Immune thrombocytopenia remains a diagnosis per exclusionem. There are no specific clinical symptoms or laboratory parameters to ensure diagnosis. In otherwise caused thrombocytopenias many authors found increased platelet-associated immunoglobin (PaIg) and platelet-binding serum immunoglobulin (PbSIg). Methods to determine PaIg and PbSIg are described and their clinical relevance is discussed. Determinations of PaIg and PbSIg with Ig-L-chain-specific ELISA are recommended. They improve diagnostic possibilities to distinguish ITP from nonimmune thrombocytopenias.