What is the diagnostic work‑up for immune thrombocytopenic purpura (ITP) in a clinically well patient with isolated thrombocytopenia?

Diagnostic Work-Up for Immune Thrombocytopenic Purpura (ITP)

In a clinically well patient with isolated thrombocytopenia, the diagnostic work-up for ITP requires a complete blood count with differential, a hematopathologist-reviewed peripheral blood smear to exclude pseudothrombocytopenia and alternative diagnoses, and mandatory HIV and hepatitis C testing in all adults regardless of risk factors. 1, 2

Essential Initial Laboratory Tests

Complete Blood Count with Differential

• Isolated thrombocytopenia (platelet count <100 × 10⁹/L) without anemia or leukopenia is the hallmark finding in ITP. 2, 3
• The CBC must demonstrate normal hemoglobin and white blood cell count; any additional cytopenias mandate investigation for bone marrow failure, myelodysplastic syndrome, or lymphoproliferative disorders rather than primary ITP. 1, 2
• Mild microcytic anemia from chronic mucosal bleeding is acceptable, but unexplained anemia suggests Evans syndrome (combined autoimmune hemolytic anemia and ITP) or alternative diagnoses. 1, 2

Peripheral Blood Smear Review by Hematopathologist

• This is the single most critical diagnostic test and must be performed before diagnosing ITP. 1, 3
• The smear serves three essential functions: confirms true thrombocytopenia by excluding EDTA-dependent platelet clumping (pseudothrombocytopenia), identifies characteristic ITP features, and excludes alternative diagnoses. 1

Expected findings in ITP:

• Platelets of normal size or only mildly enlarged (not giant platelets approaching red cell size). 1, 2
• Normal red blood cell morphology without schistocytes, which would indicate thrombotic microangiopathy (TTP, HUS, DIC). 1
• Normal white blood cell morphology without immature cells, blasts, or leukocyte inclusion bodies. 1, 2
• No platelet clumping, which would indicate pseudothrombocytopenia. 1

Red flags that exclude primary ITP:

• Schistocytes suggest thrombotic microangiopathy requiring urgent ADAMTS13 activity, LDH, haptoglobin, and coagulation studies. 1
• Giant platelets (approaching red cell size) indicate inherited thrombocytopenias such as MYH9-related disease or Bernard-Soulier syndrome. 1
• Leukocyte inclusion bodies point to MYH9-related disease. 1
• Immature or abnormal white cells raise suspicion for leukemia or myelodysplastic syndrome. 1

Mandatory Infectious Disease Testing in All Adults

HIV Antibody Testing

• HIV testing is required in every adult with suspected ITP, regardless of perceived risk factors. 1
• HIV-associated thrombocytopenia is clinically indistinguishable from primary ITP and may precede other HIV manifestations by years. 1
• Antiretroviral therapy should be considered before other ITP treatments if HIV is detected. 1

Hepatitis C Virus Serology

• HCV testing is mandatory in all adults with suspected ITP. 1
• Chronic HCV infection can cause thrombocytopenia years before other symptoms develop. 1
• Successful antiviral therapy for HCV can lead to complete hematologic remission. 1

Helicobacter pylori Testing (Context-Dependent)

• H. pylori testing should be performed in adults with typical ITP, as eradication therapy can normalize platelet counts. 1
• Use urea-breath test or stool antigen assay rather than serology, which has lower sensitivity and specificity. 1
• H. pylori testing is not routinely recommended in children except in regions of high prevalence. 1

Physical Examination Requirements

The physical examination must be normal except for bleeding manifestations (petechiae, purpura, mucosal bleeding). 1

Findings that exclude primary ITP and mandate alternative work-up:

• Splenomegaly, hepatomegaly, or lymphadenopathy indicate secondary causes such as HIV, systemic lupus erythematosus, lymphoproliferative disorders, or chronic liver disease. 1
• Constitutional symptoms (fever, unexplained weight loss, night sweats, bone pain) suggest underlying malignancy or systemic disease. 1

When Bone Marrow Examination Is NOT Required

• Bone marrow examination is not necessary in patients with typical ITP features: isolated thrombocytopenia, normal physical exam except for bleeding, and normal peripheral smear. 1
• This applies regardless of age in patients with classic presentation. 1

