Double blinding with a new placebo needle: a further validation study

Abstract

Background: The masking properties of a new, non-penetrating, double-blind placebo acupuncture needle were demonstrated. Practitioners correctly identified some of the needles; if they were confident in this opinion, they would be unblinded.

Objective: To investigate the clues that led to correct identification, and the confidence in this decision.

Methods: Ten acupuncture practitioners, blindly and randomly, applied 10 each of three types of needle to the shoulder: blunt, non-penetrating needles that pressed the skin ('skin-touch placebo needle'); new non-penetrating needles that penetrated soft material (stuffing) but did not reach the skin ('non-touch control needle'); matching penetrating needles. Afterwards, practitioners were asked to judge the type of needle, their confidence in their decision and what clues led them to their judgements.

Results: Of the 30 judgements made by each practitioner, the mean number of correct, incorrect and unidentifiable answers were 10.4 (SD 3.7), 15.2 (SD 4.9) and 4.4 (SD 6.1), respectively. There was no significant difference in the confidence scores for 104 correct (mean, 54.0 (SD 20.2)%) and 152 incorrect (mean, 50.3 (SD 24.3)%) judgements. Twelve needles were identified with 100% confidence-three correct, and nine incorrect. For needles correctly identified, the proportions of non-touch (p = 0.14) and skin-touch (p = 0.17), needles were no greater than chance, but the proportion of penetrating needles correctly identified exceeded chance (p < 0.01). 53% of judgements were made from the "feeling of needle insertion", but 57% of these were wrong.

Conclusion: Practitioners had a slight tendency to guess the penetrating needles correctly, but were uncertain about most of their judgments, posing only a very small risk to double blinding.

Figure 1

Double-blind needles. A, Means of needle insertion. B, The non-touch control (left), skin-touch placebo (middle) and penetrating needles (right) comprise an opaque guide tube and upper stuffing to give resistance to the needle body during its passage through the guide tube. The needle body of the non-touch control needle is shorter than the guide tube, and the needle body of the skin-touch placebo needle is just long enough to allow its blunt tip to press against the skin, but the needle body of the penetrating needle is longer than the guide tube by an amount equal to the insertion depth when the needle body is advanced as far as possible. The non-touch control and skin-touch placebo needles contain lower stuffing to give a similar sensation to that of skin puncture and tissue penetration. Each needle has a stopper, which prevents the needle handle from advancing further when the tip of the needle reaches the specified position. The pedestal on each needle is adhesive, allowing it to stick firmly to the skin surface. The left and right needle for each type of needle is before and after needle insertion, respectively.

Figure 2

Confidence level in the practitioners’ judgements of ‘non-touch control’, ‘skin-touch placebo’ and ‘penetrating’ for non-touch control, skin-touch placebo and penetrating needles. The top, middle and bottom lines of the boxes correspond to the 75th, 50th (median) and 25th percentiles, respectively. The whiskers extend from the 10th percentile to the 90th percentile. The circles indicate the arithmetic mean. Note: N.S. indicates that there was no significant difference.