Direct antiglobulin test
Note: this site is for informational purposes only. To view test results or book a test, use the NHS app in England or contact your GP.
The direct antiglobulin test checks whether antibodies have attached to your red blood cells. It helps doctors find out why red blood cells are being destroyed, and is used to investigate transfusion reactions, anaemia, and blood group problems in newborn babies.
Who needs this test
Your doctor may request this test if they think your red blood cells are being destroyed faster than your body can replace them. This is called haemolytic anaemia (haemolysis means the destruction of red blood cells).
You may need this test in one of these situations:
- You have symptoms of anaemia – such as tiredness, pale skin, yellowing of the skin or eyes (jaundice), or dark urine
- You have had a blood transfusion and are experiencing symptoms of a reaction, such as fever, chills, back pain, or dark urine
- You are a newborn baby, or have just given birth, and there may be a blood group incompatibility between mother and baby
The test may be requested by your GP, a hospital doctor, a haematologist (blood specialist), or an obstetrician (pregnancy specialist).
Preparing for your test
No special preparation is needed. A small sample of blood will be taken from a vein in your arm.
Tell your doctor about all medicines you are taking, including antibiotics and over-the-counter drugs. Some medicines can affect the result.
If you have had a recent blood transfusion, tell your doctor. The blood sample for this test must be correctly labelled at the time it is taken. The healthcare professional will write your details directly on the sample tube. This is a blood transfusion safety requirement.
Understanding your results
What the test measures
Your red blood cells carry proteins on their surface called antigens. These antigens determine your blood group. In some situations, antibodies – proteins made by the immune system – can attach to the surface of red blood cells.
When antibodies attach to red blood cells, the body may start to destroy them. The DAT checks whether antibodies are currently coating your red blood cells. The result is reported as either positive or negative.
A positive result does not always mean you have a serious condition. Your doctor will consider your symptoms, medical history, and other test results together. Some healthy people have a weakly positive DAT with no symptoms at all
If your result is negative
A negative result means that antibodies are most likely not attached to your red blood cells. If you have symptoms, your doctor will look for another cause.
In rare cases, a negative result can occur even when immune destruction of red blood cells is happening. Your doctor will take your symptoms and other results into account.
If your result is positive
A positive result means that antibodies are coating your red blood cells. The strength of the result gives some indication of the amount of antibody present. However, a stronger positive does not always mean more severe symptoms.
A positive DAT does not tell your doctor the exact cause or what type of antibody is involved. Further tests and a review of your history will be needed to find out why. The main causes of a positive DAT are explained below.
What a positive result can mean
Autoimmune haemolytic anaemia
Sometimes the immune system mistakenly makes antibodies that attack your own red blood cells. This is called autoimmune haemolytic anaemia (AIHA). It can happen on its own, or alongside other conditions including:
- autoimmune conditions such as systemic lupus erythematosus (lupus)
- blood cancers such as lymphoma or chronic lymphocytic leukaemia (CLL)
- certain infections, such as mycoplasma pneumonia or infectious mononucleosis (glandular fever)
If this is the cause, your DAT may remain positive over a long period. Your doctor will work with you to identify and manage any underlying condition.
Drug-induced haemolytic anaemia
Some medicines can trigger the immune system to produce antibodies that attack red blood cells. This is an uncommon side effect. Medicines that have been linked to this include some antibiotics, such as penicillin and cephalosporins.
If your doctor suspects a drug is causing the problem, they will usually stop or change the medicine. Symptoms typically improve once the drug is stopped. The DAT may remain positive for up to three months after the drug is discontinued.
Reaction to a blood transfusion
Before a blood transfusion, your blood is carefully matched with the donor blood. However, your immune system may still recognise some donor red cell proteins as foreign and produce antibodies against them.
If you develop symptoms during or after a transfusion – such as fever, chills, back pain, or dark urine – a DAT is done as part of the investigation. A positive result may mean your body is destroying the transfused red blood cells.
People who have had many transfusions are more likely to develop antibodies to donor red blood cells, as they have been exposed to more foreign blood cell proteins over time.
Haemolytic disease of the fetus and newborn (HDFN)
During pregnancy or at delivery, some of the baby’s blood cells can enter the mother’s bloodstream. If the baby has inherited blood group antigens from the father that the mother does not have, her immune system may produce antibodies against them.
These maternal antibodies can cross the placenta in a future pregnancy and attach to the baby’s red blood cells, causing them to be destroyed. This is called haemolytic disease of the fetus and newborn (HDFN).
A DAT performed on the baby’s blood at birth can show whether the mother’s antibodies have attached to the baby’s red blood cells. Symptoms in a newborn may include jaundice, paleness, and anaemia.
Pregnant women are routinely screened for antibodies during pregnancy to identify those at risk. D‑negative mothers are given a preventive treatment (anti‑D immunoglobulin injection) to reduce the risk of developing antibodies against D‑positive babies. This has made D‑related HDFN rare in the UK.
Questions to ask your doctor
What happens next
A positive DAT is a starting point for investigation, not a final diagnosis. Your doctor will use the result alongside your symptoms, medical history, and other test results to find out the cause.
You may be referred to a specialist – such as a haematologist – depending on what your doctor finds. If a medicine is suspected, it may be stopped or changed. If an underlying condition is identified, your doctor will work with you on a treatment plan.
If the DAT was done to investigate a transfusion reaction, the transfusion team will review whether it is safe to continue and what compatible blood to use in future.
What can affect your results
Certain medicines can cause a positive result, including some antibiotics and other drugs. Always tell your doctor everything you are taking.
A small number of healthy people have a weakly positive DAT without any symptoms or clinical condition. The strength of the result and your clinical picture together determine its significance.
Strict blood transfusion labelling rules apply to this test as incorrect labelling is a serious safety risk in transfusion medicine. Positive patient identification is used when taking this sample and a sample that is not correctly labelled will not be tested.
Other tests you might need
The DAT is rarely used on its own. Your doctor will often request several tests together to get a fuller picture. These may include:
- Full blood count (FBC) – to check whether you are anaemic and how severe it is
- Blood film – to look at the shape and size of your red blood cells under a microscope
- Reticulocyte count – to measure how many new red blood cells your bone marrow is making
- Bilirubin – raised levels can indicate red blood cell breakdown
- Lactate dehydrogenase (LDH) – an enzyme that rises when red blood cells are being destroyed
- Haptoglobin – a protein that falls when haemolysis is occurring
- Red blood cell antibody screen and identification – to find out exactly which antibodies are present and what they are reacting to
About haemolytic anaemia
Haemolytic anaemia occurs when red blood cells are destroyed faster than the body can make new ones. It can be caused by the immune system, infections, medicines, or inherited conditions. Symptoms include tiredness, pale skin, and jaundice.