[The value of the thrombocytopenia test in the specific diagnosis of drug allergy, especially drug-exanthema].

125 thrombocytopenia tests (so-called thrombocytopenic index) from 94 patients with suspected drug allergies and negative skin tests were critically examined to determine the value of this in vivo method in the clarification of drug allergy. A consistent decrease of 20% or more in thrombocytes was rated as significant and therefore indicative of drug sensitization. 78.4% of the tests were in agreement with the expected result. Clinically relevant drug sensitivity was apparent in 30.4% of the tests, while the possibility of allergy could be eliminated in 48%. 16.8% of the results were inconclusive and 4.8% were false negatives. A flare-up occurred after 5.6% of the tests. A leukocyte count was considered unnecessary since it proved to be unrelated to a drop in thrombocytes and the existence of drug allergy. The thrombocyte test is a practical and valuable method for investigation of drug allergies, especially as the available in vitro tests are too complex to be used routinely in a diagnostic test program. However, the results must be considered critically in conjunction with an accurate case history, the clinical symptoms and the course of the disease. In the case of blisters and erythema exsudativum multiforme-like drug exanthema, renewed exposure to the suspected drug must be avoided due to the possible danger of a drug reaction. The latest research results have helped to clarify the underlying mechanism of the thrombocyte decrease.