Initial laboratory testing for children with immune thrombocytopenia

Laboratory tests	Findings in typical ITP	Comments
Routine testing (performed in all patients with suspected ITP)
Complete blood count	Isolated thrombocytopenia (platelets <100,000/microL), without anemia or leukopenia	The presence of anemia or leukopenia mandates careful evaluation for disorders other than ITP
Differential WBC count	Normal	Suppression of total WBC count or neutrophils suggests bone marrow failure or infiltration or active viral infection*
Red blood cell indices	Normal or minimally abnormal	Macrocytosis may be seen in acquired or inherited bone marrow failure syndromes
Examination of the peripheral blood smear	Isolated thrombocytopenia, with normal-sized platelets; sometimes, platelets may be variable in size, with some large platelets No evidence of hemolysis (eg, schistocytes) No blast cells Postinfectious ITP may have activated lymphocytes that appear worrisomely like blasts	Clumping of platelets in an asymptomatic patient suggests that the automated platelet count may not be accurate Predominantly very large (giant) platelets suggest a giant platelet disorder (eg, MYH9-related disease) Predominantly very small platelets suggest Wiskott-Aldrich syndrome Schistocytes suggests a hemolytic process^¶
Additional optional testing
Reticulocyte count	Normal	An elevated reticulocyte count suggests a hemolytic process^¶ A very low reticulocyte count is consistent with bone marrow failure syndrome*
Direct antiglobulin test (DAT; formerly known as direct Coombs test)	Negative	Positive DAT suggests autoimmune hemolytic anemia if the hemolysis is ongoing and if IVIG or anti-D have not been given recently (eg, within the past 2 weeks)
Quantitative immunoglobulins	Normal	Common variable immunodeficiency is associated with age-specific reduction in serum concentrations of IgG, in combination with low levels of IgA and/or IgM

This table outlines laboratory testing recommended as part of the initial evaluation of a child with suspected ITP. Some experts obtain all of these tests (including optional tests) at the time of initial presentation. Other experts may perform more selective testing initially if the child has a typical presentation (ie, sudden onset of a petechial rash or bruising in an otherwise healthy and well-appearing child). The results of these initial tests, along with the history and physical examination, inform decisions about further testing to evaluate for other causes of thrombocytopenia. For additional details, refer to UpToDate topics on ITP in children.

ITP: immune thrombocytopenia; WBC: white blood cell; MYH9: nonmuscle heavy-chain myosin-9; DAT: direct antiglobulin test; IVIG: intravenous immunoglobulin; anti-D: anti-D immunoglobulin, also known as anti-Rho immunoglobulin; Ig: immunoglobulin.

* Bone marrow infiltration can be caused by a malignant process (leukemia or lymphoma) or inherited process (hemophagocytic lymphohistiocytosis). Bone marrow suppression may be idiopathic (aplastic anemia) or triggered by viral infection or drugs.

¶ A variety of disorders may cause hemolysis, including autoimmune hemolytic anemia, hemolytic uremic syndrome, or congenital thrombotic thrombocytopenic purpura.

Courtesy of James B Bussel, MD.

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Initial laboratory testing for children with immune thrombocytopenia