The efficacy of certain growth factors in healing various injuries and the concentrations of these growth factors found within PRP are the theoretical basis for the use of PRP in tissue repair. The platelets collected in PRP are activated by the addition of thrombin and calcium chloride, which induces the release of these factors from alpha granules. The growth factors and other cytokines present in PRP include:
The use and clinical validation of PRP is still in the early stages. Results of basic science and preclinical trials have not yet been confirmed in large-scale controlledclinical trials. For example, clinical use of PRP for nerve injury and sports medicine has produced "promising" but "inconsistent" results in early trials. A 2009 systematic review of the scientific literature stated that there are few controlledclinical trials that have adequately evaluated the safety and efficacy of PRP treatments and concluded that PRP is "a promising, but not proven, treatment option for joint, tendon, ligament, and muscle injuries".
Proponents of PRP therapy argue that negative clinical results are associated with poor quality PRP produced by inadequate single spin devices. The fact that most gathering devices capture a percentage of a given thrombocyte count is a bias, since there is significant inter-individual variability in the platelet concentration of human plasma. More is not necessarily better in this case. The variability in platelet concentrating techniques may alter platelet degranulation characteristics that could affect clinical outcomes.
Implications for doping
Some concern exists as to whether PRP treatments violate anti-doping rules, such as those maintained by the World Anti-Doping Agency. It is not clear if local injections of PRP can have a systemic impact on circulating cytokine levels, in turn affecting doping tests; it is also not clear whether PRP treatments have systemic anabolic effects or affect performance. In January 2011, the World Anti-Doping Agency removed intramuscular injections of PRP from its prohibitions after determining that there is a "lack of any current evidence concerning the use of these methods for purposes of performance enhancement".
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