A blood test taken in the morning could give subtly different results from one done in the evening, or at a different time of year. The discovery may affect whether people are diagnosed with conditions like anaemia or deemed eligible to donate blood.
Anaemia occurs when the number of red blood cells, which contain the oxygen-carrying protein haemoglobin, is lower than normal or if they have too little haemoglobin. Cases , while or can affect whether someone is judged healthy enough to give blood.
“The differences [in the study] are small, but they could matter for people whose haemoglobin sits right on a diagnostic or blood-donation threshold,” says at the University of Guelph in Canada, who wasn’t involved in the research. “As we learn more about medical chronobiology, it’s becoming increasingly clear that when we measure something can be almost as important as what we measure.”
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Components of blood fluctuate over the course of the day. For instance, red blood cells show daily changes in the chemical state of haemoglobin, which may contributing to fluctuations in body temperature.
In the latest study, at Sanquin Blood Supply Foundation in Amsterdam, the Netherlands, focused on a different aspect of blood biology: how concentrations of haemoglobin and an iron-storage protein called ferritin vary at different times of day and over the course of the year.
The researchers analysed blood measurements collected from around half a million Dutch people when they registered as donors between 2013 and 2023. They found that haemoglobin and ferritin levels vary with the seasons, peaking in December and reaching their lowest levels around June to August.
This could be due to the body’s response to warmer temperatures, says team member , also at the Sanquin Blood Supply Foundation. When it is hot, the body directs more blood towards the skin to help shed heat, while prolonged exposure to warmer temperatures can increase the volume of blood plasma, the liquid portion of blood. This then increases the volume of circulating blood, which lowers the concentration of haemoglobin and other blood biomarkers like ferritin, says van den Hurk.
The researchers also found that haemoglobin levels followed a clear daily pattern, peaking at around 10.30am and gradually falling as evening approached. The size of this fluctuation was slightly greater in men than women, and in both sexes, the daily peak occurred around an hour earlier in people aged over 45.
The age finding fits with what we already know about how our circadian rhythm changes throughout our life, says Martino. The circadian rhythm describes the natural oscillations in the activity of our tissues, driven by an internal clock over a 24-hour period.
For instance, teenagers and young adults are generally more like night owls and become more lark-like as they get older, says Martino. The sex differences could partly reflect the influence of sex hormones, she says, but the study didn’t look into this.
To what extent these daily fluctuations are driven by the body’s internal clocks, rather than other factors that vary between day and night – such as whether people are asleep or active, or how much they’ve eaten and drunk – is unclear.
Also unclear is whether these fluctuations serve any biological purpose, but they could have clinical implications. “There appears to be quite substantial variation in haemoglobin and ferritin levels, which implies that the timing of a visit to the blood establishment or a clinic may determine whether a donor or patient is considered anaemic or ineligible to donate,” says van den Hurk.
, at the MRC Laboratory of Molecular Biology in Cambridge, cautions against any change to clinical guidelines based on these findings alone. However, he says clinicians should be aware that haemoglobin levels fluctuate. “Just by chance, some people, if you draw the same blood at this time, might not make the cut [for a diagnosis], whereas at this [other] time they would do,” he says.
Preprints with The Lancet