When Bone Marrow Examination IS Mandatory

Bone marrow aspiration and biopsy (with flow cytometry and cytogenetics) are required when any of the following are present:

• Age ≥60 years, to exclude myelodysplastic syndrome, leukemia, or other malignancies. 1
• Systemic constitutional symptoms (fever, weight loss, night sweats, bone pain). 1
• Abnormal CBC parameters beyond isolated thrombocytopenia (unexplained anemia or leukopenia/leukocytosis). 1
• Atypical peripheral smear findings (schistocytes, immature cells, giant platelets, leukocyte inclusion bodies). 1
• Splenomegaly, hepatomegaly, or lymphadenopathy on physical examination. 1
• Minimal or no response to first-line ITP therapies (IVIg, corticosteroids). 1
• Prior to splenectomy in patients with persistent or chronic ITP. 1

Additional Context-Specific Testing

Direct Antiglobulin Test (DAT)

• Perform DAT to exclude Evans syndrome (combined autoimmune hemolytic anemia and ITP). 1

Pregnancy Testing

• Obtain pregnancy test in women of childbearing potential to differentiate gestational thrombocytopenia, pre-eclampsia, or HELLP syndrome. 1

Rh(D) Blood Group Typing

• Required before anti-D immunoglobulin therapy is considered. 1

Heparin-Induced Thrombocytopenia (HIT) Assessment

• Calculate the 4T score immediately in any patient who received heparin within the past 3 months. 1
• If 4T score is ≥4 (intermediate/high probability), discontinue all heparin, obtain anti-PF4 antibody testing, and initiate non-heparin anticoagulation without awaiting results. 1

Tests NOT Recommended for Routine Use

The following tests lack diagnostic utility and should not be ordered routinely:

• Platelet-associated IgG (PaIgG) or glycoprotein-specific antiplatelet antibodies—elevated in both immune and non-immune thrombocytopenia, making them non-discriminatory. 1
• Thrombopoietin (TPO) levels. 1
• Reticulated platelets or immature platelet fraction (IPF)—cannot definitively confirm or exclude ITP. 1
• Bleeding time. 1
• Antiphospholipid antibodies—only if clinical features of antiphospholipid syndrome are present. 1
• Antinuclear antibodies (ANA)—only when systemic lupus erythematosus is suspected. 1

Critical Diagnostic Pitfalls to Avoid

• Never diagnose ITP without personal review of the peripheral blood smear by a hematopathologist, as automated counters miss pseudothrombocytopenia, giant platelets, and schistocytes. 1
• Never omit HIV and HCV testing in adults, because these infections masquerade as primary ITP and may precede other symptoms by years. 1
• Missing thrombotic microangiopathy (TTP, HUS) or heparin-induced thrombocytopenia carries high mortality risk (untreated TTP >90% mortality; HIT thrombosis risk 30-50%). 1
• Failing to recognize secondary causes of ITP (HIV, HCV, H. pylori, autoimmune diseases, lymphoproliferative disorders) leads to misdiagnosis and inappropriate treatment. 1
• Overlooking inherited thrombocytopenias—family history and platelet size on smear provide critical clues. 1
• Missing drug-induced thrombocytopenia—review all medications including over-the-counter products, herbal supplements, and quinine-containing beverages. 1

Algorithmic Approach to Diagnosis

1. Confirm isolated thrombocytopenia (platelet count <100 × 10⁹/L) with normal hemoglobin and white blood cell count on CBC. 2, 3

2. Obtain hematopathologist-reviewed peripheral blood smear to exclude pseudothrombocytopenia and identify red flags. 1, 3

3. Perform mandatory infectious disease testing: HIV, HCV, and H. pylori (in adults). 1

4. Complete physical examination to exclude splenomegaly, hepatomegaly, lymphadenopathy, or constitutional symptoms. 1

5. If age <60 years, typical ITP presentation, normal smear, and no systemic symptoms: Diagnosis of ITP can be made without bone marrow examination. 1

6. If age ≥60 years OR atypical features present: Proceed directly to bone marrow examination. 1

7. Obtain DAT, pregnancy test (if applicable), and Rh(D) typing based on clinical context. 1