What Is a Blood Transfusion?

A bag of blood hanging by a hospital bed looks deceptively simple — a pouch, a tube, a slow drip. What you don't see is the chain of checks that got it there. A transfusion is the giving of donated blood, or one part of it, into a patient's vein, and almost everything interesting about it happens before the drip even starts. The video at the top of this page follows a transfusion being performed, in a little over six minutes of clinical footage, and the steps below are the ones unfolding behind it.

Step one — deciding which part of blood is actually needed

Blood is rarely given whole these days. Donated blood is usually separated into its useful parts, and you receive the part that fits your situation. Red blood cells are the oxygen carriers, given when anaemia or blood loss has left the body short of them. Platelets are the tiny fragments that help blood clot, and plasma is the liquid part that carries clotting factors of its own. Separating blood this way means one donation can help more than one person, and it means you aren't given components you don't need. Your clinical team decides which component makes sense after looking at your condition and your blood test results — it isn't a one-size choice.

Step two — matching, then matching again

Before any unit is released, your blood group is identified from a sample — the familiar A, B, AB and O groups, plus whether you're Rh positive or negative. The transfusion laboratory then selects a compatible unit and, for red cells, usually runs a further compatibility check between your sample and the donor unit, often called a crossmatch. Donated blood itself has already been through screening and testing by the blood service before it ever reaches a hospital. None of this is bureaucracy for its own sake: giving incompatible red cells can make a patient seriously unwell, so the system is built with repeated checks at every hand-over.

Step three — the checks at your bedside

When the unit arrives on the ward, staff check it against your identity and your prescription, carefully, at the bedside — your wristband, the label on the bag, and the paperwork all have to agree. The blood runs through a giving set, usually with a filter, into a cannula in a vein, most often in your arm or hand. It commonly starts slowly while the team watches you closely, because the early part of a transfusion is when a reaction, if one is going to happen, will often show itself. Your temperature, pulse and blood pressure are recorded before and during the transfusion for the same reason.

Step four — knowing what to speak up about

Most transfusions pass without any trouble at all, and you can usually read, rest, or doze while one runs. But you're part of the safety system too. Tell your nurse promptly if you start to feel feverish or shivery, itchy, flushed, short of breath, or just noticeably unwell in a new way. Staff would always rather pause a transfusion and check than carry on regardless, and pausing early is exactly what the monitoring is designed to allow. If you'd like the patient's-eye view rather than the clinical one, this Having a blood transfusion film was made for patients, families and carers learning about common blood procedures. And if blood treatments in general interest you, there's a useful contrast in our guide to how dialysis works — the other well-known treatment in which blood leaves the body, is processed outside it, and returns.

Frequently Asked Questions

Can I catch an infection from a transfusion?
Donated blood is screened and tested by blood services before it is issued, and modern testing has made transfusion-transmitted infection uncommon in regulated systems. No medical procedure is described honestly as carrying zero risk, which is one reason clinicians weigh up whether a transfusion is needed rather than giving one automatically. Your team can explain the safeguards in your own country.

Do I have to receive blood from someone with exactly my blood group?
Not always exactly — what matters is compatibility. Some groups can safely receive from a wider range of donors than others, and in an emergency, staff may use a group chosen because it is compatible with most people while your own group is being confirmed. The laboratory's matching and crossmatching steps exist to make sure whatever you receive is compatible with you.

How long does a transfusion take?
It varies with the component and your situation. A unit of red cells is typically given over a few hours rather than minutes, while other components may run at different rates, and your team sets the rate on your prescription. Expect checks before it starts and observations while it runs, whichever component you're having.

Can I refuse a blood transfusion?
Yes — transfusion requires your consent, and you can ask questions or decline. If you have concerns, including religious or personal ones, raise them with your team as early as you can, ideally before any planned surgery, so alternatives and risks can be discussed properly and your wishes recorded.

MedicalVideos.com has published trusted medical videos since 2007. The site was founded by Dr. Mohamed Ibrahim, MD, PhD — physician and researcher.

Educational only — this article is general information about blood transfusion, not medical advice. Whether you need a transfusion, and which component, is a decision for your own clinical team based on your condition and blood results. If you feel unwell during a transfusion, alert staff straight away.