This is the question:
When testing if a type II hypersensitivity reaction occurred we
learned you can use free plasma hemoglobin levels. Wouldn't haptoglobin levels
be a better estimation if what we are testing is intravascular hemolysis?
Haptoglobin is produced in the
liver. It binds free hemoglobin in serum
and it is an acute phase reactant. Its
primary function appears to be the irreversible binding of free oxy-hemoglobin
in plasma. Haptoglobin prevents the loss
of hemoglobin to urine and conserves iron.
The total circulating haptoglobin is capable of binding about 3 g of
hemoglobin and it may be completely depleted by a single severe episode of
hemolysis. If the RBC destruction is
occurring in the liver or spleen, the haptoglobin level will be normal because
the free hemoglobin is not being released into the bloodstream.
The haptoglobin test is used
primarily to help detect and evaluate hemolytic anemia and to distinguish it from
anemia due to other causes. However, it
cannot help determine the cause of the hemolysis. Serial determinations are used to detect and
monitor acute phase reactions and hemolytic states. If the haptoglobin level, RBC count,
hemoglobin, and hematocrit levels are decreased and the reticulocyte count is
increased it is likely that you have some degree of hemolytic anemia. If the haptoglobin level is decreased and
there is no sign of hemolytic anemia, then it is possible that the liver is not
producing haptoglobin. Liver disease may
also decrease the haptoglobin levels, because of decreased production. Plasma haptoglobin is increased in acute
phase reactions and in burns and nephritic syndrome. Low haptoglobin levels occur in increased
intravascular hemolysis or hemoglobin turnover, which is seen in hemolytic
anemias, transfusion reactions, and malaria.
Chronic intravascular hemolysis causes a small but continuing drop in
haptoglobin levels. Haptoglobin levels
will return to normal within 2-3 days unless the hemolysis persists.
Plasma free hemoglobin binds to
haptoglobin and this complex is rapidly removed by the parenchymal cells of the
liver. The free hemoglobin, in excess of
the binding capacity of haptoglobin, is excreted in the urine. The urine will be chocolate brown colored.
I have not found information supporting
one test over the other. I think that
haptoglobin is probably the more commonly ordered of the two tests. The haptoglobin test might be better since
the free hemoglobin is excreted in the urine.
I do know that serial determinations are better than a single measurement
to monitor the level of hemolysis.
