Immune destruction of platelets may occur in patients with certain hematological disorders, such as SLE, leukemia, & collagen vascular disorders.  Antiplatelet antibodies may be classified as autoantibodies, alloantibodies or drug-dependent antibodies.  Autoantibodies may arise as in idiopathic thrombocytopenia purpura, secondary to diseases affecting immune function, e.g., malignancy, or following a viral  infection such as rubella or infectious mononucleosis.  Alloantibodies may develop as a result of multiple transfusions or during pregnancy when the mother is immunized  against the fetal platelet and forms antibodies which cross the placenta.  Drug-dependent antibodies exhibit activity only in the presence of the offending drug.  Drugs most often implicated include quinidine, digitoxin, quinine, sulfonamides, an gold compounds.  Thrombo-cytopenia due to platelet antibodies will usually disappear when the offending drug is removed.

There are several types of assays for platelet antibodies, a direct test using patient platelets, an indirect serum test using the patient's serum, or ELISA methodology to detect specific antibodies. The direct test can answer the question, "Is there antibody (immunoglobulin IgG) present?"or, if present, "what is the target of the alloantibody?" but they do not answer the question, "Is this a functional antibody?"

HEMEX Laboratories performs all of these tests. For  autoantibodies, two types of functional serum assays are performed.  If there is an antibody present, they will show how functional or strong that antibody is.  Donor platelets are washed and incubated with the patient serum.  When an immunoglobulin  directed against platelets is present,  the antibody will lyse the donor platelets. The amount of destruction of the donor platelets is proportional to the strength of the antibody.  If no antibody is present, little or no destruction will occur.  Normal values are less than 10% destruction of the donor platelets. A second method utilizing platelet aggregation is also performed.  Washed donor platelets are incubated with the patient serum.  Platelet aggregation is measured. This method is less specific but more sensitive than the indirect serum method.  When a platelet antibody is forming, the aggregation method will become positive first, detecting the presence of the antibody.  When a patient is treated, the platelet count method decreases faster than the aggregation method.  Thus, the two methods should give similar results, except as noted.

Platelet specific antigens can be measured for alloantibodies against various platelet glycoproteins by an ELISA method, including GP Ib/IX, IIb/IIIa, Ia/IIa and HLA Class I.

If a drug-induced antiplatelet antibody is suspected, both indirect serum assays are performed in the presence and absence of the drug.  A difference in response confirms the presence of a drug-induced antiplatelet antibody.

For further information, please call HEMEX Laboratories at (800) 999-CLOT (999-2568).

Specimen requirement:  2 ml frozen serum, 1 ml aliquots submitted in 2 separate plastic vials.
If it is suspected that the antiplatelet antibody is drug-induced, include a sample of the suspected drug.  Note: If the drug is heparin, order the Heparin Induced Thrombocytopenia (HIT) Antibody.

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