Direct Coombs test vs. Indirect Coombs test

[MudPhud20XX]

The way Kaplan explains direct vs indirect Coombs test is that direct Coombs test detects antibodies bound to RBC whereas indirect Coombs test detect production of anti-RBC antibodies from mother's serum.
So I get that, but it also explains that the indirect is more specific and accurate, who would that be the case?

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[lunalight]

A woman that is not currently pregnant (or is very early in her pregnancy) that is sensitized to Rh would return a negative direct Coombs because there is no fetal blood for her antibodies to bind to. However she would return a positive indirect Coombs because she has anti-Rh antibodies in her serum.

 

[notbobtrustme]

The way Kaplan explains direct vs indirect Coombs test is that direct Coombs test detects antibodies bound to RBC whereas indirect Coombs test detect production of anti-RBC antibodies from mother's serum.
So I get that, but it also explains that the indirect is more specific and accurate, who would that be the case?

The abs may or may not bind to RBCs, so the direct Coombs will miss abs that don't bind to RBCs. Indirect Coombs will find anti-Rh (or anti-w/e) abs in the serum, whether or not those abs are bound to RBCs. That makes it more sensitive than the direct Coombs.

 

[lunalight]

The abs may or may not bind to RBCs, so the direct Coombs will miss abs that don't bind to RBCs.

I wouldn't necessarily say that direct Coombs misses unbound antibodies because that implies that it is looking for unbound antibodies. Direct Coombs is used to test for only bound antibodies. The antibodies used in direct are "generic" and will bind to any human antibody. Thus direct Coombs will return positive for any condition that causes antibodies to bind to RBCs. In the case of hemolytic disease of the newborn, it would be used to test fetal blood to confirm the diagnosis.

Indirect Coombs will find anti-Rh (or anti-w/e) abs in the serum, whether or not those abs are bound to RBCs. That makes it more sensitive than the direct Coombs.

Indirect Coombs won't find bound anti-Rh antibodies because it is only looking for unbound antibodies. The serum that is tested does not contain RBCs. For the OP's scenario, indirect Coombs would use Rh+ "donor" RBCs and will only return positive if there are anti-Rh antibodies in the maternal serum. Indirect Coombs is used to prevent hemolytic disease of the newborn.

I think the OP was asking about specificity and in this case, indirect would be more specific. Direct Coombs is actually more sensitive because if all of the antibodies are bound to RBCs, then there won't be any unbound in serum and it will return negative.

 

[notbobtrustme]

I wouldn't necessarily say that direct Coombs misses unbound antibodies because that implies that it is looking for unbound antibodies. Direct Coombs is used to test for only bound antibodies. The antibodies used in direct are "generic" and will bind to any human antibody. Thus direct Coombs will return positive for any condition that causes antibodies to bind to RBCs. In the case of hemolytic disease of the newborn, it would be used to test fetal blood to confirm the diagnosis.



Indirect Coombs won't find bound anti-Rh antibodies because it is only looking for unbound antibodies. The serum that is tested does not contain RBCs. For the OP's scenario, indirect Coombs would use Rh+ "donor" RBCs and will only return positive if there are anti-Rh antibodies in the maternal serum. Indirect Coombs is used to prevent hemolytic disease of the newborn.

I think the OP was asking about specificity and in this case, indirect would be more specific. Direct Coombs is actually more sensitive because if all of the antibodies are bound to RBCs, then there won't be any unbound in serum and it will return negative.

well, that would be unlikely, considering that Ig concentrations in the blood are in the milligrams/dL. It's extremely unlikely that all ab would be bound to RBCs. Although, on the boards, this may be true. In real life, no way would that happen.

Plus, nowadays, we would be using an ELISA anyways, and that **** can detect picomolar concentrations on a regular basis. I used to find femtomolar concentrations of protein back in the day.