How do I differentiate between primary ITP and secondary causes of thrombocytopenia in my clinical practice?

To differentiate between primary immune thrombocytopenia (ITP) and secondary causes of thrombocytopenia in clinical practice, start with a thorough clinical history and examination focusing on potential underlying conditions such as infections, autoimmune diseases, lymphoproliferative disorders, or drug exposures. Primary ITP is a diagnosis of exclusion characterized by isolated thrombocytopenia without an identifiable cause, whereas secondary thrombocytopenia occurs in the context of another disease or factor ¹.

Laboratory evaluation should include a full blood count and blood film to exclude other hematological abnormalities; in primary ITP, other cell lines are typically normal, and the blood film shows isolated thrombocytopenia without abnormal cells. In contrast, secondary causes may show additional abnormalities such as schistocytes in thrombotic microangiopathies or blasts in leukemia ¹.

Screening for secondary causes involves targeted investigations based on clinical suspicion, including tests for HIV, hepatitis C, autoimmune markers (e.g., ANA), and assessment for lymphoproliferative disorders. The presence of these markers or clinical features suggests secondary thrombocytopenia rather than primary ITP ¹.

Bone marrow examination is not routinely required for typical primary ITP but may be indicated if atypical features are present, such as age over 60, systemic symptoms, or cytopenias affecting other lineages, to exclude marrow infiltration or aplasia seen in secondary causes. This approach aligns with NICE guidance and is supported by literature emphasizing the importance of excluding secondary causes before confirming primary ITP (Del Vecchio et al., 2014).

In summary, differentiating primary from secondary thrombocytopenia relies on excluding other causes through clinical assessment, targeted laboratory tests, and selective bone marrow examination, with primary ITP being a diagnosis of exclusion characterized by isolated thrombocytopenia without underlying systemic disease. This integrated approach ensures accurate diagnosis and appropriate management ¹; (Del Vecchio et al., 2014).