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Blood typing and transfusions

Updated: May 27, 2021

We discussed last week about the NHS blood scandal, where blood transfusions were given to patients that were infected with HIV or hepatitis C. This week we are going to look at what a blood transfusion is, what types there are and what can go wrong.

Blood transfusions can be life-saving treatment both in the acute and the elective setting. One of the main components of blood that is commonly transfused is red blood cells (called packed red cells). Patients are given a blood transfusion when they have a reduced haemoglobin count. Haemoglobin is a molecule within the red blood cells that carry oxygen within the blood. Therefore, when a patient has a reduced haemoglobin count, called anaemia, they can experience symptoms such as dizziness, tiredness, shortness of breath. There are various causes of anaemia, such as iron or vitamin B12 deficiency, blood loss, chronic disease, the list goes on and on.

When a blood transfusion is necessary depends on if the patient is experiencing symptoms and their haemoglobin level. Most hospitals have different guidelines about the haemoglobin level, but these are only guidelines and clinical judgement needs to be taken into account. For example, a patient might have a haemoglobin of 80g/L (normal level is >120-130g/l depending on gender) but is young and have no symptoms so clinically might not require a transfusion. Whereas a frail elderly patient might have a haemoglobin of 90g/L however present with angina symptoms and therefore need an urgent transfusion.

Blood is transfused in units, each unit aims to increase the overall haemoglobin by 10g/L. The correct blood must also be given. Patient’s blood needs to be crossmatched if they are receiving a transfusion. This is where the blood is checked against the donors blood to try and limit any potential adverse reactions that might occur between the patients and donors blood mixing. There are various blood groups which basically correspond to the antigens that are present of the surface of the red blood cells. This is summarised in the image. Putting it simply there are 2 antigens that can be present on red blood cells, A and B. So if the red blood cell has A antigens on it’s surface then it is blood type A, if it has B antigens then it’s blood type B, if it has both A and B antigens then it is blood type AB and finally if it doesn’t have any antigens present on its surface then it is blood type O. IF an antigen isn’t present on the red blood cell then the body will create antibodies to that antigen, causing an immune system response. For example, a patient with blood type A will have anti B antibodies, therefore causing an immune reaction if it comes into contact with any B antigens, so B blood type cannot be given to a patient who is blood type A (just to clarify only the red blood cells are given on transfusion, any antibodies are removed). Just to confuse things there is another component that can be present on the surface of blood cells, this is called rhesus D factor, if this is present a + is added to the blood group e.g. B+, if it isn’t present then a – is added e.g AB-.

Understanding these blood types is crucial when giving blood in a transfusion as certain blood types will react to others. This is why O- is known as the universal donor as it contains no antigens on its surface so there is nothing to react to.

There are many risks involved in blood transfusions, and although rare some of the adverse effects of a blood transfusion can be fatal

Acute Haemolytic reaction eg ABO incompatibility

This is a condition that we have just been discussing and is the result of a blood typing error, where for example blood type A- is given to a B+ patient for example.

It results in a spike in temperature, flushing, hypotension and tachycardia, eventually resulting is disseminating intravascular coagulation, due to activation of the complement system releasing inflammatory cytokines.

If this occurs then the transfusion needs to be stopped immediately, a senior doctor needs to review the patient and an ABCDE approach taken, giving oxygen, fluids, steroids and antihistamines given IV in order to control the immune response. All the equipment used for the transfusion including the blood unit needs to be sent back to the lab so that this can be investigated and the cause determined.

Transfusion related acute lung injury (TRALI)

The mechanism of this reaction is not fully understood, but results in sudden development of shortness of breath and a drop in oxygen saturations, usually a fever develops too. Again in this the transfusion needs to be stop immediately and 100% oxygen given.


This is another potentially life-threatening issue. See our previous post for how this IgE mediated immune response occurs.

Febrile non-haemolytic transfusion reaction

This is a less serious complication, presenting with a fever usually 30mins after the transfusion was started. The treatment of this is to either slow or stop the transfusion and give something to decrease the fever such as paracetamol. Close monitoring is needed of a patient that develops this however.


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