Platelet-rich plasma (abbreviation: PRP) is blood plasma that has been enriched with platelets. As a concentrated source of autologous platelets, PRP contains several different growth factors and other cytokines that can stimulate healing of soft tissue. Platelet-rich plasma therapy is an old therapy and used extensively in specialities of dermatology, orthopedics and dentistry. Platelet rich plasma therapy utilizes growth factors present in alpha granules of platelets in an autologous manner. Main indications in dermatology for PRP are androgenetic alopecia, wound healing, face rejuvenation etc. For preparation of PRP, various protocols are used and no standard protocol exists but main principles essentially involve concentrating platlets in a concentration of 3–5 times the physiological value and then injecting this concentrated plasma in the tissue where healing or effect is desired. As of 2016, no large-scale randomized controlled trials have confirmed the efficacy of PRP as a treatment for musculoskeletal or nerve injuries, the accelerated healing of bone grafts, or the reduction of androgenic hair loss.
PRP was first developed in the 1970s and first used in Italy in 1987 in an open heart surgery procedure. PRP therapy began gaining popularity[where?] in the mid 1990s. It has since been applied to many different medical fields such as cosmetic surgery, dentistry, sports medicine and pain management.
The number of peer reviewed publications studying the PRP's efficacy has increased dramatically since 2007.
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 the mentioned factors from alpha granules. The growth factors and other cytokines present in PRP include:
- platelet-derived growth factor
- transforming growth factor beta
- fibroblast growth factor
- insulin-like growth factor 1
- insulin-like growth factor 2
- vascular endothelial growth factor
- epidermal growth factor
- Interleukin 8
- keratinocyte growth factor
- connective tissue growth factor
As of 2009[update] there have been two PRP preparation methods approved by the U.S. Food and Drug Administration. Both processes involve the collection of the patient's whole blood (that is anticoagulated with citrate dextrose) before undergoing two stages of centrifugation (TruPRP) (Harvest) (Pure PRP) designed to separate the PRP aliquot from platelet-poor plasma and red blood cells. In humans, the typical baseline blood platelet count is approximately 200,000 per µL; therapeutic PRP concentrates the platelets by roughly five-fold. There is broad variability in the production of PRP by various concentrating equipment and techniques.
In humans, PRP has been investigated and used as a clinical tool for several types of medical treatments, including nerve injury, chronic tendinitis, plantar fasciitis, osteoarthritis, cardiac muscle injury, and androgenic alopecia, for bone repair and regeneration, in plastic surgery, colorectal surgery and oral surgery
As of 2016[update] results of basic science and preclinical trials have not yet been confirmed in large-scale randomized controlled trials. A 2009 systematic review of the scientific literature found there were few randomized controlled trials that adequately evaluated the safety and efficacy of PRP treatments and concluded that PRP was "a promising, but not proven, treatment option for joint, tendon, ligament, and muscle injuries".
In 2009 a pilot study investigating the effects of PRP on 20 male athletes with a mean injury history of 20.7 months of pain due to chronic patellar tendinosis, also known as jumpers knee, found statistically significant improvements in physical function and pain levels. 6 month follow up's after treatment showed participants returned to 90% of pre-injury sports activity levels, and 80% of participants were able to return to sports within 4 months of treatment. 
As of 2011, PRP use for nerve injury and sports medicine has produced "promising" but "inconsistent" results in early trials.
A 2013 review stated more evidence was needed to determine PRP's effectiveness for hair regrowth.
A 2014 Cochrane analysis for PRT use to treat musculoskeletal injuries found very weak (very low quality) evidence for a decrease in pain in the short term, up to three months and no difference in function in the short, medium or long term. There was weak evidence that suggested that harm occurred at comparable, low rates in treated and untreated people.
A 2016 systematic review and meta-analysis of randomized controlled clinical trials for PRP use to augment bone graft found only one study reporting a significant difference in bone augmentation, while four studies found no significant difference.
Since 2004, proponents of PRP therapy have argued 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 could bias results, because of inter-individual variability in the platelet concentration of human plasma and more would not necessarily be better. The variability in platelet concentrating techniques may alter platelet degranulation characteristics that could affect clinical outcomes.
Use in horses
Implications for doping
Some concern exists as to whether PRP treatments violate anti-doping rules. As of 2010 it was not clear if local injections of PRP could have a systemic impact on circulating cytokine levels, affecting doping tests and 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".
According to the Baltimore Sun, Zach Britton had PRP injections in his left shoulder in March 2012, Orioles first baseman Chris Davis underwent two PRP injections to speed the healing and recovery of an oblique injury in April 2014, and Dylan Bundy had the procedure in April before undergoing Tommy John surgery in June 2014.
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