Question
Casoni’s intradermal test
Answer
Casoni’s intradermal test is an older, historically significant immediate (Type I) hypersensitivity skin test used for the presumptive diagnosis of hydatid disease (echinococcosis), caused by Echinococcus granulosus.
Principle: sterile hydatid cyst fluid antigen is injected intradermally; in a previously sensitized individual (i.e., someone harbouring, or having previously harboured, a hydatid cyst), this provokes an immediate wheal-and-flare reaction at the injection site within 15–20 minutes, analogous to an allergy skin-prick test — reflecting the presence of IgE antibody sensitized against hydatid antigen from prior/ongoing exposure to cyst fluid (which is highly antigenic).
Interpretation: a positive test (wheal ≥1–2 cm with surrounding erythema) supports the diagnosis of hydatid disease, but the test has significant limitations.
Limitations (why it has been largely abandoned):
- Variable and only moderate sensitivity — false negatives occur, particularly with cysts that are intact/non-leaking (since antigen exposure driving sensitization requires some degree of prior antigen leakage) or in early infection.
- Cross-reactivity/false positives with other helminth infections.
- Risk of anaphylaxis — since it involves injecting a small amount of the same highly antigenic material that can cause severe anaphylaxis if a hydatid cyst ruptures in vivo, the test itself carries a real, albeit small, risk of triggering an allergic/anaphylactic reaction in a sensitized individual — a significant safety concern.
- Can also falsely sensitize the patient to hydatid antigen for subsequent serological testing.
Modern practice: Casoni’s test has been largely replaced by safer, more reliable methods — serology (ELISA, indirect haemagglutination) for antibody detection, and, most importantly, imaging (ultrasound, CT, MRI), which is now the primary diagnostic modality for hydatid disease, providing direct visualization of the characteristic cystic lesion without any of the safety concerns associated with the Casoni test.