Immune Thrombocytopenia (ITP)


The diagnosis of immune thrombocytopenia (ITP) is one of exclusion since no specific diagnostic test for the condition exists. Other conditions and some medications may also cause thrombocytopenia and bleeding, so it is essential to first rule these out.1,2

Laboratory Testing for ITP Diagnosis

Complete Blood Cell Count

The first laboratory test recommended for the diagnosis of ITP is a complete blood cell (CBC) count to assess the number of platelets, erythrocytes, and neutrophils in a blood sample. The presence of isolated thrombocytopenia may indicate ITP. However, the presence of anemia and/or neutropenia may point to other pathologic conditions.3 

Normal platelet counts range from 150,000 to 450,000/µL, whereas platelet counts below 100,000/µL occur in ITP.4 Typically, a platelet count in a patient with ITP is below 40,000/µL for longer than 3 months.5 Significant bleeding may occur at platelet levels below 10,000/µL.4

Read more about ITP signs and symptoms

Peripheral Blood Smear 

Analysis of a peripheral blood smear should reveal erythrocytes, leukocytes, and often platelets with normal morphology. A diverse number of very large platelets along with tiny platelet fragments often indicates acute ITP. These cases may present with megathrombocytes (stress platelets), caused by the early release of megakaryocyte fragments into the circulating blood.3,5 

If most platelets appear as large as erythrocytes, lack granules, or are abnormal in color, the patient may have an inherited platelet disorder rather than ITP.3

Platelet clumping when ethylenediaminetetraacetic acid (EDTA) is used as an anticoagulant for the peripheral blood smear indicates pseudothrombocytopenia. This can be confirmed if the platelet count is normal after the test is repeated with heparin or citrate used as the anticoagulant.3

Platelet Coombs Test 

A direct antiglobulin test confirms the presence of antiplatelet antibodies bound to the patient’s platelets.5 In a patient with isolated thrombocytopenia, a positive Coombs test further suggests a diagnosis of ITP. However, a positive Coombs test result in a patient with both anemia and thrombocytopenia suggests that the patient may have Evans syndrome.3

Indirect Platelet Antibody Test

In this test, normal donor platelets are used to identify antiplatelet antibodies, usually antibodies against glycoproteins IIb and IIIa.5

Additional Platelet Antibody Tests

Other tests that can detect antiplatelet antibodies include: 

  • Radiolabeled Coombs antiglobulin test
  • Fluorescein-labeled Coombs antiglobulin test
  • Enzyme-linked immunosorbent assay (ELISA)
  • Competitive binding assay to assess for phagocytosis of platelet-associated immunoglobulin G (IgG)
  • Lymphocyte activation test with autologous platelets or platelet-antibody immune complexes.

The reliability of these platelet antibody tests depends on the methods used by the laboratory where they are performed; therefore, results should be considered carefully. Negative antiplatelet antibody assay results do not necessarily rule out ITP.3

Read more about ITP guidelines

Hepatitis C and HIV Testing

Clinicians should rule out hepatitis C and HIV infection as potential underlying causes of thrombocytopenia. Epistaxis, petechiae, and thrombocytopenia may be early hematologic manifestations of HIV infection.1,3,6,7

Systemic Lupus Erythematosus Testing

Thrombocytopenia is one of the hematologic criteria for a diagnosis of systemic lupus erythematosus (SLE). Both SLE and ITP are autoimmune disorders in which thrombocytopenia is a feature. SLE may develop later in a patient with a diagnosis of ITP.8.9

To rule out comorbid SLE, laboratory testing should include detection of antinuclear antibody (ANA), anti-cardiolipin, anti-beta2 glycoprotein I, anti-Smith, anti-double-stranded DNA, and extractable nuclear antigen (ENA) antibodies.5 

Additional SLE tests may include laboratory tests for complement 3 and 4 (C3, C4), immunoglobulins (IgG, IgA, and IgM), cryoglobulin, serum protein electrophoresis. Biopsy for the lupus band test may be helpful.5

Read more about ITP testing

Prescription Review for Diagnosis of Drug-Induced ITP

Several medications may lower platelet counts, including heparin, chemotherapy agents, valproic acid, furosemide, nonsteroidal anti-inflammatory drugs (NSAIDs), quinine, quinidine, penicillin, ranitidine, sulfonamides, linezolid and other antibiotics, gold (to treat arthritis), and statins. Drug-induced thrombocytopenia may be either immune or nonimmune. Clinicians should review a list of the patient’s medications thoroughly to rule out drug-induced thrombocytopenia.1,10

