10.     The Coomb’s Test:

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MCQ Paper 1

Sample MCQs


a. the direct test detects maternal IgM on fetal cells False
b. is used in the investigation of thrombocytopenia False
c. is positive in the baby with jaundice due to spherocytosis False
d. who wrote: “A flea hath smaller fleas that on him prey…..” Swift
e. what was the rest of the verse? See below
f. what connection has this verse with the Coomb’s test? See below
g. the indirect Coomb’s test is used to detect antibodies in maternal serum True
h. the direct test uses Anti IgG serum True

(See also MCQ9, MCQ paper 1, question 31, MCQ paper 4, question 17 and  MCQ6, question 21 and MCQ9, question 18.)


You might feel that the Coomb’s test is a bit esoteric.

But it is in the examination database, so you need to know something about it. 

It was devised by Robert Royston Amos Coombs, but this is not in the DRCOG database!


Fetal and neonatal red cells sometimes break down more than they should.

This causes neonatal jaundice (see MCQ7, question 12).

There are immune and non-immune causes.


There may be a built-in tendency to haemolysis without any immunological process.

This occurs in conditions like spherocytosis and glucose-6-phosphate dehydrogenase deficiency.


Red cells will also break down if attacked by maternal antibodies.

This happens in Rhesus disease (see MCQ4, question 17) and ABO incompatibility (see MCQ4, question 8).

These are the main immune causes.


You could be asked about the Coomb’s test in connection with ABO incompatibility or Rhesus disease.

It could also come up in the use of anti-D immunoglobulin. (See MCQ1, question 31 and MCQ 9, question 18.)


Rhesus disease of the newborn is now relatively rare.

This is due to the use of anti-D immunoglobulin.

The fact that women have fewer children also helps as each pregnancy is a potentially sensitising event.

The result is that ABO incompatibility is now a more common cause of neonatal jaundice than Rhesus disease.


When Rhesus haemolytic disease of the new-born does occur, it is due to "big D" in 95% of cases.

Maternal anti-D antibodies cross the placenta to the baby, attach to its red cells and cause haemolysis.

So, the baby’s Rh positive (D) cells have maternal anti-D attached.


The direct Coomb’s test is used to detect the maternal antibodies attached to the fetal cells.

If such antibodies are present, the Coomb’s test will show them.

This will demonstrate that the haemolysis is an immune process.

Watch out in the exam for questions restricted to either immune or non-immune haemolysis.

As the mother will have been sensitised some time before, the antibodies are IgG.


IgM is the initial response to an antigenic stimulus, with IgG replacing IgM in time.

You need to know the differences between IgM and IgG as it comes up in possible infection in early pregnancy, e.g. Rubella.

If the mother has IgM antibodies, this usually indicates recent infection.

IgG gives reassurance that she has some degree of immunity from earlier exposure or immunisation.

Be wary if the question is about cytomegalovirus infection.

Interpretation of the results is tricky due to IgM persisting much longer than usual.

See the MCQ on cytomegalovirus for a fuller discussion.

The Coomb’s test uses antibodies that can detect Rhesus antibodies.

My understanding is that these “detective” antibodies are generated by injecting sheep with Rhesus antibodies.

The resulting "detective" antibodies will attach to the maternal Rhesus antibodies that are affecting the baby's red cells.

This will produce agglutination.

So, some of the baby’s blood is treated with the sheep antibodies and agglutinates.

This is a positive test.

Reminds one of Swift. 

    “A flea hath smaller fleas that on him prey, and these have smaller still to bite ‘em, and so proceed ad infinitum”.


Agglutination takes place because the sheep antibody binds the red cells together.

Normally red cells are somewhat English in behaviour.

They are aloof and keep their distance.

What actually happens is that they have a small negative electric charge.

As "like repels like", this keeps them apart.

Apparently they never get closer than 14 nanometres.

For you and me, 14 nanometres would represent intimate contact, not just invasion of personal space.

But it is very comfortable for red cells.

The sheep IgM is big enough to span this distance and attach to more than one red cell causing agglutination.

Maternal anti-D is IgG and much smaller than IgM.

This lets it cross the placenta but it is too small to bridge the gap between red cells and cannot cause agglutination.

This is totally irrelevant to the exams, but interesting (at least to me).

There is an article with nice pictures on Wikipedia. 


The indirect test is used to detect maternal antibodies during pregnancy.

Let us say she is Rh negative with anti-D antibodies present.

Rh positive (D) cells are introduced into a sample of her serum.

They will be coated with her anti-D.

To this mix are added antibodies to anti-D, and an agglutination reaction takes place.

The test can be used with appropriate cells and antibodies to check for other antibodies.

A measure of the amount of antibody can be obtained by running the test with different dilutions of maternal serum.

This is why amounts of antibody are expressed as ratios: 1/16, 1/32 etc.

These reflect the dilution at which it can still be detected.

If you can detect antibody at a dilution of 1/ a million, there must be huge amounts present in serum.

At the other extreme, if you can only detect it in undiluted serum, there can’t be a lot there.


The Coomb’s test is unreliable in ABO incompatibility, sometimes +ve and sometimes –ve with an affected baby.

I would have thought that it would always give a negative result.

It is mainly designed to seek anti-D antibodies.

But ABO incompatibility derives from maternal anti-A and anti-B antibodies attaching to A and B fetal cells.

I have not yet found a paediatrician who could give me a satisfactory answer to why it is sometimes +ve.

One day I’ll get on to a haematologist or immunologist, but you don’t need to bother about it for the DRCOG.

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