The guaiac test is a medical screening tool used to detect the presence of blood in stool samples. It is commonly used to help diagnose conditions such as gastrointestinal bleeding, colon cancer, or hemorrhoids. The test works by applying a small amount of stool to a special card that contains guaiac, a plant resin that reacts with hemoglobin in blood, producing a color change. This test is non-invasive and can be performed at home or in a doctor's office. Patients are usually advised to avoid certain foods and medications before the test, as they can affect the results. If the test is positive, further evaluation may be necessary to determine the underlying cause.