BackgroundHematin is a state in which hemoglobin, as petechiae, is discolored to a brown coffee color by gastric hydrochloric acid. Given the nature of hematin, a close relationship between hematin and acidity has been suggested, but has not been confirmed. We investigated the clinical significance of endoscopic finding of hematin with respect to gastric acidity.MethodsA total of 501 patients were assessed for both hematin and fasting gastric juice pH by endoscopy. Endpoints were as follows: 1) the relationship between the presence of hematin and the fasting gastric juice pH; and 2) the diagnostic performance of endoscopic hematin. In addition, we performed a supplementary in vitro study to clarify the relationship between hematin formation and various acid pH levels.ResultsThe prevalence of hematin was 31.1% (142/206), 4.6% (5/109) and 45.2% (84/186) in the H. pylori-uninfected, -infected and -eradicated groups, respectively. The mean pH of fasting gastric juice in the hematin-positive cases was significantly lower than the hematin-negative cases (mean pH 1.2, 95% confidence interval [CI] 1.1-1.3 vs. 2.7 95%CI 2.5-3.0; P<0.001). The sensitivity, specificity, positive predictive value and negative predictive value of hematin for predicting strong acidic condition (pH 1 or 2 for fasting gastric juice) were 36.0%, 98.1%, 98.7% and 29.3%, respectively. Interobserver agreement was categorized as “excellent” (k=0.88). Supplementary in vitro results showed that hematin formation was only observed at a pH=1.ConclusionEndoscopic finding of hematin represent strong gastric acidity.