Referral of any group of unwell people from primary to secondary to tertiary care, causing an increase in the concentration of rare cases, more complex cases or people with worse outcomes.
Referral filter bias was listed by David Sackett in 1979 and indicates that participants in a study may not properly represent the population being looked at. Due to this the results in a study may not be applicable and may have low external validity.
People with high blood pressure studied in the community may have different characteristics compared to those studies in the hospital. These patients are more likely to have unusual features, to have more severe high blood pressure and have secondary causes. They may not, therefore, represent the population with high blood pressure as a whole.
As far as we are aware, there has been no formal evaluation of referral filter bias. However, the impact of selection bias is considered in the relevant catalogue entry.
Study design and methods to minimize selection bias can also take into account referral filter bias, so as to ensure that a properly representative sample of a population is recruited into a study. Interpretation of study results should always consider whether the inclusion criteria, or sampling frame, enable generalizability to the target population.
Catalogue Of Bias Collaboration. Bankhead C, Nunan D, Aronson JK. Referral filter bias. In: Catalogue Of Biases 2019. https://catalogofbias.org/biases/referral-filter-bias/