Bone Marrow Aspiration and Biopsy for ITP Diagnosis

Bone marrow examination is often not necessary for ITP diagnosis, although its overall value to diagnosis is still unresolved. Bone marrow aspiration and biopsy that reveals a normal-to-increased number of megakaryocytes without other noticeable abnormalities is consistent with ITP diagnosis. It suggests thrombocytopenia is secondary to increased platelet destruction rather than decreased platelet formation.5,11

However, bone marrow examinations can be helpful in the diagnosis of adults older than 60, as they can rule out leukemia or myelodysplastic syndrome. Bone marrow examination is also recommended before splenectomy to rule out hypoplasia or fibrosis.11

Baseline bone marrow references before the initiation of any corticosteroid-based treatments can be useful. Pre-steroid biopsy findings may be helpful in considering other diagnoses for patients with chronic ITP despite 3 to 6 months of steroid treatment.11

According to guidelines set forth by the American Society of Hematology, bone marrow examination is not recommended in children or adolescents with typical presentations of ITP or in children who fail intravenous immunoglobulin (IVIG) treatment. Bone marrow examination is warranted in children with atypical presentations and hematologic findings if they do not respond to treatment after 6 months.11

Read more about ITP prognosis

Imaging Studies for ITP Diagnosis 

Computed tomography (CT) and magnetic resonance imaging (MRI) can rule out other causes of thrombocytopenia, but they are not useful as components of the routine workup for an initial diagnosis of ITP. However, in emergency cases of bleeding in patients with ITP, these studies are indicated to locate the source of serious internal bleeding.12

Common Misdiagnoses

The most common misdiagnoses in patients with primary ITP include secondary ITP (related to an infection or underlying illness), myelodysplastic syndrome, drug-induced thrombocytopenia, inherited thrombocytopenia, and bone marrow failure with primary thrombocytopenia. If only a limited number of diagnostic tests are performed, the chance of an incorrect diagnosis of ITP or a misdiagnosis instead of ITP is increased as ITP is a diagnosis of exclusion.1

Read more about ITP differential diagnosis

References

  1. Miltiadous O, Hou M, Bussel JB. Identifying and treating refractory ITP: difficulty in diagnosis and role of combination treatment. Blood. 2020;135(7):472-490. doi:10.1182/blood.2019003599
  2. Immune thrombocytopenia (ITP): diagnosis. Mayo Clinic. Accessed October 22, 2022.
  3. Kessler CM. Immune thrombocytopenia (ITP) workup: laboratory studies. Medscape. Updated January 7, 2021. Accessed October 22, 2022.
  4. Idiopathic thrombocytopenia purpura. Johns Hopkins Medicine. Accessed October 22, 2022.
  5. Justiz Vaillant AA, Gupta N. ITP-Immune thrombocytopenic purpura. StatPearls [Internet]. Updated July 8, 2022. Accessed October 22, 2022. 
  6. Dahal S, Upadhyay S, Banjade R, Dhakal P, Khanal N, Bhatt VR. Thrombocytopenia in patients with chronic hepatitis C virus infection. Mediterr J Hematol Infect Dis. 2017;9(1):e2017019. doi:10.4084/MJHID.2017.019
  7. Lai SW, Lin HF, Lin CL, Liao KF. Immune thrombocytopenic purpura might be an early hematologic manifestation of undiagnosed human immunodeficiency virus infection. Intern Emerg Med. 2017;12(2):157-162. doi:10.1007/s11739-016-1520-9
  8. Jung JH, Soh MS, Ahn YH, et al. Thrombocytopenia in systemic lupus erythematosus. Medicine (Baltimore). 2016;95(6):e2818. doi:10.1097/MD.0000000000002818
  9. Ahn SM, Choi EJ, Oh JS, et al. Prognostic factors for the development of systemic lupus erythematosus in patients with immune thrombocytopenia. Arthritis Res Ther. 2022;24(1):213. doi:10.1186/s13075-022-02901-y
  10. Drug-induced thrombocytopenia. Medline Plus. Accessed October 22, 2022.
  11. Kessler CM. Immune thrombocytopenia (IPT) workup: bone marrow examination. Medscape. Updated January 7, 2021. Accessed October 22, 2022.
  12. Kessler CM. Immune thrombocytopenia (ITP) workup: imaging studies. Medscape. Updated January 7, 2021. Accessed October 21, 2022.

Reviewed by Debjyoti Talukdar, MD, on 10/28/2022.

READ MORE ON ITP