Coombs test?

[Anonymous]

when do you use direct vs. indirect ? I guess this question confused me:

A pregnant 38 yr. old woman with two presents at the office concerned about her pregnancy. She is Rh (-), and her husband is Rh (+). Both of her children are also Rh (-). She has had 2 spontaneous abortions and carried a 3rd pregnancy to term, but the child died at birth, diagnosed with erythroblastosis fetalis. The test of choice to determine the presense of circulating anti-Rh antibody in the mother is:

A.) direct Coombs test to measure IgG anti-Rh antibody

B.) direct Coombs test to measure IgM anti-Rh antibody

C.) indirect Coombs test to measure IgG anti-Rh antibody

D.) indirect Coombs test to measure IgM anti-Rh antibody

E.) Ouchterlony test to measure precipitin IgG anti-Rh antibody





****The answer is given as C, but I don't understand why do u use indirect vs. direct. My guess was A. . . can someone please explain if you can? I'm so confused

[Anonymous]

Direct coomb's test.. for antibodies "directly" present on RBC's surface.
Indirect coomb's test... for antibodies present in serum ( indirect).

[Anonymous]

but can't u take the mother's RBCs to detect?

[Anonymous]

This is a rather important concept. If you look at the history, you'll notice that the Rh- babies were fine, but she also had a couple of spontaneous abortions (likely because of Rh incompatability with fetuses that were Rh+). So she has probably developed the antibodies against Rh + blood.

A direct coombs on mom would only detect the Abs which may have attached to her RBC's, but this would not happen to her own RBC's because she is Rh- and the Abs are against Rh+ blood. One option might be to check the babies RBC's directly to see if it has any of mom's Abs attached to it, but it's not so practical to get a fetal RBC sample for a direct coombs test. So a better option is to get an indirect coombs test to look for Abs floating around (not attached to RBCs) in mom's plasma instead. If the results are positive, it means that she has Abs in her blood which can cause damage to any future Rh+ fetus that she carries since the Abs would cross the placenta and attach to the babies RBCs (which would cause a + dirct coombs in the fetus).


Normal
No antibodies are detected.

• 1. Direct Coombs' test.
  A negative test result means that your blood contains no antibodies that are already attached to your red blood cells.
  
  2. Indirect Coombs' test.
  A negative test result means that your blood is compatible with the blood you are to receive by transfusion. A negative indirect Coombs' test for Rh factor (Rh antibody titer) in a pregnant woman means that she has not developed antibodies against the blood of her fetus (if her fetus has Rh-positive blood). This means that Rh sensitization has not occurred.

Abnormal test results may mean

• 1. Direct Coombs' test.
  The detection of antibodies attached to your red blood cells indicates that your blood has antibodies against red blood cells in your bloodstream.
  
  2. Indirect Coombs' test.
  A positive test result, or the detection of antibodies against red blood cells from a donor, may indicate that your blood is incompatible with the donor's blood and you cannot receive blood from that person. If the Rh antibody titer test is positive in a woman who is pregnant or is planning to become pregnant, it means that her immune system has formed antibodies against Rh-positive blood (Rh sensitization). She will be tested to determine the blood type of her fetus. If the fetus has Rh-positive blood, close monitoring throughout the pregnancy is needed to prevent serious damage to the fetus's red blood cells caused by the mother's immune system. If sensitization has not occurred, it can usually be prevented by an injection of the Rh immune globulin vaccine.

ref: peacehealth.org