An antigen test may be done if a person's medical history and symptoms suggest giardiasis.

Older antigen tests detect the surface Lancefield group A carbohydrate.

However, the likelihood of a positive antigen test depends on the number of adult female worms present.

Even with female worms, an antigen test will usually only become positive seven to eight months after infection.

The antibody test is more sensitive than the antigen test, but it does not provide direct evidence of adult infection.

The antigen test can also be used to monitor the effectiveness of drugs.

An antigen test also may be used to screen people who are at high risk for having giardiasis.

Known as the prostatic-specific antigen test, it costs £15 and is currently used by about 20% of GPs.

Most antibody and antigen tests are not very helpful because they cannot tell the difference between current and past infection.

To know for sure, you need a culture or rapid antigen test, which takes less than 20 minutes and can be done while you wait.