🧫 Entamoeba histolytica IgG Test: Complete Clinical Guide

Diagnosis of Amebiasis · ELISA Method · Interpretation · Precautions

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1. Overview of Entamoeba histolytica

✔ Characteristics of the Pathogen

• Protozoan parasite causing amoebiasis
• Exists in two forms:
  • Cyst (infective, resistant to gastric acid)
  • Trophozoite (invasive form causing colon and liver damage)
• Invades colonic mucosa → may disseminate to the liver → amoebic liver abscess (ALA)

✔ Transmission

• Fecal–oral route
• Contaminated food/water
• Person-to-person spread
• Common in regions with poor sanitation

✔ Major Clinical Presentations

• Amoebic dysentery: abdominal pain, fever, bloody diarrhea
• Amoebic liver abscess: right upper quadrant pain, fever, hepatomegaly
• Stool microscopy often fails to detect trophozoites in ALA → serology is essential

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2. Purpose of the Entamoeba histolytica IgG Test

✔ Why is IgG testing essential?

• Highly sensitive for invasive amebiasis, especially amoebic liver abscess
• Helps diagnose cases where:
  • Stool microscopy is negative
  • PCR is unavailable
  • CT/ultrasound shows liver lesions
• Useful for evaluating:
  • Travelers returning with fever + liver mass
  • Patients with suspected ALA vs. pyogenic liver abscess
• Helps determine past vs. recent invasive infection

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3. Testing Method – ELISA

The Entamoeba histolytica IgG test is predominantly performed using ELISA (Enzyme-linked immunosorbent assay).

✔ Principle of ELISA

1. Microplate coated with E. histolytica-specific antigens
2. Patient serum added → antigen-antibody binding
3. Enzyme-labeled secondary antibody binds IgG
4. Substrate reaction → color change
5. Optical density (OD) measured → compared to cut-off values

✔ Strengths

• Sensitivity >90% in amoebic liver abscess
• Detects antibodies well in invasive disease
• More reliable than stool testing for ALA

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4. Reference Range (General)

(Cut-offs vary by manufacturer)

Result	Interpretation
Negative	Index < 0.9 or OD below cut-off
Equivocal	0.9–1.1 → repeat testing
Positive	>1.1 → strong suspicion for invasive amoebiasis

Always follow the reference values provided by your laboratory’s kit.

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5. Clinical Significance

✔ 1) Diagnosis of Invasive Amebiasis

• IgG is positive in >90% of patients with amoebic liver abscess
• Appears 7–10 days after symptom onset
• Essential for early diagnosis when imaging suggests ALA

✔ 2) Intestinal Amebiasis

• IgG may be weakly positive
• Non-invasive infections may show low antibody titers

✔ 3) Past Infection vs. Recent Infection

• IgG persists for years, sometimes lifelong
• Cannot reliably distinguish old vs. new infection
• Must correlate with:
  • Clinical presentation
  • Imaging findings
  • CBC, CRP
  • Stool PCR if available

✔ 4) Differentiation from Pyogenic Liver Abscess

• Pyogenic abscess → IgG negative
• ALA + IgG positive combination → highly diagnostic

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6. Interpretation of Results

✔ IgG Positive

• Strongly suggests invasive amebiasis, especially when:
  • RUQ pain
  • Fever
  • Liver lesion on imaging
• But:
  • Past infection may also remain positive → clinical correlation required

✔ IgG Negative

• Early stage infection possible
• Non-invasive amebiasis may have low titers
• If suspicion remains high → repeat test + stool PCR + imaging

✔ Equivocal

• Repeat in 1–2 weeks
• Parallel stool testing recommended

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7. Critical Precautions

⚠ 1) IgG does not distinguish old vs. current infection

• Antibodies persist for years
• Must integrate imaging and clinical picture

⚠ 2) Noninvasive intestinal infection may show only low IgG

• Full seroconversion sometimes absent

⚠ 3) Cross-reactivity with other Entamoeba species

• E. dispar, E. moshkovskii
• These species are noninvasive → false-positive possible

⚠ 4) Immunocompromised patients

• Slower or weaker antibody response
• Consider PCR or antigen tests when available

⚠ 5) Stool tests often negative in liver abscess

• Only <30% of ALA patients have positive stool results
• Serology is more dependable for diagnosis

Hantaan Virus Antibody Test: Clinical Interpretation

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8. Quick Summary

Category	Key Points
Test purpose	Diagnose invasive amoebiasis & liver abscess
Method	ELISA (IgG antibody detection)
IgG kinetics	Rises 7–10 days, persists for years
Positive IgG	Strongly suggests invasive disease
Precautions	Cannot differentiate old vs new infection; cross-reactivity

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📚 References

• Stanley SL Jr. Amoebiasis. Lancet. 2003.
• WHO. Amoebiasis: Diagnostic and Epidemiologic Guidelines.
• Fotedar R, Stark D, et al. Diagnostic techniques for Entamoeba species. Clin Microbiol Rev. 2007.
• Haque R, et al. Amebiasis. N Engl J Med. 2003.
• CDC. Amebiasis – Diagnosis & Laboratory Testing.