What is bruising?
Bruising is caused by bleeding under the skin. For example, a hard knock which does not break the skin can damage fragile blood vessels which lie just beneath. These damaged blood vessels leak a small amount of blood, which collects in the area as a bruise. With time, the familiar blue-black discoloration changes to green, then yellow and eventually fades and disappears. This may take up to three weeks if the bruise is large.
Why can it happen after blood donation?
When the needle is taken out of the arm bleeding will continue until the small hole in the vein closes up. The way to prevent this is to apply pressure to the arm over the site where the needle was inserted. This must continue until all signs of bleeding have stopped. Failure to maintain this pressure is the most common cause of bruising.
Secondly, when the donation needle is put into the arm, damage to the opposite wall of the vein may occur, causing a small hole through which blood can escape. This is not always seen during the donation but may become apparent afterwards.
Thirdly there are tiny fragile blood vessels running just under the skin, as well as the larger veins from which a blood donation is obtained. When the donation needle is inserted into the arm, one of these small vessels may be damaged and bleeding occurs. It is impossible to predict this, as such vessels are not usually visible.
What can be done?
The single most important way of preventing a bruise is to apply pressure to the place where the needle was, until the bleeding has stopped. A plaster will be applied to the area to keep it clean. It should be kept on for a minimum of 6 hours. Additionally, if you are wearing a tight sleeve, we may ask you to remove that article of clothing. A tight sleeve can act as a tourniquet and cause congestion in the vein increasing the chance of bruising. If a bruise occurs during the donation, we may discontinue the donation to prevent the bruise worsening. You may also be more likely to develop a bruise if your donation is more difficult than usual.
What can you do?
Bruising may be painful and you should avoid heavy lifting as this could aggravate the pain in the arm. However, gentle movement may be beneficial.
Applying something cool to the area can help to relieve any pain or discomfort. A cold cloth or flannel is ideal. If you require more pain relief, we recommend taking paracetamol (according to the manufacturers instructions).
This page has been taken from National Blood Service, UK