Casoni Test for Echinococcus granulosus
The Casoni intradermal skin test is no longer recommended for diagnosing hydatid disease and has been replaced by modern serological methods combined with imaging studies. 1
What the Casoni Test Is
The Casoni test is an intradermal skin test that was historically used to diagnose cystic echinococcosis (hydatid disease) caused by Echinococcus granulosus. 2, 3 The test involves:
- Intradermal injection of hydatid cyst fluid antigen into the forearm 2, 3
- Reading both immediate (15-30 minutes) and delayed (24-48 hours) reactions 2
- A positive result shows a wheal and flare response at the injection site 3
Why It Is No Longer Recommended
Current UK and international guidelines do not include the Casoni test in diagnostic algorithms for echinococcosis. 1 The test has been abandoned for several critical reasons:
Poor Diagnostic Performance
- Specificity is unacceptably low at only 47-87%, meaning high rates of false-positive results 2, 4
- Sensitivity ranges from 63-70%, missing approximately one-third of true cases 2, 4
- False-positive reactions occur in patients with other parasitic infections and non-hydatid cystic lesions 2, 3
Risk of Severe Adverse Reactions
- The test can trigger anaphylaxis in sensitized patients, particularly those with leaking or ruptured cysts 1, 5
- This risk is unacceptable when safer diagnostic alternatives exist 5, 6
Lack of Standardization
- Antigen preparations varied widely between manufacturers, making results unreliable and non-reproducible 3, 7
- No standardized interpretation criteria were ever established 7
Current Recommended Diagnostic Approach
Modern diagnosis of cystic echinococcosis relies on imaging combined with serology, not skin testing. 1, 8
First-Line Investigations
- Ultrasound is the initial imaging modality of choice for suspected hepatic hydatid cysts 1, 8
- Serology using ELISA or immunoblot has >90% sensitivity for liver cysts and does not carry anaphylaxis risk 5, 9
- CT or MRI with contrast (arterial and venous phases) for unclear cases or staging 6, 8
Key Diagnostic Features
- WHO classification system (CE1-CE5) based on ultrasound appearance guides treatment decisions 6, 8
- Pathognomonic findings include daughter cysts, "water lily sign" (detached membrane), and wall calcification 6, 8
- Serology is not invariably positive, especially in intact, non-leaking cysts 1
Important Caveats
- Never perform percutaneous aspiration or biopsy without reviewing serology first due to anaphylaxis risk 5, 6
- Eosinophilia is present only with leaking cysts, not in most asymptomatic cases 1
- Liver cysts have higher serological sensitivity (81-90%) than lung cysts 2, 3
Historical Context
While the Casoni test had some utility in resource-limited settings decades ago 4, 7, the availability of safer, more accurate serological tests (ELISA, immunoblot) and imaging has rendered it obsolete. 9, 7 The test is mentioned only in historical literature reviews and is not part of any contemporary diagnostic algorithm. 1, 8