Diagnosis of giardiasis is usually based on:

• Medical history and physical exam. If your history and physical exam strongly suggest the symptoms are due to giardiasis, some doctors may prescribe a single course of treatment before doing further tests.
• Stool analysis (also known as ova and parasite test).
• Antigen tests that detect proteins (antigens) from the Giardia parasite.
• String test, which may be used if the stool analysis is inconclusive. The string test collects a sample of the contents of the small intestine so it can be examined for the presence of parasites. This test is rarely done.

Fluid from the upper part of the small intestine (duodenal fluid) may also be looked at under a microscope to look for the parasite or to do an antigen test. A duodenal fluid sample is collected by endoscopy.

Other tests to help detect giardiasis or similar conditions may sometimes be done if you have long-term symptoms, especially if problems with malabsorption are occurring. These tests are used to examine the small intestine (upper GI series) or to collect a sample (biopsy) of the upper small intestine to identify what is causing the infection.

Early Detection

When outbreaks of giardiasis occur in nursing homes or day care centers, it may be appropriate to check those who may be infected. People who are infected but don't have symptoms can pass the infection to others.