The oipA (outer inflammatory protein A) virulence factor is a genetic marker associated with more aggressive strains of Helicobacter pylori. Its presence helps evaluate the inflammatory potential of an H. pylori infection and the likelihood of gastric irritation or tissue damage.

Hydrogen is produced when certain bacteria in the small intestine consume sugars and produce hydrogen gas as a byproduct.

Hydrogen levels are considered abnormal when they rise greater than ≥20 ppm (parts per million) from the baseline within 90 minutes.

In healthy humans, hydrogen gas is exclusively produced by intestinal bacteria - primarily a result of carbohydrate fermentation by anaerobic bacteria in the colon. In SIBO, fermentation of the malabsorbed lactulose substrate by bacteria residing in the small intestine results in elevated concentration of exhaled hydrogen (H2).

Trio-smart measures a third fermented gas, hydrogen sulfide (H2S). Which is produced by sulfate-reducing bacteria utilizing H2 to produce H2S. Clinical trials have noted that H2S is associated with diarrhea in patients. In a 2021 study, it was found that healthy subjects had H2S levels of <3.00 ppm. Levels of hydrogen sulfide ≥3.00 ppm are associated with diarrhea and indicative of excess hydrogen sulfide. Higher levels of hydrogen sulfide predict more severe diarrhea.

Hemoglobin (Hb) is the iron-containing oxygen transportation protein in red blood cells. It's rate of binding oxygen depends on the number oxygen molecules already bound.

Haemophilus is a genus of bacteria that can inhabit mucosal surfaces, including the gut. While often present at low levels, abnormal abundance may signal dysbiosis, impaired gut barrier function, or immune activation, particularly in inflammatory gut environments.

Hafnia is a genus of bacteria naturally present in the human gut and considered a potential biomarker of gut health. Both low and elevated levels may reflect microbiome imbalance, altered immune signaling, or metabolic stress, making Hafnia a useful indicator on comprehensive stool testing.