What is exactly Platelet-Rich Plasma (PRP)?

Platelet-Rich Plasma (PRP) is a biomaterial derived from our own blood that is rich in platelets and in other bioactive molecules. It was first mentioned in medical literature in the 1950s and its therapeutic potential was discovered in the 1960s.

Platelets are tiny cells found in blood which are involved in clotting. A normal platelet count is generally between 150,000-450,000 / μL of blood and their lifespan is just 7 to 10 days on average; they make up around 0.15% of total blood by volume and also contain several granules that hold a wide range of growth factors and bioactive molecules within them: when platelets are activated, they release such granules and their content in the surrounding tissues. Platelets not only stop the bleeding when we are injured, but also control the healing process by releasing the content of their granules, like the growth factors, in the injured tissues.

platelet rich plasma (PRP)

What are the growth factors in platelet-rich plasma (PRP)?

Growth factors are proteins that stimulate cell growth, cell differentiation, cell migration to the injury site, and angiogenesis (the formation of new blood vessels); some of the cells targeted by the growth factors are the:

  1. Endothelial cells, which will form new blood vessels bringing more oxygen and nutrients to the healing tissues
  2. Fibroblasts, which produce the extracellular matrix in which cells sit in and that produce proteins like collagen, elastin and hyaluronic acid.
  3. Osteoblasts, which form bone tissue
  4. Chondroblasts, which form cartilage
  5. Mesenchymal stem cells (MSC), which are multipotent undifferentiated cells that can develop into a wide range of specialized cells including those that constitute bone, cartilage, muscle and fat tissue.

Some of the growth factors regulate the lifespan of the connective tissue proteins like collagen. Overall, their synergic work is fundamental and drives the wound healing and tissue repair processes.

The following are some of the growth factors released by the platelets:

  • Platelet-derived growth factor (PDGF-A and PDGF-B)
  • Transforming growth factor beta (TGF-beta)
  • Vascular endothelial growth factor (VEGF)
  • Epidermal growth factor (EGF)
  • Fibroblast growth factor (FGF)
  • Connective tissue growth factor (CTGF)
  • Insulin-like growth factor (IGF-1 and IGF-2)
  • Endothelial cell growth factor (ECGF)
  • Tumor necrosis factor (TNF)

What other bioactive molecules are found in PRP?

The platelet granules and the plasma fraction of blood also add more bioactive molecules other than the growth factors which are injected in the treatment area. Bioactive means that the compound exerts a documented effect on cells, tissues or living organisms. Among the many substances found in the PRP cocktail there are:

  • Signaling molecules capable of downregulating the inflammatory response of white blood cells.
  • Antimicrobial compounds having effect against some strains of bacteria and fungi.
  • ADP, ATP, Vitamins and electrolytes like chloride, sodium, potassium, calcium and magnesium.
  • Hormones such as ACTH, thyroxine, HGH, estrogens and androgens.
  • Clotting factors
  • Bioactive amines like serotonin and histamine

 There are hundreds, if not thousands, of different proteins: so many that we currently don’t even know all of them nor we know exactly how they interact between each other and with our tissues, adding to the complexity of PRP and its understanding.

PRP preparation methods

The preparation protocol for PRP varies depending on the tools used by the doctor and on the final product that they are trying to obtain. Dozens of different preparation kits from manufacturers are available producing a wide range of results after the processing of the blood sample; while these kits make the PRP preparation easier and with consistent results, they also increase costs for the physician and for the patient. To keep it simple and not specific to any kit manufacturer, I’ll describe PRP preparation without going into too many details.

First, blood is drawn from the patient. This is a quick and almost painless procedure, like when blood is collected for a routine blood analysis through venipuncture. The volume of blood collected can vary depending on many factors, including the treatment sought by the patient, but it’s generally very low and won’t cause any side effect, complication or downtime.

The blood is then transferred into test tubes with an anticoagulant to prevent clotting. The tubes are placed inside a centrifuge and are spun at a certain speed for a set amount of time, all factors that vary depending on the protocol followed. Having different weight, all blood components separate and stratify into 3 main layers: the red bottom layer consists of mainly red blood cells and is discarded, the “buffy coat” in the middle mainly contains white blood cells and platelets, and the yellow top layer is the plasma. The middle and top layer are drawn from the tube using a syringe and can then be injected into the tissues. Sometimes, after discarding the red blood cells layer the sample goes through a second spin for further refining and concentration; other protocols employ an activation method to boost the concentration of growth factors in the sample.

By KnuteKnudsen at English Wikipedia, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=3986752

PRP is effective when the concentration of platelets in the syringe is at least 1.5 times that of platelets in the original sample (blood). When the concentration is increased between 1.5 and 8 times, the effect on the tissues seems to be the same, while the more concentrated it gets the harder it is to prepare PRP. When the concentration is 8 times that of blood or above, then the effect of PRP seems to change and produce a more intense stimulation of the stem cells in tissues which in turn increases the level of regeneration and the effectiveness of the treatment.

