Assessment of acceptability is a central component of most pre-exposure prophylaxis (PrEP) and microbicide trials. It stands to reason that many participants will not be adherent to product use during the course of a trial, nor will many of those at risk of HIV use the product when it becomes available if it is considered unacceptable. However, although numerous PrEP and microbicide studies have investigated product acceptability, a consensus regarding the definition and operationalization of the concept does not exist [1,2]. A theoretical muddiness plagues discussions and investigations of acceptability as many researchers equate acceptability with use and focus studies that investigate acceptability on identifying factors that motivate or undermine use. Indeed, a recent review study considered use-associated factors and willingness to use in the future to be key elements in an ‘integrated’ approach to acceptability . The observation from contraceptive research that perceived method attributes are only weakly predictive of product use  and the finding in a study of barrier methods and sexually transmitted disease prevention, that acceptability – measured either as hypothetical willingness to use or as product satisfaction – was not associated with subsequent method use , may also have contributed to skepticism regarding the utility of measuring acceptability narrowly and to the incorporation of product usage in recent definitions. In this paper we will review current definitions and frameworks employed in microbicide and PrEP acceptability research. We will then examine recent empirical studies that include assessments of product acceptability – variously operationalized – and will summarize trends in acceptability research based on 18 papers (from 15 unique studies) published in 2011 and 2012. We are focusing primarily on studies in which the product, whether actual or hypothetical, is a drug or chemical barrier, although the issues we discuss apply to acceptability of physical barrier methods as well. We will conclude by offering a new framework for investigating product acceptability within clinical trials, one which avoids the inherent circularity of current definitions and considers product acceptability to be conceptually distinct from adherence.
CURRENT DEFINITIONS AND FRAMEWORKS
Fifteen years ago, in an attempt to ‘re-orient’ research on contraceptive choice, Heise (: p. 7) argued that acceptability is not a feature ‘inherent in a product or a method’ that could be adjusted if it is lacking; rather it is ‘relative, conditional, and user-driven.’ She considered acceptability to be an ‘interplay’ between the user, the technology, the service delivery system and the social environment. Context – type of partnership (primary or casual), nature of relationship (new or established), and gender–power dynamics – mattered. This conceptualization has influenced behavioral and social scientists involved in microbicide acceptability research. Elias and Coggins  posited that acceptability is more than just the ‘method continuation rate,’ and broadened their definition to include the knowledge regarding potential benefits, correct usage, potential side effects, and alternative products. Equally important is the service delivery system, that is, ‘whether and how the product is provided to potential users’ (: p. 164). In 2005, Severy and Newcomer (: p. 49) argued for ‘a sense of acceptability demonstrated by long-term use,’ noting that use depends on product characteristics (which will affect individuals differently across cultures), partner dynamics, and other actors, including family members and healthcare providers. Their definition of acceptability – ‘the voluntary sustained use of a method in the context of alternatives’ – still prevails today. Severy et al. (: p. 127) subsequently proposed an ‘expanded framework’ for assessing acceptability, one that re-conceptualized sustained acceptability as a dynamic and sequential process, and incorporated a leading behavior change theory, the AIDS Risk Reduction Model [9,10]. Their framework also goes beyond product attributes ‘to identify the individual, couple, and sociocultural factors that make [such sustained] use possible’ (: p. 127). More recently, Woodsong and Alleman  developed a framework for clinical trials that views acceptability as a continuum of behavioral and attitudinal changes strongly influenced by context. It has been applied and adapted in several qualitative inquiries in clinical research, for different types of products and delivery modes [12▪,13▪,14]. The most recent conceptual model of microbicide acceptability, developed by Coly and Gorbach , emphasizes stages of product development and also considers context and trial phases to be important. They, however, note that despite its importance, context has ‘received little attention in the literature.’ Acceptability ‘as measured by single or short-term use of a product in a phase I or phase II trial, should be measured as adherence in phase IIb or phase III trials that require extended product use. . . .’ (: pp. 584–585). Coly and Gorbach do not provide their own definition of acceptability; rather their model outlines the factors that influence acceptability.
MEASUREMENT OF ACCEPTABILITY: REVIEW OF RECENT STUDIES
The 15 unique studies presented in the 18 publications reviewed vary markedly in terms of sample size, population, product, definition of acceptability, and methodology (see Table 1). As such, it is not possible to generalize about acceptability, specifically which product attributes – for example, color, smell, texture, viscosity, size – delivery mode, or dosing regimen might be viewed as desirable or problematic in specific settings and among particular populations. However, one can summarize components of the study designs and make some general statements about findings:
1. Only 7 of the 18 papers [12▪,17▪,19,21,26,27,29] investigate acceptability of actual candidate products.
2. Eight publications examine hypothetical acceptability [15,16,18▪,20,22–24,30] – typically investigating whether respondents would be willing to use a product, and attitudes and beliefs about the product.
