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Background Iron deficiency (ID) is common in patients with heart failure (HF). Current guidelines define ID based on serum ferritin and transferrin saturation (TSAT) rather than soluble transferrin receptor (STFR) or ratios such as iron/STFR or TSAT/STFR. We investigated the associations between these biomarkers and prognosis in patients with new-onset HF with reduced ejection fraction (HFrEF).Methods All patients hospitalised in 2016–2020 at Sahlgrenska University Hospital with new-onset HFrEF who had iron biomarkers measured within 6 months of discharge were included, with follow-up from the date of the iron biomarker test until 31 December 2021. The primary composite event of interest was first re-hospitalisation for HF (HHF) or all-cause mortality. The associations between ID biomarkers and endpoints were analysed using Cox regression adjusted for age, sex, previous HF, left ventricular ejection fraction, HF medications and comorbidity. Bonferroni-Holm correction was applied for multiple testing.Results Of 325 patients included (median age 68 (57–76) years, 70.2% men), 168 (52%) had ID using current guideline criteria and STFR was available for 224 (69%) patients. Median follow-up was 2.2 years. In the prespecified analysis plan, biomarkers of ID were not statistically significantly associated with the primary composite endpoint, although each SD increase in TSAT was associated with 56% lower risk of HHF (0.44 [0.27–0.71]) as were increased ratios of iron/STFR (0.58 [0.40–0.86]) and TSAT/STFR (0.55 [0.37–0.83]). However, in a post hoc sensitivity analysis in which patients with extreme values were excluded, each increase in SD of TSAT was associated with 47% lower risk of the primary composite endpoint (0.53 [0.35–0.79]).Conclusions Lower TSAT is associated with a greater risk of clinical events in patients with new-onset HF, as were the ratios of iron/STFR and TSAT/STFR. However, neither ratio appeared to offer a substantial advantage compared with TSAT alone.