A diagnostic reference level (DRL) is a specified quantity for a given imaging procedure designed to be used as an aid to optimization of radiation exposures. Diagnostic reference levels are not limits and are not intended to be applied to exposures of individual patients.
The quantity used to define DRLs varies with modality:
Entrance skin dose (ESD) and dose area product (DAP) are typically used to set diagnostic reference level values in radiography and fluoroscopy.
CT dose index and dose length product are typically used to set diagnostic reference level values in CT.
Diagnostic reference levels are set for a particular procedure based on audit data from exposures of a large number of patients at a local, regional or national level.
National DRLs (NDRLs) use data from multiple hospitals in a country and allow for comparison of radiation exposures for a particular procedure across different hospitals. Local DRLs (LDRLs) use data from an individual hospital and allow comparison between different equipment / rooms within that hospital. The ICRP recommend using the median average in determining diagnostic reference levels as this is less susceptible to outlying data points than using the mean average.
If an audit for a particular procedure demonstrates that doses are consistently significantly higher or lower than a DRL this should prompt an investigation into why this is the case. This may include a review of radiographic technique or equipment performance or may be due to the characteristics of the patient cohort being imaged.
If local values are higher than a published DRL this may imply that patient exposures could be optimized by reducing exposure factors however the effect on patient outcome from reduced image quality would also have to be considered. Conversely if local values are significantly lower than published DRLs this may imply that exposure factors may need to be increased to improve image quality.
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