The effect of placebo in split-scalp and whole-head platelet-rich plasma trials for androgenetic alopecia differs: Findings from a systematic review with quantitative evidence syntheses.

BACKGROUND: Some studies have shown that platelet-rich plasma (PRP) improves androgenetic alopecia (AGA), while others do not. We determined whether the placebo effect significantly varies between split-scalp and whole-head trials on PRP monotherapy for AGA. Our rationale was based on the plausibility of PRP diffusing to the control (i.e., "placebo") side of split-scalp trials. This is not possible in whole-head studies. METHODS: We systematically searched the literature for available data. Our choice of analyses and outcomes were determined by the available data. RESULTS: Our endpoint was change in total hair density 6 months after baseline. Our regression showed that total hair density after 6 months was significantly (p < 0.05) higher in the placebo arm of split-scalp trials, compared to whole-head studies, by 37 hairs/cm2 . Our one-arm meta-analyses showed that the pooled change in total hair density between the PRP side and placebo side in split-scalp studies was -3 hairs/cm2 (p = 0.37), that is, a slight decrease in hair density in the placebo side of the scalp. For whole-head studies, the corresponding difference in total hair density between patients receiving PRP and those on placebo was -30 hairs/cm2 (p = 0.000017), that is, a much larger decrease in hair density. Patients in the placebo group in whole-head trials lost significantly more hair than in the placebo side of the split-head trials where hair loss was comparatively reduced - presumably because of PRP diffusing from the treatment side of the scalp. CONCLUSIONS: The association between design (i.e., split-scalp vs. whole-head) and outcome, in placebo arms of AGA trials on PRP monotherapy, had never been reported. This "design effect" could partly reconcile the incongruent conclusions across the PRP literature for AGA; furthermore, clinical guidelines can consider "design effect" when selecting evidence to base care practices on.