If your haemoglobin came back at 12.4 g/dL as a man, or 12.1 g/dL as a woman, most lab reports print it in black rather than red. It sits inside the reference range, so nothing gets flagged. New research published on 8 September 2026 suggests that reassurance may be a little too quick.
A University of Glasgow team followed close to half a million UK Biobank participants for more than a decade and found that people whose haemoglobin sat just above the anaemia threshold had a higher risk of death than people further up the same normal range. Nothing about this changes a diagnosis on its own, and it is an observational finding rather than proof. But it does change how a borderline number deserves to be read.
What the new study actually found
The researchers compared haemoglobin concentration with deaths from any cause, from cardiovascular disease and from cancer, over a median follow-up of more than ten years. The shape of the relationship was a U: risk rose at both the low end and the high end, and bottomed out somewhere in between.
The interesting part is where that bottom sat. It was not immediately above the anaemia cut-off. It was 1 to 3 g/dL above it, in both men and women. People with what the authors call borderline anaemia, meaning a value just clearing the threshold, carried more risk than people sitting in the upper part of the normal band.
The team is careful about what this does and does not mean. An association is not a cause. A borderline haemoglobin may simply be a marker that something else is going on, rather than a problem in itself. The authors explicitly say the findings do not justify redrawing the anaemia thresholds today, and that whether investigating these people improves their outcomes still has to be tested directly.
Where the anaemia threshold comes from
Anaemia is currently defined by World Health Organization cut-offs that have barely moved in fifty years: haemoglobin below 13 g/dL in men and below 12 g/dL in women. Everything above those numbers is reported as normal.
A single line has obvious advantages. It is simple, it is the same everywhere, and it tells a busy clinician when to act. The limitation is equally obvious: biology does not change at a decimal point. Two people separated by 0.2 g/dL get very different reports, and one of them is told nothing is wrong. That is the tension this study puts a number on.
The three zones on a haemoglobin result
Reading the result as three zones rather than two is closer to what the data show.
| Zone | Haemoglobin (men / women) | How to read it |
|---|---|---|
| Below the threshold | Under 13 / under 12 g/dL | Anaemia by the current definition. Warrants a cause being looked for. |
| Borderline | Roughly 13 to 14 / 12 to 13 g/dL | Reported as normal. The zone where risk already started to rise in this study. |
| Comfortably normal | Roughly 14 to 16 / 13 to 15 g/dL | Where the lowest mortality was observed in the cohort. |
| High | Above the upper reference limit | Uncommon, and also associated with higher risk. Needs medical assessment. |
Reference ranges differ slightly between laboratories, and altitude, pregnancy and age all shift them, so the numbers on your own report are the ones that count. For the wider picture of what this marker does, you can read our complete guide to haemoglobin levels.
Why a borderline result is often an iron story
One of the study authors made a practical point: borderline anaemia comes with a high prevalence of iron deficiency. That matters, because iron runs down in stages, and haemoglobin is the last thing to fall.
Iron stores empty first. Red cells then start being built with less haemoglobin in them, which is why the mean corpuscular volume often drifts down before anything else. Only at the end does the haemoglobin itself drop below the line. Someone in the borderline zone may already be well into that sequence.
What the recent research adds
A 2025 review in JAMA on iron deficiency in adults set out how common this is: in high-income countries, roughly 38 percent of non-pregnant women of reproductive age have iron deficiency without anaemia, compared with about 13 percent who have iron-deficiency anaemia. In plain terms, for every woman whose blood test flags anaemia, there are two or three whose iron is low while the haemoglobin still reads normal. The review also gives the number that identifies it: a ferritin below roughly 30 ng/mL, in the absence of inflammation.
A smaller 2023 hospital series in Blood looked specifically at people whose haemoglobin was above 12 g/dL. Their average haemoglobin was 13.4 g/dL, entirely unremarkable, yet their average ferritin was 16 ng/mL, and 83 percent of them reported fatigue. Most were treated with iron, and roughly six in ten said their symptoms improved, without their haemoglobin changing much.
