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MI3.5-8 | Enteric Fever, PUO & Sepsis — SDL Guide (Part 2)

Laboratory Diagnosis: Which Test on Which Day?

This is the single most high-yield examination topic in enteric fever microbiology. The choice of test depends on which week of illness the patient presents in:

Week of Illness	Organisms in	Best Test	Sensitivity
Week 1 (Days 1–7)	Blood, bone marrow	Blood culture / Bone marrow culture	80–90% (BM) / 60–80% (blood)
Week 2 (Days 8–14)	Blood still positive; antibodies rising	Blood culture + Widal	Blood culture peak sensitivity
Week 3 (Days 15–21)	Urine, stool; antibodies high	Widal / Stool culture	Widal most reliable here
Week 4	Stool, urine	Stool/urine culture	Carrier detection

Blood culture:
- Best specimen in first 2 weeks — take 10 mL blood in 100 mL broth (1:10 dilution to dilute out serum bactericidal factors)
- Transport: bile broth, brain-heart infusion broth, or tryptic soy broth at 37°C (NOT room temp, NOT refrigerated)
- Incubate 7–14 days (Salmonella may take longer than staphylococci)
- Bone marrow culture is the gold standard at ANY stage of illness (90% sensitivity even after antibiotics)

Widal test:
- Tube agglutination method (quantitative, preferred) or slide method (qualitative, rapid screening)
- Detects agglutinating antibodies against O antigen and H antigen of Salmonella
- Significant titres (single specimen): O ≥ 1:160, H ≥ 1:160 (endemic areas — baseline titres higher, so use 4-fold rise in paired sera)
- O antibodies: rise early, fall quickly — indicate acute/current infection
- H antibodies: rise later, persist longer — indicate past infection or immunisation
- Vi antibody — detected in chronic carriers (chronic biliary carrier screening)

Stool/urine culture:
- Weeks 3–4 of illness and for carrier detection
- Selective media: Wilson-Blair bismuth sulphite agar (black metallic sheen — S. Typhi), MacConkey agar (NLF colonies), XLD (Xylose Lysine Deoxycholate) agar (red/pink colonies with black centres for Salmonella)
- Enrichment broth: Selenite F broth (before plating on selective agar)

Carrier detection:
- Stool culture × 3 (over 3 separate days)
- Vi agglutination: Vi titre ≥ 1:20 suggests carrier state → confirm with multiple stool cultures
- Vi-phage typing for epidemiological strain tracing

Diagnostic Window in Enteric Fever

Interpreting Widal Test Results

Critical principle: The Widal test is non-specific and fraught with interpretation pitfalls in India — endemic baseline titres are high due to widespread exposure, prior immunisation (TAB vaccine), and cross-reactions.

Step-by-step Widal interpretation:

Step 1 — Is there a significant titre?
- In non-endemic areas: O ≥ 1:80, H ≥ 1:160 considered significant
- In endemic areas (India): O ≥ 1:160, H ≥ 1:320 — or better yet, 4-fold rise between acute (Day 7) and convalescent (Day 21) paired sera

Step 2 — What pattern do you see?

Pattern	Interpretation
O high, H low	Acute current infection
O high, H high	Current infection or recent illness
O low, H high	Past infection OR prior TAB vaccination
O low, H low	Not typhoid (if truly in Week 3+)

Step 3 — Consider cross-reactions:
- Other Salmonella infections (group D, C1, C2)
- Malaria, liver disease, collagen disorders — may give false positive O
- Prior TAB vaccine — raises H titre only, not O
- Dengue — documented non-specific elevation

When is a positive Widal USEFUL?
- In resource-limited settings without blood culture capability
- When paired sera show a 4-fold rise
- Combined with clinical picture (Week 3 fever, rose spots, splenomegaly)

Newer tests (replacing Widal progressively):
- Typhidot / Typhi IgM dot-ELISA — detects IgM and IgG against S. Typhi 50 kDa OMP; sensitive in first week
- Typhidot-M — IgM only (eliminates IgG background — more specific in endemic areas)
- Tubex test — detects anti-O9 (S. Typhi specific LPS) antibodies; colorimetric; rapid
- PCR/qPCR — detects flagellin/Vi gene; available in reference laboratories