Comparative performance of symptoms questionnaire, hydrogen test and genetic test for lactose intolerance Background: Genetically programmed adult-type hypolactasia affects 56% of Chilean population.Ideally, diagnosis should be confirmed.Aim: To compare diagnostic yield of genetic test, hydrogen (H 2 ) expiratory test and a validated symptomatic structured survey (SS).Material and Methods: Patients submitted to H 2 test answered a historic (anamnestic) and current SS (after the ingestion of 25 g of lactose). A blood sample was obtained for determination of genetic polymorphisms C/T_13910, C/G_13907 and G/A_22018 by polymerase chain reaction. The gold standard for diagnosis of lactose intolerance (LI) was the agreement of at least two of three tests.Results: Sixty-one participants aged 39 ± 12 years (21 males), were studied.Anamnestic SS was diagnostic of LI in all cases (score ≥ 7), while current SS detected LI in 27/61 (46%).H 2 test (an increase ≥ 15 ppm after ingestion of 25 g of lactose) showed LI in 31/61 (51%).The locus C/G_13907 showed no polymorphism and locus G/A_22018 was in complete linkage disequilibrium with C/T_13910.Genotype C/C_13910, associated to hypolactasia, was present in 30/58 (52%).According to the gold-standard, 32/61 (52.5%) patients were diagnosed as LI.Sensitivity and specificity were, respectively, 79% and 69% for current SS, 93% and 93% for H2 test and 97% and 93% for the genetic test.The last two showed a positive likelihood ratio (LR) > 10 and a negative LR < 0.1, figures within the range considered clinically useful.Conclusions: Genotype C/C_13910 is responsible for hypolactasia in this population.Anamnestic report of symptoms after milk ingestion and symptoms after lactose ingestion, are not accurate enough.H 2 and genetic tests are simple and similarly accurate to diagnose lactose intolerance in adults.