Having a venous blood test is a safe and common procedure that should only be performed by a health professional who is trained in phlebotomy. Here are some side effects that can be experienced during or after a blood draw.
When you have your blood taken a tiny hole is left where the needle passed through the wall of your vein. This will usually lead to a small amount of bruising at the site of your blood test. Sometimes the bruising is accompanied by swelling, this can develop quite quickly during your blood test and is called a haematoma. Whilst the appearances can be quite dramatic this is a relatively common event when having a blood test. You can help to limit the amount of bruising by pressing firmly down on the blood test area after the blood sample has been taken. Using an ice pack can help to reduce the amount of swelling and pain that is experienced. You can also try elevating the arm that has the haematoma so that it is higher than your heart. Haematomas are not dangerous and will usually resolve without treatment, but they can be quite painful and unsightly for 1-2 weeks after a blood test. On very rare occasions an infection may form in a haematoma.
Infection happens more rarely than bruising or haematoma after a blood test. The needle will leave a small hole in the skin which may allow bacteria to cause a skin infection. If you notice that you have redness developing around the blood test site in the days following the blood test then we recommend seeing a doctor so that they can assess whether there is an infection present.
A small amount of pain or discomfort is normal after a blood test. Sometimes pain can be more intense than expected. Veins often sit close to the other structures such as tendons and nerves. When the phlebotomist is trying to pass the needle into the vein then sometimes these other structures can experience some damage. This will in most cases resolve without any treatment. If it continues to worsen then you should see your doctor.
Feeling faint or dizzy during the procedure is common. If you have experienced this in the past, tell the phlebotomist at the beginning and they should get you to lie down while taking your sample. Feeling faint is caused by the blood vessels relaxing, this causes blood to collect in the veins of the legs, and less blood (and oxygen) to flow to the brain. If you start to feel faint or dizzy during the procedure, then tell the phlebotomist straight away. They will ask you to either sit or lie down, and they may even ask you to raise your legs in the air. These manoeuvres help to relieve the feeling of faintness by helping blood to flow to the brain.