Stool antigen refers to specific proteins or molecules derived from pathogens that can be detected in a patient's stool sample, commonly used for diagnosing infections such as those caused by Helicobacter pylori or Giardia lamblia. This diagnostic tool is crucial in clinical practice as it allows for non-invasive testing, guiding appropriate treatment decisions. A key association is its role in confirming eradication of H. pylori after therapy, helping to prevent complications like peptic ulcers.