Background: The prevalence, characteristic and determinants of anemia, at the time of inflammatory bowel disease (IBD) diagnosis have yet to be fully elucidated.

Methods: Retrospective cross-sectional study. Analytical data and disease characteristics obtained upon diagnosis of 1278 IBD patients [Crohn's disease/ulcerative colitis (CD/UC): 718/560] were collected.

Results: Anemia was present in 41.2% of patients at diagnosis (47% and 33.8% of CD and UC patients, respectively; p<0.001), being severe in 5.5%. Iron deficiency anemia represented 69.6% of cases, with no differences between CD and UC. Female sex was the strongest risk factor for anemia in both CD and UC (OR 7.11; 95%CI 4.18-12.10 and 6.55; 95%CI 3.39-12.63, respectively), followed by elevated (≥2mg/dL) C-reactive protein (OR 4.08; 95%CI 2.39-6.97 and 4.58; 95%CI 2.26-9.27, respectively). Current smoking was a risk factor for anemia in CD (OR 2.23; 95%CI 1.24-4.02), but a protective one in UC (OR 0.36; 95%CI 0.14-0.92). A penetrating CD behavior increased the risk of anemia (OR 3.34; 95%CI 1.36-8.21); in UC, anemia increased with disease extension (E2+E3) (OR 1.80; 95%CI 1.13-2.86).

Conclusions: Female sex and disease activity are major determinants of anemia at IBD diagnosis. Anemia is associated with disease behavior in CD and with disease extension in UC.