Scientists in Glasgow found patients with very high and very low levels of red blood cells (haematocrit) were more at risk of early death.
They identified the haematocrit range with the lowest risk as between 42% and 44% in men and between 38% and 40% in women.
Researchers say this could allow patients identified as at risk to be managed with more aggressive blood pressure treatments.
A team of researchers from the Glasgow Blood Pressure Clinic analysed data on 10,951 hypertensive patients.
They found that risk of early death increased at both the upper and lower end of the haematocrit range.
Hypertension is the most common preventable cause of premature death in the UK affecting over a quarter of adults – and over half of those over 60 years of age.
The Glasgow clinic, at the Western Infirmary, is one of the largest and the longest established blood pressure clinics in the world.
More than 16,000 patients have been managed at the clinic since it opened in 1968.
Dr Sandosh Padmanabhan, Reader in the Institute of Cardiovascular and Medical Sciences at the University of Glasgow, who led the study, said: "Finding new biomarkers that predict disease or risk is the Holy Grail for medical researchers.
"This study shows that there is valuable predictive information that can be obtained from blood tests that are routinely performed in out-patient clinics and primary care."