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What is Coombs test?

Coombs test is a blood test that is used to detect antibodies against red blood cells (RBCs). These antibodies destroy RBCs and can result in a condition known as haemolytic anaemia which is characterised by jaundice, fever and weakness. 

Coombs test is of two types: direct Coombs test and indirect Coombs test.

  • Direct Coombs test measures the antibodies bound to RBCs
  • Indirect Coombs test measures the free antibodies in the serum (the liquid portion of the blood excluding the blood cells) that are not yet bound to RBCs
  1. How do you prepare for a Coombs test?
  2. Why is a Coombs test performed?
  3. How is a Coombs test performed?
  4. What do Coombs test results mean?

One does not need to take any special precautions before this test. However, inform your physician if you are on any medications or if you have a disease or infection. Results of this test may be affected if you are pregnant (three months) or have undergone a blood transfusion in the past. Certain medications such as tetracycline, cephalosporin, sulpha or tuberculosis medicines and insulin may also affect the test results.

Doctors usually recommend a coombs test to people who are suspected of having haemolytic anaemia or jaundice. This test is advised:

  • To detect antibodies present on the surface of the RBCs due to a specific disease or drug
  • To detect antibodies floating in the blood, which could act against certain RBCs
  • To find out if an individual has developed an allergic reaction due to transfusion of incompatible blood
  • In newborns showing the signs of haemolytic anaemia
  • To diagnose the condition of erythroblastosis fetalis
  • To check for blood antibodies during pregnancy
  • To detect or diagnose anaemia or jaundice (yellowish discolouration of the skin) in case of drug toxicity in patients. The signs and symptoms include the following:

While a direct Coombs test is generally used to detect jaundice and anaemia, an indirect test is usually ordered for detecting blood transfusion reactions and erythroblastosis fetalis

A laboratory technician will collect a blood sample from a vein in your arm. A heel prick is generally done for infants. After the test, you might experience temporary pain, stinging sensation, throbbing or slight bruising at the site of needle insertion. Most of these symptoms subside on their own. If you feel persistent discomfort talk to your doctor at the earliest.

Normal results: 

A negative Coombs test result is considered normal. In case of direct Coombs tests, it means that no RBC clumping or RBC bound antibodies were found in your blood. 

In indirect coombs test, a negative result indicates that you are safe to receive blood from a donor. 

For an Rh-negative pregnant mother, negative Coombs test results mean that her immune system has not made antibodies against her baby’s Rh-positive blood. In such cases, the doctor may give a shot of Rh immunoglobulin to the mother to prevent erythroblastosis fetalis in the future.

Abnormal results:

A positive Coombs test result is considered abnormal. It indicates the presence of RBC bound antibodies in case of direct Coombs test and the presence of serum antibodies in indirect Coombs test. An abnormal result may be due to any of the following conditions:

  • Haemolytic anaemia (autoimmune) or haemolytic disease in the newborn baby
  • Erythroblastosis fetalis, which is anaemia in the baby caused by the antibodies in the mother’s blood attacking the red blood cells of the baby. This leads to high levels of bilirubin in the infant’s blood, in turn causing jaundice. This condition is potentially fatal
  • A blood transfusion with incompatible blood
  • Malignant diseases such as lymphoma or leukaemia (chronic lymphocytic leukaemia)
  • Drug toxicity (cephalosporin, levodopa, dapsone, nitrofurantoin, ibuprofen and quinidine)
  • Syphilis, infectious mononucleosis or mycoplasma infection
  • Systemic lupus erythematosus (Read more: Lupus causes)

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor.

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References

  1. National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; Hemolytic Anemia
  2. McPherson R, Pincus M. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, PA: Saunders Elsevier. 2011. Pp:712-715.
  3. Michigan Medicine: University of Michigan [internet]; Health Library
  4. MedlinePlus Medical Encyclopedia: US National Library of Medicine; Coombs test
  5. Johns Hopkins Medicine [Internet]. The Johns Hopkins University, The Johns Hopkins Hospital, and Johns Hopkins Health System; Hemolytic Anemia