Laboratory Reports (Investigations) for Dengue

1. Routine Blood Tests (CBC + Metabolic)

Serial CBC is the backbone of dengue monitoring. A full blood count should be done at first visit to establish the patient's baseline haematocrit. A rapidly decreasing platelet count in parallel with a rising haematocrit is suggestive of progression to the plasma leakage/critical phase. - Park's Textbook of Preventive and Social Medicine

2. Specific Dengue Diagnostic Tests

A. Acute Phase Tests (Days 1-5 of fever)

• Detects dengue viral RNA in serum
• Highest sensitivity in the first 4-5 days (~85-90%)
• Also identifies the specific serotype (DENV 1-4)
• Turnaround: 1-2 days
• Gold standard for early diagnosis alongside NS1

• NS1 (non-structural protein 1) is released into serum during early infection
• Detectable from Day 1 up to Day 6-9 of illness
• Available as rapid test (minutes) or ELISA (1 day)
• Works in both primary and secondary dengue infection
• Specific to dengue virus; does not differentiate serotypes
• Tietz Textbook of Lab Medicine notes: "NS1 is specific to dengue virus and detectable in serum during the acute febrile phase, filling a key role for early diagnosis."

• Inoculation into mosquito or mosquito cell culture
• Very high specificity but time-consuming (≥1 week)
• Specimens: acute serum/plasma, washed buffy coat, autopsy tissue (liver, spleen, lymph nodes, thymus), or mosquitoes from the affected area
• Practical only in reference/research labs

B. Convalescent Phase Tests (After Day 5)

• Best single test after ~Day 4-5 of fever
• A single strongly positive serum sample suggests recent dengue infection
• Sensitivity peaks between Day 7 and Day 90
• Can remain positive for 2-3 months
• Cross-reactivity with other flaviviruses (Zika, yellow fever, West Nile) may give false positives

• IgG appears later and persists for years
• Indicates past infection, not necessarily current
• A fourfold or greater rise in IgG between acute and convalescent paired sera (collected 15-21 days apart) confirms diagnosis
• HIA (Haemagglutination Inhibition Assay): titre ≥1:1280 on a single sample supports probable diagnosis

• Helps distinguish primary from secondary dengue infection
• High IgM/low IgG = primary infection
• Low IgM/high IgG = secondary infection (higher risk for severe disease)

• Most specific serological test; used to confirm positive IgM
• Distinguishes between dengue serotypes and other flaviviruses
• Labour-intensive; restricted to reference labs

3. Summary Table: Dengue Diagnostics by Phase

4. WHO Case Classification Criteria (Diagnostic Thresholds)

• Supportive serology (HI titre ≥1280, or positive IgM)
• Occurring in same location/time as confirmed cases

• Isolation of dengue virus from serum/CSF/autopsy tissue, OR
• Fourfold+ rise in IgG (HI test) or rise in IgM, OR
• Detection of antigen/virus by IHC, immunofluorescence, or ELISA, OR
• Detection by RT-PCR

• Fever 2-7 days
• Haemorrhagic manifestations (positive tourniquet test, petechiae, mucosal bleeding)
• Platelet count ≤100,000 cells/mm³
• Evidence of plasma leakage (haematocrit rise ≥20%, or pleural effusion/ascites/hypoproteinaemia)

• Park's Textbook of Preventive and Social Medicine, p. 293-294
• Goldman-Cecil Medicine, 26th Ed.
• Henry's Clinical Diagnosis and Management by Laboratory Methods

5. Practical Testing Algorithm

Day 1-4 of fever:
  → RT-PCR (dengue RNA) + NS1 Antigen test
  → Serial CBC (platelet, haematocrit, WBC)

Day 5+ of fever / after defervescence:
  → IgM ELISA (MAC-ELISA)
  → Continue serial CBC - watch for platelet drop + haematocrit rise

Equivocal IgM or in flavivirus-endemic area:
  → Confirm with PRNT (plaque reduction neutralization test)

Suspected DHF/DSS:
  → Urgent platelet count + haematocrit
  → CXR / ultrasound for pleural effusion/ascites