Sir, I read with interest the paper evaluating PAD in endodontics (BDJ 2006; 200: 337). Although the aim of the study was to assess the effect of PAD when used as an adjunct to conventional disinfection (using 2.25% sodium hypochlorite solution) the methodology appeared flawed in one aspect which was left unmentioned in the authors' discussion.

The paper described sampling, following sodium hypochlorite irrigation, using a 0.02 taper hand file one size larger than the MAF file. This would have altered the canal geometry, increasing its diameter and hence volume. Consequently the flow, volume and depth of penetration of the PAD solution, which was subsequently introduced, would have been facilitated.

It follows that the conditions in the canal comparing the effect of sodium hypochlorite and PAD were distinctly different. In fact the conditions were biased in favour of the PAD, and it is my assumption that this may have accounted for the observed fall in microbes following PAD treatment. If sodium hypochlorite was reintroduced having enlarged the canal a similar effect may have resulted.

Therefore, it is difficult to conclude from this study that PAD is more effective than sodium hypochlorite as a disinfectant, even though the authors' discussion implied that it was. If the authors' results were obtained without altering the canal dimensions during sampling then indeed PAD could be shown (taking into account the small sample size) to be a useful adjunct to sodium hypochlorite but this was clearly not the case.