What this changes for you is modest but concrete. A haemoglobin sitting near the bottom of its range, especially alongside tiredness, breathlessness on exertion, restless legs or hair and nail changes, is a reasonable prompt to ask whether iron stores have been measured. The same blood draw usually allows a ferritin blood test. These are small studies and a narrative review, not treatment recommendations, and iron supplements should never be started without a measured deficiency.
When to talk to a doctor
A borderline haemoglobin on its own, in someone who feels well, is not an emergency and rarely needs urgent action. It is worth raising with a doctor when it comes with any of the following.
- Fatigue that is new, persistent, or out of proportion to your routine.
- Breathlessness climbing stairs, palpitations, or unusual pallor.
- Heavy periods, or any visible blood loss from the gut or urine.
- A haemoglobin that has been drifting down across successive tests, even inside the normal range.
- A known condition that affects red cell production, such as chronic kidney disease or an inflammatory disease.
Trend matters more than a single reading. A value of 13.2 g/dL means something different in someone who was at 15 two years ago than in someone who has always been there. The lab also reports your haematocrit, and the report shows the mean corpuscular volume, both of which help place a borderline number in context.
Glossary
- Haemoglobin: the iron-containing protein inside red blood cells that carries oxygen from the lungs to the tissues.
- Anaemia: too little haemoglobin to meet the body’s oxygen needs, defined by the WHO thresholds of 13 g/dL in men and 12 g/dL in women.
- Borderline anaemia: a haemoglobin sitting just above those thresholds, reported as normal.
- Ferritin: the protein that stores iron. It falls before haemoglobin does, which makes it the earlier signal of iron deficiency.
- Cohort study: research that follows a large group over time and records what happens. It can show associations but cannot prove cause.
- U-shaped association: a relationship where risk is higher at both the low and high ends of a measurement, and lowest in the middle.
Frequently asked questions
Does this mean the anaemia threshold is wrong?
No. The authors are explicit that their findings do not justify changing the WHO thresholds. What they suggest is that a single cut-off cannot capture everything, and that the zone just above it deserves more research.
My haemoglobin is 12.5 and I am a woman. Should I be worried?
Worried, no. Curious, reasonably. That value is normal by the current definition. If you feel well and it has been stable, it is very likely nothing. If you are tired, have heavy periods, or it has been falling, it is worth mentioning at your next appointment.
Should I take iron supplements if my haemoglobin is borderline?
Not on the basis of a haemoglobin number alone. Iron overload causes its own harm, and iron should only be taken when a deficiency has been measured, typically on ferritin or transferrin saturation, and when the reason for it has been considered.
Does a borderline haemoglobin always mean iron deficiency?
No. Vitamin B12 or folate deficiency, chronic kidney disease, chronic inflammation and blood loss can all produce the same picture, and some people simply run at the lower end of normal. That is exactly why the cause is investigated rather than assumed.
How often should haemoglobin be rechecked?
There is no universal interval for someone who feels well. Where a doctor is following a borderline value or has started treatment, a recheck after a few weeks to a few months is usual. The timing is a clinical decision.
Sources
- University of Glasgow — Borderline low haemoglobin levels are linked to a higher risk of death (8 September 2026)
- Annals of Internal Medicine — Association Between Blood Haemoglobin Concentration and Mortality by Age and Sex: A Prospective Cohort Study
- MedlinePlus (NIH / National Library of Medicine) — CBC blood test
- Auerbach M. et al. — Iron Deficiency in Adults: A Review, JAMA, 2025
- Sevgi M. et al. — Symptomatic Iron Deficiency without Anaemia: An Underrecognised Phenomenon, Blood, 2023
Other articles
- Haemoglobin levels: what your blood test results mean
- Ferritin blood test: what your results mean
- Serum iron test: how to read high, low and normal levels
- Haematocrit test: what your results mean and normal ranges
- MCV blood test: what it measures and what results mean
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