Direct and Indirect Coombs test:
Their use is to detect antibodies that BIND to the surface of red blood cells (RBC).
Anti-Human Globulins (AHG) are mixed with the patient’s RBCs. If the RBC has the antibodies attached, the AHG will bind to those antibodies and bind RBCs together. This will lead to agglutination – a visible clumping of red blood cells.
Direct Coombs’ test:
Is used to detect antibodies already attached onto the red blood cell. This could be for cases such as immune mediated haemolytic anaemias.
- Add AHG to washed red cell
- Washed = removing the patient’s own plasma
- Wait and see if there is agglutination.
Indirect Coombs’ test:
This test is used to determine antibodies in patient’s serum that is unbound to their red blood cells. This is useful in cases such as crossmatching prior to transfusion, and also in pre-natal screening of pregnant women.
- Serum from the patient is cultured with RBCs of a known antigenicity from another patient.
- This process allows binding of antibodies to RBCs in vitro
- AHG then added to RBC
- Wait and see if there is agglutination
An indication of serum haptoglobin is to determine if hemolysis is present.
Haptoglobin is a protein produced by the liver and is found in the blood. Haptoglobin binds to free hemoglobin released from red blood cells. The Haptoglobin-hemoglobin complex will then be removed by the reticulo-endothelial system.
Hemoglobin is released in large quantities into the blood during hemolysis. This will cause lots of serum haptoglobin to bind to hemoglobin and consequently be removed.
Thus, during hemolysis, it is reasonable to expect haptoglobin levels to decrease.
Lactate Dehydrogenase (LDH) levels
Is an enzyme found in almost all cells. There are many indications to test for LDH levels. During haemolytic anaemias, the lysis of red blood cells causes a release of LDH into the blood. Thus it is reasonable to expect LDH levels as a marker of hemolysis.
Another indication for observing LDH levels is when the patient experiences muscle trauma or injury as it can also indicate tissue damage.
- Kumar P, Clark M. Kumar & Clark Clinical Medicine. 5th ed. London: Elsevier limited; 2002.
- Hoffbrand A, Moss P. Essential Haematology. 6th ed. Oxford: Wiley-Blackwell; 2011.