For patients with heart failure and a reduced left ventricular ejection fraction (LVEF) who are thought to have iron deficiency, intravenous (i.v.) administration of iron increases haemoglobin, improves symptoms, reduces hospitalizations for heart failure, and might also reduce cardiovascular mortality.1 However, uncertainty exists about which blood markers best reflect iron deficiency and the response to i.v. iron.2 Administration of i.v. iron to patients who are not iron deficient may not be effective, which will dilute the benefit observed in clinical trials leading to underestimation of benefit and, potentially, a neutral outcome.