LAUSR

Iron‐deficiency contributes to a ∼50% of anemia prevalence worldwide, but reference intervals for iron status tests are not optimized for anemia diagnosis. To address this limitation, we identified the serum ferritin (SF) thresholds associated with hematologic decline in iron‐deficient patients, and the SF thresholds from which an SF increase was associated with hematologic improvement. Paired red blood cell and SF measurements were analysed from two adult cohorts at Massachusetts General Hospital (MGH), from 2008‐2011 (N = 48 409), and 2016‐2018 (N = 10 042). Inter‐patient measurements in the first cohort were used to define optimal SF thresholds based on the physiologic relationship between SF and red cell measurements. Intra‐patient measurements (1‐26 weeks apart) in the second cohort were used to identify SF thresholds from which an SF increase was associated, with an increase in red cell measurements. The identified optimal SF thresholds varied with age, sex and red cell measure. Thresholds associated with a ∼5% decline in red cell index were typically in the range 10‐25 ng/mL. Thresholds for younger women (18‐45 year) were ∼5 ng/mL lower than for older women (60‐95 years), and ∼10 ng/mL lower than for men. Thresholds from which a subsequent increase in SF was associated with a concomitant increase in red cell measure showed similar patterns: younger women had lower thresholds (∼15 ng/mL) than older women (∼25 ng/mL), or men (∼35 ng/mL). These results suggest that diagnostic accuracy may be improved by setting different SF thresholds for younger women, older women, and men. This study illustrates how clinical databases may provide physiologic evidence for improved diagnostic thresholds.