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Lab Diagnosis - MBBS Exam Answer Format

Diseases Covered: HIV/AIDS | Dengue | Chikungunya | KFD | Ebola

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HOW TO WRITE LAB DIAGNOSIS IN MBBS EXAM (Full Marks Format)

1. Specimen
2. Screening Tests
3. Confirmatory/Supplemental Tests
4. Antigen Detection
5. Molecular Methods
6. Virus Isolation
7. Non-specific / Hematological findings
8. Gold Standard - mention it explicitly

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1. HIV/AIDS - Laboratory Diagnosis

Specimen

A) Screening Tests (Antibody Detection)

• Most commonly performed test at blood banks and tertiary care sites
• Sensitivity and specificity >99.5%
• 3rd Generation ELISA - uses recombinant/synthetic peptides to detect HIV antibodies
• 4th Generation ELISA - detects BOTH HIV antibodies AND p24 antigen; reduces the window period significantly

• Immunochromatography (ICT / Lateral Flow) - e.g., Tri-dot test
• Particle Agglutination Assays (latex, gelatin, RBCs)
• Dip stick / Comb tests

Note for exam: India follows NACO protocol - 3 rapid tests using different principles

B) Confirmatory / Supplemental Tests

• Detects individual antibodies against specific HIV proteins (gp41, gp120, gp160, p24, p31, p51, p55, p65/66)
• Works on immunoblot principle
• Previously recommended by NACO; now largely replaced by molecular tests
• Positive if antibody to two of three: gp120/gp160, gp41, p24

• Confirmatory test
• Detects both IgM and IgG separately

C) p24 Core Antigen Detection

• Detected 12-26 days after infection
• Elevated during late stage (advanced AIDS)
• CDC criteria: presence of any two out of p24, gp120, gp160
• WHO criteria: presence of at least two envelope bands

D) Viral RNA Detection (Molecular Methods) ⭐ GOLD STANDARD

"Viral RNA Detection is the GOLD STANDARD method for confirmation of HIV diagnosis"

• Real-time RT-PCR (Quantitative) - quantifies viral load (10-14 days post-exposure)
  • Most sensitive and specific; best method for monitoring response to ART
• NASBA (Nucleic Acid Sequence-Based Amplification) - RNA detection
• bDNA (Branched DNA assay)
• RT-PCR - best for early diagnosis and confirmation
  • Most sensitive method; detects even few copies of viral RNA
  • Detects HIV earlier than all other methods
  • Best method for confirmation of HIV
  • Best for diagnosing HIV in infants <18 months (immune not reliable)

E) Non-specific Tests

• Low CD4 count
• Altered CD4:CD8 ratio
• Hypergammaglobulinemia
• Low beta-2 microglobulin

Uses of p24 Antigen Detection

• Screening of blood donations
• Confirmation of HIV/AIDS diagnosis
• Diagnose HIV in infants <18 months
• Diagnose late stage HIV/AIDS
• Monitor the window period

EXAM TIP - Window Period

• The period between entry of virus into body and detectable antibodies
• p24 antigen appears 12-26 days after infection
• Antibodies appear after 3-12 weeks
• 4th gen ELISA reduces the window period

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2. DENGUE - Laboratory Diagnosis

Specimen

A) NS1 Antigen Detection ⭐ GOLD STANDARD (Early Diagnosis)

• Available in ELISA and ICT formats
• Detectable from Day 1 of fever, remains positive up to 7 days
• Highly specific - differentiates between flaviviruses; specific to dengue serotypes

B) Antibody Detection

• Kits supplied by NIV, Pune
• Double sandwich ELISA using anti-human IgM antibody + dengue virus envelope proteins (all 4 serotypes)
• Signal enhanced by avidin-biotin complex (ABC)
• Sensitivity: excellent; Specificity: good (cross-reactivity with other flaviviruses is a limitation)

• IgM: appears after 5 days of fever, disappears within 90 days
• IgG: detectable at low titer in 14-21 days, slowly increases

• IgG rises rapidly (cross-reactive with other flaviviruses - can give false positives)
• IgM titer is LOW or undetectable

• Most specific serologic tests
• Cumbersome; used in reference labs

C) Rapid Diagnostic Tests (RDT)

• ICT for dengue IgM antibodies or NS1 antigen
• Poor sensitivity and specificity
• Government of India 2016 order: A positive RDT must be confirmed by ELISA

D) Virus Isolation

• Blood collected from -1 to +5 days of onset
• Inoculation into mosquito cell lines (C6/36 and AP61) or mouse
• Confirmation by ELISA
• Available only in reference centers (NIV Pune)

E) Molecular Methods (RT-PCR)

• Detects viral RNA (3'-UTR region)
• Most sensitive (80-90%) and specific assay (95%)
• Can be used for genotype detection
• Negative PCR = interpreted as "indeterminate"

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3. CHIKUNGUNYA - Laboratory Diagnosis

Specimen

A) Viral Isolation

• Done in mosquito cell lines
• Takes 1-2 weeks
• Useful for early diagnosis (0-7 days of illness)
• Available only in reference centers

B) Antibody Detection

• Uses virus lysate
• Sensitivity: 95%, Specificity: 98%
• Best available format

• IgM: Appears after 4 days of infection, lasts for 3 months
• IgG: Appears after 2 weeks, persists for years
• Detection of IgM OR fourfold rise in IgG titer = clinically significant

