Interesting comment by @swooping about anagen/telogen ratio.
There's a good study about this effect of PRP. I think it's worth describing here in detail. But if you want to skip the details, the TL;DR is that PRP increased hair density.
It was conducted by the University of Catalonia in Barcelona, Spain and published last year (https://www.ncbi.nlm.nih.gov/pubmed/27035501). I think it's much better structured than most I've seen. They studied 25 patients. Each patient received 3 treatments of PRP, 1 month apart. Half the patients received treatment with PRP on the right half head and the placebo on the left half head, while the other half of patients received treatment with PRP on the left half head and the placebo on the right half head. One injection was made into each of the four corners of the top of the patients scalp; so in total, two injections of PRP and two injections of placebo were made on each patient on 3 separate times. The treating physician had to know which side was PRP and which placebo, but the analyzing physician and the patients were blinded as to which was which until the end of the study.
Treatments were made at month 0, month 1, and month 2. Measurements were made at month-3 (so one month after the last of the three treatments) and at month-6 (i.e., four months after the last treatment).
The results were solid:
The changes in hair density area are most striking to me. At the 3 month point, the mean density of the placebo area decreased by 0.7 hairs/cm2 (from 167.1 to 167.8), while the mean density in the treated areas increased by 14.8. The improvement was still present at the 6 month point as well, which is interesting to me because that was 4 months after the last treatment, showing that it seems to have some lasting effect. It's also notable that this improvement at both 3-months and at 6-months was small but significant in terms of terminal hair density. The range in improvement between patient results is fairly wide, but the results seem pretty solid and positive.
In addition to density, a change in anagen/telogen ratio is seen in the data. At the 3-month mark, the PRP-treated area increased from 62% anagen hairs to 68%, while the placebo-treated area only increased to 63.5%. What's interesting, though, is that at the 6-month point, that difference had evened out and both the placebo area and the PRP area were fairly close in their anagen/telogen ratio.
The authors' discussion of the possible underlying mechanism:
They also comment that there's a lot of differences in how PRP is prepared and it's important to find the best methods and standardize.
Personally, these studies seem good enough by now that I decided to give it a try. After speaking to a few clinics, I can confirm that there really is a huge range in methods to prepare PRP. It's really hard to get information from most clinics about the details of their PRP; truth is, I think most of them just buy a centrifuge and kit and don't really know a lot. There've been only a handful of PRP hair studies, but there have been a ton of studies on PRP in joints, so I'm using the info from all of that in looking for a clinic that prepares the PRP well.
(At some point, I've even thought of doing the PRP myself. It's not as crazy as it sounds if you're fairly comfortable with lab work. And for money, after a little upfront investment, it'd be a huge savings over time, especially as it seems that regular injections every month or so are better than just doing a one-time one-and-done injection.)
If anyone's interested, let me know and I'll report back once I have some info.