The Giardia antigen test is used to make a diagnosis of giardiasis  the digestive tract illness caused by Giardia lamblia A doctor may order the test if your child has symptoms such as watery diarrhea  abdominal pain  large amounts of intestinal gas  appetite loss  and nausea or vomiting  especially if there's been an outbreak of giardiasis at your child's school or daycare center  your child recently drank untreated water  or if your family recently visited a developing country