3. Three of the 18 [13▪,25,28▪] examine acceptability of a placebo or proxy product.
4. Three of the 18 [12▪,19,28▪] ask participants to compare multiple products or dosing regimens and indicate their preference, rather than to evaluate a single product or individual features of a product.
5. To the extent that an overall measure of acceptability is included in studies, the proportion of participants who indicate a ‘willingness to use’ the product is one of the most common summary constructs. Fifteen publications [13▪,15,16,17▪,18▪,19–24,26,27,28▪,30] contain such a measure with some variability in responses; in 5 [13▪,15,22,27,28▪] of the 15 studies at least 80% of participants expressed a willingness to use. In another [17▪], 59% reported that they would be likely to use in the future. In yet another [18▪], which interviewed nearly 1800 high-risk participants in seven countries, 61% indicated they would definitely use. In the one reviewed acceptability study that was conducted in China  over two-thirds indicated a willingness to use. Another study  asked about overall satisfaction and whether the participant would recommend the product to a friend; here again, the product was considered ‘highly acceptable.’ Interestingly, despite the range in the percentages willing to use that are reported, the conclusion is the same, namely that participants find the products under investigation to be acceptable.
6. It is the rare study in which researchers feature negative findings; only one study [17▪,21,26] devoted considerable space to problems, noting that participants reported issues with the three gels under investigation (one active, two placebo), complaining about leakage, and interference and decreased satisfaction with sex.
7. Two studies [18▪,20] used conjoint analysis to assess the relative importance participants assign to various attributes or features of a product, whether positive or negative. Participants were presented with hypothetical scenarios with different combinations of attributes involving such dimensions as cost, efficacy, side effects, duration of administration, dosing regimen, ‘dispensing process’, frequency of resupply, and frequency of HIV testing. They were then asked to rate these scenarios. One of these studies  reported the results for eight different hypothetical scenarios and observed enormous variability across scenarios – ranging from nearly 20% to over 80% acceptable – with out-of-pocket cost, efficacy, and side effects exerting the greatest effect on acceptability. These results suggest that acceptability is determined by numerous attributes and that an overall measure of ‘willingness to use’ or ‘liking the product’ may be insufficiently nuanced to explain eventual adherence.
8. Implicitly acknowledging the limitations of narrow measures of acceptability, such as physical properties or side effects, all but 1 of the 18 publications we reviewed assesses multiple, broader dimensions of the concept. Although the measures are not standardized, the results are informative. The most common elements considered include ‘use attributes’ (15 publications), followed by ‘partner's attitudes’ (13 publications), ‘efficacy’ – hypothetical – (11 publications), and ‘effects of product on the sexual encounter’ (10 publications).
LINKING ACCEPTABILITY TO PRODUCT USE
As Table 1 indicates, only 1 of the 18 publications attempted to link acceptability to use. van der Straten et al.[12▪] examined whether dosing preference – precoital or daily – was associated with self-reported adherence for the Duet, a cervical barrier and microbicide delivery system; self-reported adherence was not found to vary significantly by use regimen, although women preferred the precoital regimen. Of note, this study had a small sample size, and was of short duration. A couple of studies, published prior to 2011, have analyzed the association between acceptability and adherence. Joglekar et al. assessed whether various dimensions of acceptability, including physical characteristics, use-related issues, sexual experiences with the product, genital symptoms, and partner acceptability, affected self-reported adherence to a candidate vaginal microbicide tablet; only sexual experiences with the product were significantly associated with adherence in a multivariate model. van der Straten et al. examined whether consistent use of a gel with or without a diaphragm was associated with several dimensions of acceptability – liking product, sexual pleasure, reasons for use, recommendation to a friend, partner's knowledge of use, partner's reaction to product, and experiencing a problem with study product. No association was observed. A major limitation of both these studies is the reliance on self-reports to measure adherence.
NEW DEFINITION AND CONCEPTUAL MODEL FOR INVESTIGATING MICROBICIDE ACCEPTABILITY
As asserted above, consensus is lacking about the definition and measurement of acceptability. It is a complex construct, which has been equated to a behavior (use or sustained use) combined – or not – with a set of attitudes influenced by a given context. The reduction of acceptability to attitudes or hypothetical intentions (e.g. willingness to use in the future), which may have little relevance to (future) reality, has not strengthened support for the value of measuring this concept in clinical trial research.
