We take for granted the value of the care continuum (or ‘cascade’), now increasingly seen as the key measure of health system response to HIV (Cassell (STIs editorial)).   The application of this model to HIV has provided a benchmark for evaluation in contexts as diverse as Moscow (Wirtz & Beyrer (STIs)), South Africa (Schwartz & Baral (STIs)) or the Netherlands (van Veen & van der Sande (STIs)).   But could the same model also offer a means of evaluation in the case of other complex sexual health interventions such as PrEP (Pre-Exposure Prophylaxis)?

An on-line soon-to-be-published paper by Nunn & Chan (N&C), building on an earlier attempt by Kelley & Rosenberg (K&R), does precisely this.  An important difference from the earlier paper seems to be the more concrete definition of a larger number of steps (nine as against five) – especially in the central area of ‘uptake’ and engagement in care.  Here K&R define three stages: ‘need for awareness of PrEP and willingness to use it’, ‘need for good access to healthcare’, and ‘need for a prescription for PrEP.  N&C replace these with a more concrete conceptualization of the process in five stages involving: an occasion where PrEP access is facilitated (4); an appointment arising from that occasion where the assessment is performed (5); the prescription of PrEP, where indicated (6); the actual initiation of PrEP (i.e. when the client starts taking the pills) (7).  Also important is N&C’s substitution of two final steps – adherence (8)) then retention (9) for K&R’s single final step of ‘adherence’.  N&C point out that, whereas, with ART, ‘adherence’ is once-and-for-all and secures the ultimate goal of viral suppression, in the case of PrEP, we can envisage multiple trajectories depending on whether PrEP continues to be indicated (e.g. the client may no longer be exposed to risk).  Finally, K&R’s first step – ‘identifying at risk MSM’ – gives way to three: identifying at risk individuals (1), enhancing HIV awareness (2), enhancing PrEP awareness (3).

Is this nine-stage definition of a PrEP cascade overly “complex” (EECAAC2018)?

Answering such a question requires us to reflect on the function that the ‘cascade model’ is called upon to perform.  If the model divides up the total course of an intervention into a series of staged tasks, this is presumably because the health benefit depends on the completion of the whole intervention, yet the accomplishment of each step is necessary to the achievement of subsequent ones.  The idea of the cascade can provide a fair way of evaluating the progress of an intervention before its potential health benefits have been delivered – and can also identify the precise points at which the intervention is failing (i.e. where clients become ‘disengaged’).

It follows that each step should correspond to a potential outcome that is not inferable from previous or later outcomes but is worthy of independent evaluation.  If everyone who accesses PrEP (4) also attends an appointment at which suitability of PrEP is discussed (5), or everyone who adheres to PrEP (8) is also retained in PrEP (9), then steps (4) and (5), or steps (8) and (9), can be merged.  This is not stated in so many words by the authors of the model.  However, I would assume that it must lie at the basis of their thinking.