Although no guidelines recommend that patients taking acid inhibitors be screened for B12 deficiency, ascertainment bias is potentially a concern if patients taking acid suppressing medications are more likely to be tested for B12 deficiency. We would expect this bias to influence the results primarily if patients taking medications were being screened at a higher rate for asymptomatic disease, leading to chance discovery, but not if the exposure actually caused symptomatic disease and pa- tients subsequently received a diagnostic evaluation. Although the rate of testing was higher among PPI users in the general population, the formal analyses adjusted for many fac-
tors (eg, duration of membership and age) not accounted for by these crude estimates, and the reasons for testing among the cases appeared to be symptom driven: only 10% of the cases sampled had no symptomatic indication for vitamin B12 test- ing. Thus, cases appeared to be defined by symptom-driven testing rather than testing from asymptomatic screening.