A platelet antibody screen is performed to diagnose posttransfusion purpura (PTP), which occurs 5–14 days after a platelet-containing transfusion. It can also help diagnose alloimmune neonatal thrombocytopenic purpura, idiopathic thrombocytopenic purpura (ITP), paroxysmal hemoglobinuria, and drug-induced immune thrombocytopenia (DITP).
PLA1 (antigen-negative platelets): negative
ALTP (negative drug-dependent platelet antibodies): negative
PAIgG (platelet-associated IgG antibody): negative
PAIgM (platelet-associated IgM antibody): negative
Obtain a 10- to 30-mL venous blood sample. Obtain 30 mL of venous blood when platelet count is 50,000–100,000/mm3; 20 mL of venous blood when platelet count is 100,000–150,000 mm3; and 10 mL of venous blood when platelet count is >150,000/mm3. Observe standard precautions.
Label the specimen with the patient’s name, date, and test(s) ordered and place in a biohazard bag for transport to the laboratory.
Antibodies to platelet antigens are of two types: autoantibodies develop in response to one’s own platelets as in ITP, and alloantibodies develop following exposure to foreign platelets during transfusion.
Antiplatelet antibody, usually having anti-PLA1 specificity, occurs with PTP.
A persistent or rising antibody titer during pregnancy is associated with neonatal thrombocytopenia.
PLA1 incompatibility between mother and fetus appears to account for more than 60% of alloimmune neonatal thrombocytopenic purpura. A finding of a PLA1-negative mother and a PLA1-positive father provides presumptive diagnostic evidence.
PAIgG is present in 95% of both acute and chronic cases of ITP. Patients responding to steroid therapy or undergoing spontaneous remission show increased circulatory times that correlate with decreased PAIgG levels.
The platelet hyperlysibility assay measures the sensitivity of platelets to lysis. This test is positive in and specific for paroxysmal hemoglobinuria.
With DITP, antibodies that react only in the presence of the inciting drug can be detected. Quinidine, quinine, chlordiazepoxide, sulfa drugs, and diphenylhydantoin most commonly cause this type of thrombocytopenia. Gold-dependent antibodies and heparin-dependent platelet IgG antibodies can be detected by direct assay. Approximately 1% of persons receiving gold therapy develop thrombocytopenia as a side effect. Thrombocytopenia is also a well-known side effect of heparin.
Platelet compatibility typing is done to ensure that hemostatically stable platelets can be transfused (e.g., for aplastic anemia and malignant disorders). This is important because most patients repeatedly transfused with platelets from random donors become partially or totally refractory to further platelet transfusion because of alloimmunization. Platelet typing also provides diagnostic evidence of PTP. Platelets are routinely typed for PLA1, HLH-A2, and PLEI. Those matched for HLA antigens generally produce satisfactory posttransfusion improvement. A standard platelet count performed 1 hour after the end of a fresh platelet concentrate transfusion is a sensitive indicator for the presence or absence of clinically important antibodies against HLA antigens.
Pretest Patient Care
Explain test purpose and procedure.
Follow guidelines in Chapter 1 for safe, effective, informed pretest care.
Posttest Patient Care
Review test results; report and record findings. Modify the nursing care plan as needed. Counsel the patient regarding abnormal findings; explain the need to assess and monitor the patient for bleeding tendencies. Assess for prescribed medications as cause of purpura.
Follow guidelines in Chapter 1 for safe, effective, informed posttest care.