Despite the evidence that resistance testing can predict treatment response, many doctors are cautious about its use in the clinical setting. Most see it as a potential adjunct to existing ways of determining drug regimens and decisions about switching therapy. This caution has been shaped by early results of clinical trials of resistance testing which showed, at best, only moderate benefit in the short term. These factors, combined with the limited availability of resistance assays in the United Kingdom, have meant these tests were not widely used in the past.