Borderline low haemoglobin levels, usually considered normal, are linked to a higher risk of death
Published: 8 September 2026
People with borderline anaemia (haemoglobin levels just above the World Health Organization (W.H.O.) threshold used to define anaemia) face a greater risk of death than others who have higher haemoglobin levels within a ‘normal range’, according to new research.
People with borderline anaemia (haemoglobin levels just above the World Health Organization (W.H.O.) threshold used to define anaemia) face a greater risk of death than others who have higher haemoglobin levels within a ‘normal range’, according to new research.
The major new study, led by the University of Glasgow and published in the Annals of Internal Medicine, analysed data from nearly half a million UK Biobank participants who had been followed for more than 10 years, making it one of the largest investigations of its kind to assess the potential links between haemoglobin and mortality risk.
Overall, researchers found a U-shaped association between haemoglobin concentration and mortality, with values at both extremes linked to the highest risk. However, the lowest risk of death from any cause was found at haemoglobin concentrations 1 to 3 g/dL above current W.H.O. thresholds, in both men and women.
Notably, people with borderline anaemia were found to have a greater risk of death than those who had higher haemoglobin concentrations within the W.H.O.-defined normal range. The research team say these findings highlight the importance of building a more nuanced understanding of anaemia, particularly around the potential health impacts of borderline anaemia.
Haemoglobin is the protein in red blood cells that carries oxygen around the body. Its concentration is one of the most commonly measured blood tests and is routinely assessed in both primary care and hospitals.
In older adults both high and low haemoglobin levels are associated with a range of negative health outcomes, including cognitive decline, cardiovascular disease, and more broadly, an increase in the risk of hospitalisation and death. However, while high haemoglobin (polycythaemia) is very rare, impacting only about 7 people out of every 100,000 in the UK, low haemoglobin (anaemia) is very common.
Currently, anaemia is diagnosed using thresholds set by the W.H.O. that have been largely unchanged for 50 years: a haemoglobin below 12 g/dL for women and 13 g/dL for men. Only people below these levels are conventionally classified as being anaemic.
Dr Malek Ahmad, clinician-scientist at the University of Glasgow and the first author of the study, said: “Our data show that the lowest risk of death is associated with a ‘definitely normal’ haemoglobin, 1 to 3 g/dL above the conventional W.H.O. definition of anaemia. Risk already starts to increase with ‘borderline anaemia’, before haemoglobin falls below the current W.H.O. threshold.”
Dr Pierpaolo Pellicori, Consultant Cardiologist at Queen Elizabeth University Hospital, Senior Lecturer at the University of Glasgow, and the corresponding author said: “The current W.H.O. thresholds have served clinical practice for decades, but our findings suggest that the relationship between haemoglobin concentration and long-term health is more nuanced than a single diagnostic cut-off can capture. Whether this should influence clinical practice is an important question for future research.”
Anaemia is usually caused by iron deficiency or, more rarely deficiencies in vitamin B12 or folate, or diseases that reduce blood cell production, such as severe kidney disease, any chronic inflammatory disease. Iron deficiency may be caused by poor diet, medicines that block the absorption of iron, including treatments for indigestion, or increased blood losses from stomach ulcers or cancer of the gut or kidneys.
Professor John Cleland, Professor of Cardiology and senior author of this study, said: “The most important purpose of clinical diagnostic criteria is to inform the management of patients. Currently, many clinicians do not even recognise a person has anaemia until its moderately severe, for example less than 11 g/dL. Showing that borderline anaemia is associated with increased risk and a high prevalence of iron deficiency should encourage investigation and treatment of a broad range of patients with mild/borderline anaemia.”
Professor Paul Welsh, Professor of Molecular Epidemiology and Director of the Glasgow Biomarker Research Unit at the University of Glasgow, said: “The very large study population, long follow-up and consistency across several statistical approaches make this a robust association. However, this remains an observational study. The next question is whether low-normal haemoglobin can help identify treatable underlying conditions and improve patient care.”
The researchers emphasise that this study shows an association, not proof that investigating or treating people just above the threshold would improve wellbeing or outcomes, which needs to be tested directly in future research. Their findings instead point to where that research should focus and add to a wider debate about how well diagnostic thresholds, many set decades ago, reflect the risks people actually face today.
The study entitled, “Association between blood haemoglobin concentration and mortality by age and sex: a prospective cohort study,” is published in the Annals of Internal Medicine.
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Enquiries: ali.howard@glasgow.ac.uk or elizabeth.mcmeekin@glasgow.ac.uk
First published: 8 September 2026