + intestinal secretions)
Rapid POC tests	Tubex, Typhidot — sens 70–80%, spec 80–90%
Widal test	Unreliable alone; fourfold rise in paired titers meaningful
PCR	40–100% sensitivity; increasingly available

Sources:

Harrison's Principles of Internal Medicine 22E (2025), Chapter 171
Tintinalli's Emergency Medicine: A Comprehensive Study, Chapter 162
Jawetz, Melnick & Adelberg's Medical Microbiology, 28th Ed., Chapter 15
Sleisenger and Fordtran's Gastrointestinal and Liver Disease, Chapter 110

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