C) Molecular Methods ⭐ GOLD STANDARD

• Most sensitive and specific

D) Hematological (Non-specific) Findings

• Leukopenia with lymphocyte predominance
• Thrombocytopenia (rare)
• Elevated ESR
• Elevated C-reactive protein (CRP)

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4. KYASANUR FOREST DISEASE (KFD) - Laboratory Diagnosis

Specimen

A) Virus Isolation and Serology

• Virus isolation from blood
• IgM antibody detection by ELISA

B) Molecular Methods ⭐ GOLD STANDARD

• Nested RT-PCR and Real-time RT-PCR (recent development)
• Targets NS-5 non-coding region of viral RNA in serum
• Early, rapid, and accurate diagnosis

C) Non-specific Findings (Second Stage of Disease)

• Leukopenia
• Thrombocytopenia
• Decreased hematocrit
• Albuminuria (protein in urine)
• Abnormal CSF findings

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5. EBOLA VIRUS DISEASE - Laboratory Diagnosis

Specimen

Caution: All work must be done in Biosafety Level-4 (BSL-4) cabinets due to extreme risk of laboratory spread

A) Serum Antibody Detection

• ELISA detects both IgM and IgG separately using:
  • Recombinant nucleoprotein (NP) antigens
  • Glycoprotein (GP) antigens
• IgM: Appears after 7 days of symptoms, lasts 3-6 months
• IgG: Appears after 2 weeks, persists 3-5 years or more
• Other assays: Immunofluorescence test (IFT), antibody-phage indicator assay

B) Serum Antigen Detection (Capture ELISA) ⭐ GOLD STANDARD

• Target proteins: NP, VP40, GP
• Also: Immunohistochemistry and histopathology to localize viral antigen in tissue

C) Molecular Methods ⭐ GOLD STANDARD

• RT-PCR and Real-time RT-PCR
• Detects specific RNA: NP gene and GP gene
• Virus detectable after 3 days of fever onset, remains positive for 2-3 weeks
• Most useful early method

D) Electron Microscopy

• Shows typical filamentous viruses (characteristic J-shaped/shepherd's crook morphology)
• Useful for identification

E) Virus Isolation in Vero Cell Line

• Must be done in BSL-4 facility only
• Shows typical cytopathic effect

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QUICK COMPARISON TABLE - Gold Standard Methods

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EXAM WRITING TIPS - To Score Full Marks

1. Always start with specimen - examiners look for this
2. Classify tests clearly - Screening → Confirmatory → Molecular → Isolation
3. Mention Gold Standard explicitly - write "Gold Standard = ..." in bold
4. For ELISA - always mention generation (3rd vs 4th for HIV)
5. For antibody tests - always mention timing: when IgM appears, when IgG appears
6. Window period (HIV) - examiners frequently ask about this
7. For Dengue - mention NIV Pune for MAC-ELISA; mention 2016 govt. order for RDTs
8. For Ebola - always mention BSL-4 requirement - this is a high-value point
9. For RT-PCR - mention what gene/region is targeted (NP gene for Ebola, NS-5 for KFD, nSP1/nSP4 for Chikungunya)
10. Non-specific findings - always include hematological findings at the end
11. Draw a flowchart if asked for 10-mark question - examiners appreciate diagrams

Lab Diagnosis - Short & Easy to Remember

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1. HIV/AIDS

• 4th Gen ELISA - detects antibody + p24 antigen (best, reduces window period)
• Rapid tests - ICT, Tri-dot, Particle agglutination

• Western Blot - detects antibodies to gp41, gp120, p24

• Most sensitive, detects earliest, used for viral load monitoring
• Best for infants <18 months

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2. DENGUE

• MAC-ELISA = recommended in India, kits from NIV Pune
• IgM appears Day 5, lasts 90 days
• IgG appears Day 14-21 (cross-reactive in secondary infection)
• RDT positive must be confirmed by ELISA (Govt. India 2016 order)
• Virus isolation in mosquito cell line C6/36 (reference labs only)

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3. CHIKUNGUNYA

• RT-PCR ⭐ Gold Standard - detects nSP1, nSP4 genes
• MAC-ELISA - best serological test; sensitivity 95%, specificity 98%
• IgM: appears Day 4, lasts 3 months
• IgG: appears after 2 weeks, lasts years
• Virus isolation in mosquito cell line (1-2 weeks, reference only)

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4. KFD (Kyasanur Forest Disease)

• RT-PCR (real-time/nested) ⭐ Gold Standard - targets NS-5 non-coding region
• ELISA - IgM antibody detection
• Virus isolation from blood

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5. EBOLA

• RT-PCR ⭐ Gold Standard - detects NP gene, GP gene (positive from Day 3, up to 3 weeks)
• Capture ELISA - detects NP, VP40, GP antigens
• ELISA antibody - IgM (Day 7), IgG (after 2 weeks)
• Electron microscopy - shows filamentous virus
• Virus isolation in Vero cell line (BSL-4 only)

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MASTER COMPARISON (One Look Revision)

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EXAM MUST-WRITE POINTS (Never Skip These)

1. Specimen - always first line
2. Gold Standard - write it explicitly
3. IgM vs IgG timing - IgM = early/acute, IgG = late/past
4. For Dengue - NIV Pune + 2016 Govt. order
5. For Ebola - BSL-4 mandatory
6. For HIV - window period + NACO protocol
7. For KFD - NS-5 gene target in RT-PCR