The analytical lab methods that we use to grow antibiotic-resistant bacteria make a big difference in terms of recovery. However, ‘pre-analytical’ factors are just as important in determining the sensitivity of prevalence studies. We are used to the idea of studies to work out the most sensitive anatomical site to sample for detecting colonisation with antibiotic-resistant bacteria. However, there are other ‘pre-analytical’ factors that may skew the findings of prevalence studies. A study from my old research group at KCL highlights how staff and patient choices, behaviours, and demographics can be pre-analytical factors that could skew prevalence studies.