We propose that a revised definition of acceptability be adopted for clinical trials that clearly and operationally separates product acceptability from product adherence, and the factors that may influence these two constructs (see Fig. 1). We define acceptability as the product-related attributes, perceptions, and experiences that potentially influence (i.e. act as facilitators or barriers) to product adherence. In this revised framework, product acceptability is a proximal determinant to use. Background and contextual factors also may influence use directly or indirectly through product acceptability. We adapted a socio-ecological model, which recognizes that various agents and factors are operating at different levels (individual, household, partner, organizational, contextual/structural) and may influence an individual's behavior (adherence to product use) as well as his/her opinion of acceptability. While erring on the side of being exhaustive, Table 2 presents examples of variables that could be measured under each level of influencing factor and component of product acceptability. This model recognizes the importance of participants’ agency in the use of the product or an alternative, as even in clinical trials, participants may choose not to use the product, or to use only condoms. Finally, as previously defined  adherence is operationalized as initiation (uptake or not of the product), execution (whether the product is taken as directed), and discontinuation. These components as well as persistence, the time from initiation to discontinuation, highlight the dynamic aspect of adherence as a behavioral process taking place over time . The importance of trial phases in identifying factors that affect adherence, noted by Coly and Gorbach , is highlighted in several ways; whereas timely initiation and persistence (nondiscontinuation) will be more critical to assess in longer-duration trials, execution is critical in all trial phases. Furthermore, some acceptability attributes are more important to measure in early phase I trials (e.g. physical characteristics and delivery mode), when product modification may still be feasible, whereas others will be more relevant to measure in later phases (e.g. sexual encounter attributes, partner's attitude about product, community-associated norms about products). Finally, some levels of influence may figure more prominently in different trial phases (e.g. individual characteristics in phase I when participants are typically asked to be abstinent, versus partner and contextual factors in later-phase trials) .
We believe this model is an improvement over previous models because it clearly separates acceptability from adherence, and highlights how influencing factors can affect product use directly or indirectly through acceptability. It also incorporates participants’ agency and choice (or lack of thereof) as a central concept on the pathway between acceptability and adherence. This model may help explain why unsatisfactory products, such as condoms, can still be used, despite low acceptability [5,34], or, alternatively, why despite ‘high’ reported acceptability, study products in clinical trials may not be consistently used because of contextual influences, alternative choices, or competing life priorities [13▪,35].
In their 2008 review of microbicide acceptability research, Coly and Gorbach  observed that: a consensus did not exist with regard to the definition and operationalization of acceptability; ‘as currently measured’ (: p. 581) vaginal microbicides have been found to be acceptable; and the association between microbicide acceptability and adherence in clinical trials has yet to be determined.
Four years later, these same conclusions hold; moreover, we can extend the second point to include PrEP. Whereas several trials have demonstrated efficacy since that earlier review – CAPRISA 004 (Tenofovir gel), iPrEX (Truvada), Partners PrEP (oral Tenofovir and Truvada, and TDF2 (Truvada) – two trials or arms of a trial investigating the same products have been stopped for futility – FemPrEP (Truvada) and VOICE (oral Tenofovir and Tenofovir gel). One partial explanation for the disparate findings is differential adherence . Efficacy has been shown to be associated with detectable levels of study drug in plasma . Indeed, fewer than 40% of participants had significant plasma drug levels in FemPrEP. Given the critical role of adherence to the success of clinical trials, it is important to identify the extent to which acceptability is a factor in product usage and to ascertain which dimensions of acceptability – product attributes, dosing regimen, delivery mechanism, use attributes, partner's attitudes, effect of product on the sexual encounter, product-related norms – if any, affect adherence. We have been hampered in the past by reliance on self-reported measures of adherence, which have been shown to be flawed [37–39]. And we have been hindered in our investigation of the linkages between acceptability and adherence by defining the former in terms of the latter. With the validation of biomarkers and increased ability to measure drug levels in trials, as well as inclusion of measures of product use that are more objective than self-report (e.g. unannounced product counts, electronic monitoring, applicator insertion assays), researchers will be in a better position to understand the degree to which adherence is driven by acceptability and the degree to which it is influenced by individual and contextual factors.
Finally, similar to Severy and Newcomer [7: p. 49], who contend that ‘we cannot be certain about a product's acceptability’ in the absence of choice, we think an important case can be made for consideration of choice and product preferences, whenever feasible in clinical trials, as a means of advancing our understanding of product acceptability.
The authors would like to thank Barbara Miller for her assistance in the preparation of this manuscript.
Conflicts of interest
Source of Support: Support for preparation of this manuscript was provided in part by a Professional Development Award from RTI International for Ariane van der Straten and the United States Agency for International Development (USAID) for Lauren L. Katzen. Additional funding was provided by the Microbicide Trials Network (MTN); Barbara Mensch's and Ariane van der Straten's work on the MTN is funded by the NIMH (U01 A1068633). The contents of this manuscript are the sole responsibility of the authors and do not necessarily reflect the views of USAID, the NIMH or the United States government.
In selecting studies of special interest, authors did not vote on their own work. There are no other conflicts of interest.
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
▪ of special interest
▪▪ of outstanding interest
Additional references related to this topic can also be found in the Current World Literature section in this issue (pp. 609–610).
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Keywords:© 2012 Lippincott Williams & Wilkins, Inc.
acceptability; adherence; microbicide; PrEP