The blinding status and characteristics in acupuncture clinical trials: a systematic reviews and meta-analysis

This systematic review with meta-analysis represents the first comprehensive assessment of blinding status in acupuncture clinical trials, a methodological issue fundamental to the validity of studies on this intervention. The researchers analyzed 64 randomized controlled trials published between 1999 and 2024, extracted from databases such as PubMed, Embase, and Web of Science, all containing blinding assessments with reported results. Blinding in acupuncture trials presents unique challenges compared with pharmacological studies. While medications can be easily masked through identical appearance and taste between active drug and placebo, acupuncture requires simulation of needling sensations, making blinding more complex.

Sham acupuncture seeks to convince participants that they are receiving real treatment, often by simulating the sensation of needle penetration. The researchers used Bang's Blinding Index (Bang's BI) as the primary measure, analyzing the data through hierarchical Bayesian models. This index ranges from -1 to +1, where values near zero indicate perfect blinding, positive values suggest that participants correctly identified their treatment group, and negative values indicate 'wishful thinking' (believing one is receiving real treatment when actually in the control group). The main results showed that the Bang Index was 0.41 for the true acupuncture group and -0.24 for the sham group.

This indicates that most participants in the true acupuncture group correctly identified that they were receiving active treatment, while those in the sham group tended to believe they were receiving real acupuncture — a pattern considered indicative of successful blinding in acupuncture trials. The analysis identified four distinct blinding scenarios. Scenario 1, where the index was positive for true acupuncture and negative for sham (indicating 'wishful thinking'), represented 62.50% of trials and is considered successful blinding. Scenario 3, with positive indices in both groups, occurred in 28.15% of trials and suggests possible blinding compromise, requiring special attention.

Factors that influenced blinding success included: study region (Asian populations showed greater difficulty in blinding, possibly due to greater cultural familiarity with acupuncture), number of research centers, number of treatment sessions, number of acupuncture points used, and timing of blinding assessment. Longer treatments or those with more points increased the likelihood of participants correctly identifying their group. The subgroup analysis revealed that studies in Asian countries, use of penetrating sham needling, and questioning participants about their group during the study increased the risk of unblinding. Surprisingly, no significant correlation was found between blinding success and treatment effect size, suggesting that participants' perceived effectiveness did not substantially influence their ability to identify the treatment group.

The clinical implications are significant for the future design of acupuncture trials. The researchers recommend rigorous monitoring of blinding status and transparent reporting of assessment results. They also suggest that factors such as Asian populations, penetrating sham needling, and questioning during the study should be carefully considered in the design of sham controls. Limitations include inclusion only of English-language articles, high heterogeneity between studies, and the inability to assess blinding of parties other than participants (such as acupuncturists, who typically cannot be blinded due to the nature of the intervention).

Most of the included studies had 'high' or 'some concerns' risk of bias according to the ROB2 tool, mainly due to the inability to blind acupuncturists. This study provides important evidence to improve the methodological quality of future acupuncture trials, establishing clear guidelines for blinding assessment and reporting that are fundamental to the scientific credibility of acupuncture research.