Ferritin across long-term conditions in England: cross-sectional primary care study

Background Iron deficiency (ID) is a readily treatable condition once identified. Ferritin is the primary diagnostic marker, but cut-offs vary and inflammation complicates interpretation in patients with long-term conditions (LTCs). Aim To describe ferritin distribution and the prevalence of threshold-defined low ferritin in adults with and without LTCs in primary care. Design and setting Cross-sectional observational study using routinely collected electronic health records from a national primary care database in England (1st January 2015 to 31st December 2021). Method Adults with >1 ferritin test in Clinical Practice Research Datalink (CPRD) Aurum were included. LTCs were identified using validated primary-care code lists. Outcomes included ferritin distribution and threshold-defined ID prevalence using World Health Organization (WHO) (<15 ug/L; <70 ug/L if inflammation) and National Institute for Health and Care Excellence (NICE) (<30 ug/L) cut-offs, stratified by sex and, in women, by age <50 versus >=50 as a proxy for menopausal status. Results 4,489,594 individuals were included; 55% (n=2,469,882) had >1 LTC. Ferritin was lowest in women <50 and in LTCs characterised by impaired absorption or blood loss (coeliac disease, inflammatory bowel disease). Among women <50 with an LTC, 80% had ferritin <70 ug/L versus 47% <30 ug/L, leaving 33% in the 30 to 70 ug/L range potentially missed by standard cut-offs; equivalent figures were 28% in women >=50 and 17% in men. Conclusion Threshold-defined low ferritin is very common across LTCs and disproportionately affects women, particularly those under 50. Condition-specific, inflammation-adjusted ferritin thresholds may improve detection, management, and equity in primary care.