A major UK study has found that hundreds of thousands of Britons could have a hidden form of anaemia that current medical guidelines fail to detect, leaving them at greater risk of cardiovascular disease, cancer and premature death. Researchers from the University of Glasgow say people with "borderline anaemia", whose blood tests still fall within the "normal" range under longstanding World Health Organisation thresholds, face significantly increased health risks that doctors currently overlook, meaning many patients are not being investigated or treated until their condition becomes more severe.

The study, published in the Annals of Internal Medicine, analysed haemoglobin levels from almost half a million UK Biobank participants over ten years, making it one of the largest investigations of its kind. It found that both unusually high and unusually low haemoglobin were linked to greater risk, and that even mildly reduced levels, not just those below the official diagnostic cut-off, were associated with earlier death. Anaemia, usually caused by iron, vitamin B12 or folate deficiency, already affects an estimated 3% of men and 8% of women in the UK and leads to over 57,000 hospital admissions annually; the researchers now want current guidelines reviewed to account for factors such as diet, obesity and stress, potentially widening treatment to many more patients.