b. Outcome of Platelet Transfusion.
(1) Some patients produce platelet antibodies as a result of previous
pregnancy or transfusion. Platelets collected from random donors will have a shortened
survival in those patients and, thus, may not be effective in preventing or controlling
bleeding. For further discussion, see platelet compatibility below.
(2) It has been shown that from 1 to 3 hours after infusion, the platelet count
increases approximately 12,000/mm3 when 1 x 1011 platelets are transfused into a
patient with 1 m2 body surface area (for example., a 30-kg, 6-year-old child). A platelet
concentrate usually contains approximately 0.6 x 1011 platelets. This would be
expected to increase the platelet count approximately 4,000/mm3 in an average adult
with 1.8 m2 surface area. Thus, if it is desired to elevate the platelet count from 5,000 to
40,000/mm3, 9 units of platelet concentrate would be required, [(40,000--5,000) 4,000
= 9 (rounded)]. The observed increment is somewhat lower if the platelet concentrate
has been stored "in vitro" before transfusion.
(3) Many patients do not show the expected increment in peripheral blood
platelet count following transfusions because platelet survival is affected by the clinical
condition of the patient. If active bleeding is occurring or splenomegaly exists, the
transfused platelets are sequestered at the bleeding site or in the spleen and do not
remain in the circulation. In patients who have platelet antibodies, such as those with
idiopathic thrombocytopenic purpura or sensitization to antigens of the HLA system,
survival of circulating platelets is extremely brief, sometimes only a matter of minutes.
Fever, infection, and disseminated intravascular coagulation are additional clinical
conditions that cause a shortened platelet survival.
c. Selection of ABO and Rh Type for Platelet Transfusion.
(1) ABO antigens are present on the surface of the platelet and the recovery
of A1 platelets transfused into group O patients may be decreased. In patients with
lymphocytotoxic antibodies against donor cells, however, the increment in peripheral
blood platelet count is the same following transfusion of HLA-compatible platelets,
whether ABO-compatible or ABO-incompatible. Until these inconsistencies can be
resolved, it is advisable to transfuse ABC-compatible platelets whenever possible. If
ABO-compatible platelets are not available, ABO-incompatible platelets should be used
rather than withholding platelet transfusions.
(2) Incompatibility between donor plasma and recipient RBCs usually is not
clinically important because of the small volume of plasma (20 to 50 ml) from each
individual platelet concentrate. If large numbers of platelet concentrates are being
transfused to an adult or the patient is a small child, incompatible donor plasma may
cause a positive direct antlglobulin test and RBC hemolysis. When group-compatible
platelets are unavailable, consideration may be given to removing additional plasma
from the platelet concentrate before transfusion or to giving