The hydrogen breath test is a non-invasive diagnostic tool used primarily to identify lactose intolerance, small intestinal bacterial overgrowth (SIBO), and other carbohydrate malabsorption disorders by measuring hydrogen levels in the breath after carbohydrate ingestion. Elevated hydrogen levels indicate improper digestion or absorption, as undigested carbohydrates are fermented by intestinal bacteria, producing hydrogen that is absorbed into the bloodstream and exhaled in the breath.