Different forms of PRP

Depending on the preparation method and on the portion of blood extracted after the centrifugation process, the current consensus divides platelet concentrates products into 4 main categories:

  1. Leukocyte Rich PRP (L-PRP)
  2. Leukocyte-poor PRP (LP-PRP or P-PRP)
  3. Leukocyte-rich PRF (L-PRF)
  4. Leukocyte-poor PRF (LP-PRF or P-PRF)

Many more categories have been proposed, but I won’t mention them in this article for simplicity.

What is PRP Activation?

PRP activation is a technique that stimulates the release of growth factors from the platelet granules through one of many different methods. This happens naturally when we are injured and platelets in blood encounter a variety of naturally occurring molecules in tissues like collagen.

The most common method to pre-release the growth factors is activation with the addition of 10% calcium chloride (CaCl2) to the PRP after centrifugation, but many different methods exist including freeze-thaw cycles and sonication, which is the use of ultrasounds to disrupt the membrane of the platelets.

Platelet-Rich Plasma (PRP) vs Platelet-Rich Fibrin (PRF)

Platelet-Rich Fibrin is a second generation PRP developed in 2001. it is important to note that being a newer version or PRP does not make it necessarily better: the use of PRP or PRF has advantages and disadvantages in both cases and while some of the differences in their mechanism of action are known, it is still unknown if one is superior to the other.

The main difference and advantage of PRF is that it is not only easier to prepare, but also cheaper and faster when compared to PRP.

A second difference is that unlike PRP, platelet rich fibrin is prepared by spinning whole blood without adding any anticoagulant to the vials: this way, a gel-like substance forms which is known as fibrin matrix. Physiologically, when we are injured, this is what happens in tissues and the fibrin matrix serves as a scaffolding for cells like fibroblasts and for mesenchymal stem cells that gets recruited to the injury site.

Another considerable difference is the timeframe in which growth factors are released in the treatment area: with PRP, growth factors are released immediately with high level release at 15 minutes which is sustained over the first 8 hours following the injection and lower levels from 8 hours to 10 days; with PRF, growth factors release is slower, with low levels in the first 8 hours and higher levels over the next 10 days. Moreover, PRF seems to release more growth factors in total over the 10-days period when compared to PRP. Whether this finding makes one better than the other is not yet clear but may suggest different applications for the two platelets concentrate preparations.

Finally, there seems to be a difference also in the concentration of different growth factors from the same blood sample when preparing PRP or PRF.

Factors influencing the efficacy of Platelet-Rich Plasma (PRP)

Plenty of factors influence the efficacy of PRP treatment. Platelets are found in blood at a normal concentration between 150,000 to 450,000 platelets per μL; this notion alone already shows that some subjects physiologically may have 3 times more platelets than others. Other factors that vary from patient to patient are:

  • Age. The older we get, the less potent PRP is due to lower growth factors concentrations.
  • Gender. The female population appears to have higher growth factor concentrations than the male one.
  • Diet.
  • Stress levels and sleep patterns.
  • Smoking, alcohol and drugs use.
  • Use of medications.
  • Medical history and underlying conditions.

The absence of a standardized preparation protocol means that different kits, different spin times, different spin speed (gravitational force or Gs), different anticoagulants, different activation methods and different fraction of blood and PRP collected will affect the final product.

Different treatment protocol, such as number of treatments, depth of injection, amount injected and interval of time between sessions also influence the final results.

Side effects and complications of PRP therapy

Since PRP is a product of our own organism, the risk of an allergic reaction is almost non-existent with only a couple of case reports in the medical literature. Platelet-Rich Plasma is in general considered very safe and most of the side effects are caused by the injection of PRP in tissues to provide the treatment like bleeding, pain, swelling, tissue damage, infection and nerve injuries.

Soreness and bruising at the injection site are the most common complications that may occur and resolve within a few days without needing any treatment, while the more serious complications are very rare.

Patient preparation before PRP Treatment

To optimize the treatment efficacy, the patient should follow a healthy lifestyle which includes diet, exercise, avoiding drugs, tobacco and alcohol, reducing stress and having proper sleep patterns. Moreover, several authors suggest avoiding NSAIDs (non-steroidal anti-inflammatory drugs) 1-week before and a few days after the treatment as one of their effects is to inhibit platelet function which could decrease treatment efficacy. For more information on what you should or shouldn’t do before and after the treatment always refer to your treatment provider.

Current applications of Platelet-Rich Plasma (PRP) in regenerative medicine

PRP is a very promising treatment in several different fields of medicine. Among the many current applications of platelet-rich plasma there are:

PRP is employed also in the trauma setting for wound healing of soft tissues and hard tissues, to improve post-surgical healing and to treat chronic wounds like ulcers.

PRP is a very complex cocktail of molecules and it will take more time and research to fully understand its mechanism of action, best preparation methods and applications in the medical field.

Besides the many challenges and the absence of a consensus on treatment protocols it seems to show efficacy in a wide variety of fields.



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