Chi square tests and Fisher exact test were used to assess the differences between categorial data. Relative risk was calculated as a/(a + b)/c/(c + d), where a = number of patients with previous antibiotic hypersensitivity with positive skin tests for NMBAs, b = number of patients with previous antibiotic hypersensitivity and negative skin tests for NMBAs, c = number of controls with positive skin tests for NMBAs, and d = number of controls with negative skin tests for NMBAs