The in vitro contracture test (IVCT) remains the standard test for the diagnosis of malignant hyperthermia (MH) susceptibility. The aim of this study was to investigate whether results of the IVCT varied between two diagnostic centers. The study took place at the national MH centers in Denmark and Sweden. Forty-three patients investigated for MH gave informed consent to have four extra muscle specimens excised. These were sent to the other center and immediately used for a parallel IVCT, according to the protocol of the European MH Group. Results of the IVCTs performed in the two centers on muscle samples from the same patients were compared. Each patient was assigned a diagnosis according to the result obtained in the "mother-center." Identical diagnostic results were obtained for 56% of the patients. The differing diagnostic outcomes were almost exclusively seen in cases with contractures of <5 mN (0.5 g) and abnormal results in only one or two muscle strips. We suggest different criteria for the interpretation of results for clinical and scientific purposes. The clinical criteria should remain unchanged. The scientific designation of susceptibility should be used in cases with contractures of > or =5 mN and abnormal results in at least 75% of the tested muscle strips.
Implications: The diagnostic outcomes of tests for malignant hyperthermia susceptibility were compared between two laboratories by using muscle tissue from the same patients. Identical outcomes were found for 56% of the patients. Almost all diverging outcomes were seen in cases with a few small contractures near the cutoff limit. Different diagnostic criteria for clinical and scientific purposes are suggested.