This systematic review presents the first empirical account of the influence of blinding on intervention effect estimates in dry needling trials. Dry needling differs from acupuncture because while acupuncture needles are used, they are inserted into clinically identified locations in muscles (such as tender areas, palpable nodules or bands) rather than the largely pre-determined insertion sites based on traditional Chinese medicine used in acupuncture. As such, dry needling aims at local effects whereas acupuncture aims at systemic effects. The aim of this review was to determine the influence of blinding effectiveness and blinding adequacy on pain in sham-controlled dry needling trials. Blinding effectiveness was determined by participant beliefs about group allocation relative to actual allocation, and blinding adequacy was determined by critical appraisal. This review posed two questions: (1) ‘Does blinding effectiveness moderate intervention effect on pain?’ and (2) ‘Does blinding adequacy moderate intervention effect on pain?’

Blinding is widely considered critical to the internal validity of clinical trials because it allows separation of specific intervention effects from effects due to bias. This separation is possible because blinding equalises all factors between groups except for the proposed mechanism of action of the intervention under investigation ( Hróbjartsson et al., 2014 ).

Methods

The methods complied with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist (Moher et al., 2009). The protocol was prospectively registered with the International Prospective Register of Systematic Reviews (PROSPERO) (registration number: 42016029340; URL: http://www.crd.york.ac.uk/prospero/display_record.asp?ID=CRD42016029340).

Disclosure of deviations from prospectively registered protocol Following original registration on March 2, 2016, two changes were made to the protocol of this review: (1) the data extraction template was pilot tested using an iterative process rather than a sample of 10 included trials, and percentage agreement was used to determine agreement, rather than an intra-class correlation coefficient (ICC); and (2) the time-point that was used to investigate the influence of blinding effectiveness on pain outcomes was the time-point at which blinding was assessed (instead of the pre-defined time-points of immediate, short-term, and long-term pain assessments), because the authors agreed that this time-point would most accurately reflect intervention beliefs (i.e., blinding effectiveness) as beliefs can change over time (Bang et al., 2010; Rees et al., 2005). The second change was updated in PROSPERO prior to data analyses (revision posted on February 5, 2017). This review presents only review questions 1 and 2 of the protocol; review questions 3 and 4 will be reported elsewhere.

Information sources and search strategy One reviewer (FAB) executed the search strategy. Databases (MEDLINE, EMBASE, AMED, Scopus, CINAHL, PEDro, The Cochrane Library) were searched from inception to February 2016. The general search terms were (needl* OR acupuncture OR intramuscular stimulation) AND (sham OR placebo*), and Medical Subject Headings (MeSH) were used where possible. The full electronic search strategy for MEDLINE is presented in Table 1. Searches were modified to suit the functionality of each database. Thesis databases (Trove, ProQuest) and clinical trial registries (Australian New Zealand Clinical Trials Registry (ANZCTR), Clinicaltrials.gov, World Health Organization International Clinical Trials Registry Platform (WHO ICTRP)) were crosschecked with database searches to identify further potential trials. The reference lists of systematic reviews identified by the search were examined to locate additional or unpublished trials. There were no limits on year, language, or publication status. Search terms Limits applied 1. Needl*.tw Humans only

Keyword searches limited to title/abstract/keyword fields 2. *Acupuncture therapy/ 3. Acupuncture.tw 4. Intramuscular stimulation.tw 5. Sham.tw 6. *Placebo effect/ 7. *Placebos/ 8. Placebo$1.tw 9. #1 OR #2 OR #3 OR #4 10. #5 OR #6 OR #7 OR #8 11. #9 AND #10 DOI: 10.7717/peerj.5318/table-1