IEC TR 61948-4:2019 covers the routine testing of radionuclide calibrators used in nuclear medicine. Such devices utilise ionisation chambers of the well type and a direct readout in units of activity. Requirements and specific methods to determine performance parameters are described in IEC 61303. These methods are primarily designed for acceptance testing. IEC TR 61948-4:2019 cancels and replaces the first edition published in 2006. This edition constitutes a technical revision. This edition includes the following significant technical change with respect to the previous edition: the test method to determine system linearity has been updated to reflect the technical developments of radionuclide calibrators.