Complete Blood Count Abnormalities in ITP
The hallmark CBC finding in ITP is isolated thrombocytopenia (platelet count <100 × 10⁹/L) with otherwise normal hemoglobin, white blood cell count, and red blood cell indices. 1
Primary CBC Characteristics
Isolated thrombocytopenia is the defining feature - ITP presents with a platelet count below 100 × 10⁹/L without accompanying anemia or leukopenia. 1, 2 The key diagnostic principle is that thrombocytopenia occurs in isolation, distinguishing it from other hematologic disorders.
Platelet Count Ranges and Clinical Significance
The severity of thrombocytopenia varies considerably:
- Diagnostic threshold: Platelet count <100 × 10⁹/L defines ITP 1
- Severe thrombocytopenia: Counts <30 × 10⁹/L indicate increased bleeding risk and may warrant treatment 1, 3
- Critical thrombocytopenia: Counts <10 × 10⁹/L carry the highest risk of severe bleeding 3
Importantly, patients presenting with platelet counts between 100-150 × 10⁹/L have only a 6.9% chance of developing persistent thrombocytopenia <100 × 10⁹/L over 10 years, which is why the diagnostic threshold was set at 100 × 10⁹/L rather than the traditional 150 × 10⁹/L. 1
Peripheral Blood Smear Findings
The peripheral smear should show a reduced number of large platelets with normal morphology - this is critical for confirming the diagnosis. 2
Key smear characteristics:
- Platelet morphology: Large, well-granulated platelets (megathrombocytes) are typical 2
- Absence of schistocytes: There should not be an increased number of fragmented red cells, which would suggest microangiopathic hemolysis 2
- Normal red and white cell morphology: No dysplastic features, blasts, or abnormal lymphocytes 1, 4
Critical Red Flags: What Should NOT Be Present
Any CBC abnormality beyond isolated thrombocytopenia mandates investigation for alternative diagnoses:
Anemia
- Evans syndrome presents with autoimmune thrombocytopenia plus coincident hemolytic anemia 1
- Anemia with thrombocytopenia suggests bone marrow failure, myelodysplastic syndrome, or lymphoproliferative disorders 4, 5
Leukopenia or Leukocytosis
- White blood cell abnormalities exclude primary ITP and suggest systemic lupus erythematosus, lymphoproliferative disorders, or bone marrow pathology 4, 2
- Atypical lymphocytes on smear indicate viral infection or lymphoma 1, 4
Pancytopenia
- Multiple cytopenias definitively exclude primary ITP and require bone marrow examination for aplastic anemia, myelodysplasia, or marrow infiltration 4, 5
Diagnostic Pitfalls to Avoid
Do not diagnose ITP if the CBC shows anything other than isolated thrombocytopenia - the presence of anemia, leukopenia, or abnormal cell morphology mandates further investigation including bone marrow examination, flow cytometry, and evaluation for secondary causes. 4, 2
Do not miss pseudothrombocytopenia - always review the peripheral smear to exclude platelet clumping, which can cause falsely low automated platelet counts. 2
In refractory cases, reconsider the diagnosis - patients with persistent thrombocytopenia despite appropriate therapy may have myelodysplastic syndrome with isolated thrombocytopenia (MDS-IT) rather than true ITP, requiring bone marrow examination with cytogenetics. 5
Monitoring Parameters
For diagnosed ITP patients, the CBC should be monitored to:
- Assess treatment response: Complete response is defined as platelet count ≥100 × 10⁹/L on two occasions 7 days apart with absence of bleeding 1
- Detect evolution of disease: Periodic CBC monitoring is essential in observed patients to exclude development of other hematologic disorders 1
- Guide bleeding risk assessment: While platelet count <30 × 10⁹/L increases bleeding risk, the correlation is imperfect and clinical bleeding assessment is equally important 6, 3