BackgroundProvider-initiated HIV testing and counselling (PITC) increases HIV testing rates in most settings, but its effect on testing rates varies considerably. This paper reports the findings of a process evaluation of a controlled trial of PITC for people with sexually transmitted infections (STI) attending publicly funded clinics in a low-resource setting in South Africa, where the trial results were lower than anticipated compared to the standard Voluntary Counselling and Testing (VCT) approach.MethodThis longitudinal study used a variety of qualitative methods, including participant observation of project implementation processes, staff focus groups, patient interviews, and observation of clinical practice. Data were content analysed by identifying the main influences shaping the implementation process. The Normalisation Process Model (NPM) was used as a theoretical framework to analyse implementation processes and explain the trial outcomes.ResultsThe new PITC intervention became embedded in practice (normalised) during a two-year period (2006 to 2007). Factors that promoted the normalising include strong senior leadership, implementation support, appropriate accountability mechanisms, an intervention design that was responsive to service needs and congruent with professional practice, positive staff and patient perceptions, and a responsive organisational context. Nevertheless, nurses struggled to deploy the intervention efficiently, mainly because of poor sequencing and integration of HIV and STI tasks, a focus on HIV education, tension with a patient-centred communication style, and inadequate training on dealing with the operational challenges. This resulted in longer consultation times, which may account for the low test coverage outcome.ConclusionLeadership and implementation support, congruent intervention design, and a responsive organisational context strengthened implementation. Poor compatibility with nurse skills on the level of the clinical consultation may have contributed to limiting the size of the trial outcomes. A close fit between the PITC intervention design and clinical practices, as well as appropriate training, are needed to ensure sustainability of the programme. The use of a theory-driven analysis promotes transferability of the results, and the findings are therefore relevant to the implementation of HIV testing and to the design and evaluation of complex interventions in other settings.Trial registrationCurrent controlled trials ISRCTN93692532