Thrombocytopenia
A reduced platelet count is called thrombocytopenia and is caused by reduction in production, increased destruction or a process which moves platelets out of the circulation.
Reduced production of platelets may be due to congenital or acquired bone marrow failure. Thrombocytopenia following chemotherapy is a common cause. Bone marrow infiltration with leukaemia or cancer, and bone marrow failure syndromes such as Fanconi Anaemia and Severe Aplastic Anaemia can all cause thrombocytopenia.
A platelet transfusion may be indicated when a patient has bleeding or is at risk of bleeding during a period of reduced platelet production e.g. following chemotherapy. National and international guidelines can assist clinical decision-making regarding platelet transfusion.
Increased destruction of platelets occurs in a variety of conditions such as immune thrombocytopenic purpura (ITP), sepsis, consumptive coagulopathies such as disseminated intravascular coagulopathy (DIC), and thrombotic thrombocytopenic purpura (TTP).
Platelet transfusion may be contraindicated in disorders primarily due to platelet destruction. Their role in ITP and TTP is limited to life- or limb-threatening bleeding. Specific expert clinical advice is required.
Platelet transfusion may have a role in treatment of DIC and sepsis. Specific expert clinical advice is required.
Sequestration is when platelets are removed from the circulation and are not destroyed. A very large spleen, irrespective of cause, is a common cause of sequestration and is called hypersplenism. Platelet transfusions may have reduced effect in patients with hypersplenism and is one cause of platelet refractoriness.
The following conditions cause thrombocytopenia predominantly due to platelet destruction:
Neonatal alloimmune thrombocytopenia
Neonatal alloimmune thrombocytopenia (NAIT), or feto-maternal alloimmune thrombocytopenia (FMAIT) is caused by maternal IgG alloantibodies against the fetal or neonatal platelet specific alloantigen. These antibodies cross the placenta and cause a risk of thrombocytopenia and intracranial haemorrhage.
The antibodies are directed against fetal platelet antigens in the human platelet antigen (HPA) system. Approximately 75% of cases in a Caucasian population are caused by anti-HPA-1a and 20% by anti-HPA-5b. In Asian populations, anti-HPA-4b is more common than anti-HPA-1a.
Unlike haemolytic disease of the newborn, fetal platelets may cause sensitisation which can lead to NAIT in the first pregnancy. The condition is self-limiting, usually resolving within 2 weeks. It occasionally persists for up to 6 weeks.
Urgent treatment with platelets lacking the relevant HPA antigen (HPA matched) is required when there is bleeding or a platelet count is <30 x 109/L. In the absence of suitable donor platelets, the mother's platelets may be used. They must be plasma-reduced (to remove maternal antibody) and irradiated.
In the absence of HPA-matched platelets, platelets of unknown HPA type can be used and may effectively treat or prevent severe bleeding.
Subsequent pregnancies should be carefully monitored. Regular intravenous immunoglobulin to the mother may prevent fetal thrombocytopenia and intracranial bleeding in utero. When possible, compatible platelets should be maintained on hand at the expected delivery time.
Maternal serum is the test sample usually required for the reference laboratory investigation of possible NAIT. Usually, the mother’s serum is tested against both panel cells from known HPA-typed donors and against platelets from the father.
Reactivity with only the father’s platelets may indicate the presence of antibody directed to a low-incidence antigen.
Antibodies to HLA antigens have also been reported to cause NAIT although the extent of their involvement is controversial.
Post-transfusion purpura (PTP)
Post-transfusion purpura is a rare delayed adverse reaction. The patient’s platelet count falls due to antibodies to platelet specific antigens, most frequently anti-HPA-1a. Treatment involves intravenous immunoglobulin and transfusion of antigen-negative platelets.
Platelet refractoriness
Platelet transfusion refractoriness (PTR) refers to the repeated failure to achieve satisfactory responses to platelet transfusions. This places the patient at increased risk of bleeding, longer hospitalisation and increased risk of mortality.
There are two methods to calculate whether a patient has developed platelet refractoriness, the post-transfusion platelet increment (PPI) and the corrected count increment (CCI). For both methods obtain the pre-and post-platelet transfusion platelet count. The post-transfusion count can be taken between 10 and 60 minutes after the transfusion. It’s best to calculate the PPI or CCI on two occasions.
PPI = post-transfusion platelet count – pre-transfusion platelet count
CCI = (PPI x body surface area (m2)) ÷ platelet dose (x 1011)
PTR can be defined as a PPI <10 x 109/L or CCI <5-10
It should be noted that 1 unit (1 standard adult dose) of apheresis or pooled platelets would be expected to raise the platelet count of a 70 kg adult by 20-40 x 109/L.
Approximately 80% of cases are due to non-immune causes such as sepsis, fever, disseminated intravascular coagulation (DIC), splenomegaly, active bleeding, some medications (e.g. vancomycin, amphotericin B, heparin), graft versus host disease and veno-occlusive disease.
Approximately 20% of cases are due to antibodies caused by prior antigen exposure in pregnancy, or from transfusion or transplantation. The majority of detectable antibodies are anti-HLA antibodies (80-90% of cases) and the remainder are due to anti-HPA antibodies or both anti-HLA and anti-HPA.
The treatment of non-immune PTR is to transfuse ABO compatible platelets to control bleeding risk. Patients with specific antibodies may require HLA or HPA-matched platelets.
Contact Lifeblood, collect samples for HLA typing and antibody testing and complete a Request for HLA/HPA compatible platelets – Clinical information form.
Contact Lifeblood for assistance interpreting results and planning therapy.
Thrombotic thrombocytopenic purpura (TTP)
Thrombotic thrombocytopenia purpura (TTP) is a life-threatening disorder characterised by clotting in small blood vessels resulting in reduced blood supply to organs such as the central nervous system and kidneys. TTP is associated with thrombocytopenia and microangiopathic haemolytic anaemia.
TTP is due to a marked deficiency of the von Willebrand cleaving protein called ADAMTS13 due to either a congenital deficiency or development of autoantibodies. Acquired TTP may be associated with pregnancy, malignancy, autoimmune diseases, drugs but most of the time the underlying cause is unknown.
TTP is a medical emergency and treatment should be started as soon as possible. Treatment is with plasma exchange using either fresh frozen plasma or cryodepleted plasma both of which contains replacement ADAMTS13.
Platelet transfusion is generally contraindicated.
Platelet dysfunction
Platelet dysfunction results from a variety of congenital and acquired defects. Medication is the most common cause with prostaglandin inhibitors, such as aspirin being most frequently implicated. The bleeding tendency in platelet dysfunction defects is extremely variable and the platelet count is not a reliable indicator.
Prevention of bleeding by stopping antiplatelet medications 7–10 days prior to surgery is important. The administration of platelets may be appropriate depending on clinical features and the clinical setting.
Patients with congenital platelet function disorders such as Bernard–Soulier syndrome, Glanzmann thrombasthenia or other congenital platelet disorders where development of HLA alloantibodies may make future platelet support very difficult.
