Background Realist evaluation is increasingly used in healthcare research, and theories can provide plausible explanations of why interventions work or do not work in certain circumstances such as the effect of the opt-out hepatitis C virus (HCV) testing policy in English prisons.

Aim To present the process of constructing the middle-range theories (MRTs) developed as part of an evaluation of hepatitis C test uptake in an English prison as a resource for researchers using realist evaluation.

Discussion MRTs are propositions that can explain a particular behaviour or outcome. In this evaluation, the MRTs emerged from a realist evaluation, a theory-driven approach for understanding what interventions work, in what circumstances and how. The mixed-methods data collected during the realist evaluation and the sociological theory of prisonisation were used to create the MRTs. Combining prisonisation with the qualitative data illustrates how healthcare interventions may be viewed by people in prison who may have adopted either the ‘deprivation’ or ‘importation’ processes of adaptation to cope with their incarceration. Their views may affect the acceptance of HCV tests.

Conclusion The development of MRTs is a creative and iterative process, requiring an in-depth understanding of the data collected and the subject area. MRTs permit us to see relationships among phenomena that might otherwise seem disconnected, thereby aiding the development of more efficacious interventions.

Implications for practice The MRT developed presents an evidence base for selecting interventions to increase the uptake of HCV tests in prisons. This paper explains how a MRT was developed and how HCV test uptake in prisons can be explained using a sociological theory.