Outpatient PRP preparation systems exist with the ability to concentrate platelets from two to eight times. There is some controversy about what the “optimum” platelet concentration should be, but a level of at least 1 million platelets per μL appears to be the “magic number.” Since the average patients platelet count is 200,000 +/- 75, a four to five times concentration appears to be the desired level. When levels are in the 5x range, the influx of adult stem cells has been noted to increase by over 200%.
In 2008, Kajikawa et al concluded that PRP enhances the initial mobilization of “circulation-derived cells” in the early stage of tendon healing. “Circulation-derived cells” are defined as mesenchymal stem cells that have the potential to differentiate into reparative fibroblasts or tenocytes as well as macrophages. Under normal circumstances, circulation-derived cells last only a short time after tendon injury. The authors suggest this as one of the main reasons for the known low healing ability of injured tendons. If the circulation derived cells could be activated and their time-dependant decrease stalled with PRP, then the wounded tendon could more fully heal. One study found an increase in the circulation-derived cells with the PRP group, as well as increased production of types I and III collagen in the PRP group versus control. This finding of additional fibroblast proliferation and type I collagen production enhanced by increasing platelet concentrations concur with an earlier study by Lui et al. This provides evidence that PRP stimulates the chemotactic migration of human mesenchymal stem cells to the injury site in a dose-dependent manner – i.e., the more concentrated the platelets, the more stimulation.