The haemoglobin reading problem

Every donor has their haemoglobin levels checked via a finger-prick test before each donation. This reading determines whether you are eligible to proceed. Intense exercise in the hours before this test can cause a phenomenon called plasma expansion, where the body draws extra fluid into the bloodstream in response to exercise-induced dehydration and inflammation.

This plasma expansion temporarily dilutes the concentration of red blood cells in the blood, making your haemoglobin reading appear lower than your true resting level. If the reading falls below the minimum threshold of 125 g/L for women or 135 g/L for men, you will be deferred even if you are genuinely fit and healthy. This deferral is entirely avoidable by timing your training away from your donation appointment.

Dehydration and vein access

Exercise causes fluid loss through sweating. If you attend a donation appointment without fully rehydrating after training, your blood will be more viscous and your veins will be less prominent and more difficult to access. This makes the insertion of the donation needle harder, can require multiple attempts, and increases the risk of bruising at the site.

It also increases your risk of feeling faint during or after the donation itself. A donor who is already mildly dehydrated from morning exercise and then has 470ml of blood removed is in a physiologically compromised state. The post-donation period will be less comfortable and recovery will take longer.

How long to wait after exercise

NHS Blood and Transplant advises avoiding strenuous exercise immediately before donation. The practical guidance most experienced donors follow is to avoid intense training in the 12 hours before their appointment. For most people, this means booking donation appointments on rest days or on days when only light activity is planned.

If you train in the morning and have an afternoon or evening appointment, full rehydration, a proper meal and several hours of rest after training is usually sufficient for a successful donation. The key indicators are that your heart rate has returned to resting, you have had a full meal, and you have consumed at least an additional litre of water since training ended.

What exercise is acceptable before donating

A leisurely walk to the donation centre, gentle stretching at home or a slow yoga session before your appointment are all unlikely to cause any problems. The concern is specifically with activities that significantly elevate heart rate, cause heavy sweating or stress the musculoskeletal system in a way that triggers inflammation.

If you feel your normal rested self, your veins are prominent when you look at the inside of your elbow, and you have eaten and drunk well, your donation is likely to proceed without any difficulty regardless of whether you did some light movement earlier in the day.

The simplest solution

The cleanest approach is to treat donation day as a rest day. Regular donors who train seriously and donate every three to four months almost universally report that booking appointments on scheduled rest days removes every pre-donation complication. There is no conflict with haemoglobin readings, no dehydration concern and no recovery pressure after donation.

If your training schedule does not have formal rest days, choose a day when you are scheduled for a very light session and donate before the session rather than after it. This preserves your training plan while ensuring you arrive at the centre well-rested and hydrated.