In the UK, the diagnosis of brainstem death (BSD) is confirmed by clinical testing. Guidelines regarding the timing of clinical tests after sedative drug use allow for variable interpretation. In some countries, ancillary tests are used as an alternative to clinical diagnosis. We conducted a survey of the use of ancillary tests within UK neurocritical care centres. Twenty-three centres had used or would consider using ancillary tests in addition to clinical tests where these could not be completed (eg, in cases of facial trauma). Nineteen centres had used or would consider ancillary tests when drug levels were unavailable. Twelve centres had used or would consider using ancillary tests to confirm BSD where raised drug levels precluded clinical testing. Following sedative drugs, timing of clinical testing varied considerably. While nine centres always measure thiopentone levels, six centres never do. There is considerable variation in UK opinion and practice. Further consensus is needed.