BACKGROUND: Antibiotics are used prophylactically in surgery to prevent postoperative infection. However, antibiotics administered in large doses can cause a bleeding diathesis as a result of platelet dysfunction. We wondered whether these antibiotics might impair platelet function by interfering with the initial step of platelet activation: the binding of agonists to their specific receptors on the platelet surface. METHODS: In 30 patients (male 18, women 12) undergoing primary elective knee arthroscopic surgery, the whole blood coagulation system was prospectively evaluated before, and 10 and 40 minutes after administration of 1 g of augmentin. All patients who had abnormal preoperative coagulation profiles or who received anticoagulant or antiplatelet, antibiotics therapy within 7 days prior to surgery were precluded. RESULTS: At 10 minutes after augmentin administration 25 of 30 patients had a significant impairment in all phases of whole blood coagulation as monitored by thromboelastography. In contrast, three of 30 patients had a significantly decreased coagulation time. Two of 30 patients had no significant changes of TEG variables. TEG variables were restored toward baseline in fourty minutes after augmentin administration. CONCLUSIONS: Augmentin can cause a significant but transient change in the viscoelastic properties of blood. Coagulation parameters of the TEG should be measured prior to augmentin administration to prevent and prospect a bleeding diathesis as a result of platelet dysfunction.