AbstractObjectivesLactose malabsorption is generally assessed by hydrogen breath testing (HBT). However, this test is not recommended in patients with high baseline hydrogen concentrations (H2B). In addition, breath testing is not recommended in the current situation created by the COVID-19 pandemic, due to the potential infectiveness of the samples. The objective is to assess concordance between HBT and lactose tolerance test (LTT) depending on H2B concentrations.MethodsA total of 430 patients (40 years, Q1–Q3 = 28–54 years; 66.7% women) suspected of lactose malabsorption were included in the study. Breath and heparinized blood samples were collected at baseline and sequentially after the intake of 50 g of lactose, to measure hydrogen in breath and glycemia in blood, respectively.ResultsH2B was <10 ppm in 69.5% of subjects; 10–20 ppm in 14.7%; and >20 ppm in 15.8% of subjects. In patients with H2B <20 ppm, concordance between HBT and LTT was moderate and consistently improved when the cut-off in LTT was set at 15 mg/dL. The increase in hydrogen and glucose correlated negatively (r=−0.389; p<0.05). The increase in glycemia during LTT was not influenced by H2B levels obtained in HBT.ConclusionsLTT emerges as an alternative to HBT to assess lactose malabsorption in the presence of high H2B levels or when breath testing is not recommended by the circumstances. The best concordance was obtained when the cut-off for LTT was set at 15 mg/dL.