How do you approach a conversation with a patient with a markedly positive PETH (>200 ug/L) who is insistent that they do not consume any alcohol?

Falsely positive PETh results are very rare. A possible cause is transfusion of PRBC from a donor who was a drinker. False positives from lab error are possible, so repeating in a few days after patient denial is worth it. The endogenous production of small amounts of alcohol by gut microorganisms has been reported, but usually results in negligible systemic concentrations, so I doubt a PETH value above 200 is possible. The so-called auto-brewery syndrome, where again microbial overgrowth leads to excessive fermentation, may result in symptoms of alcohol intoxication and can be seen in MASLD/MASH, resulting in theoretically high PETh levels above 200 mcg/L, but rarely sustained at those levels. So, in summary, repeating the level in patient...