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Platelet-rich plasma (PRP) is an autologous biological preparation produced by centrifuging whole blood in the presence of an anticoagulant to concentrate platelets – and, depending on protocol, leukocytes – within the plasma fraction. It is not a single standardised substance but a spectrum of preparations, unified by platelet concentration above baseline and the growth factor payload delivered when those platelets are activated and degranulate. The primary regenerative mechanism is the release of PDGF, TGF-β1, VEGF, EGF, IGF-1, and FGF from platelet alpha granules, which bind receptors on fibroblasts, keratinocytes, and endothelial cells to drive proliferation, collagen synthesis, and angiogenesis. Preparation protocol determines the final biological profile of any given PRP batch: centrifugation speed and duration, anticoagulant choice, leukocyte inclusion or exclusion, and activation method all produce measurably different preparations with different clinical behaviour. Understanding PRP as a variable biological substance rather than a pharmaceutical product is essential to interpreting the heterogeneity of clinical outcomes in the literature and in practice.

Platelet-rich plasma is the plasma fraction of autologous whole blood with platelet concentration raised above systemic baseline – typically to 3–5× baseline levels, with some high-speed protocols achieving 8–10× concentration. Platelets are anucleate cell fragments derived from megakaryocytes in bone marrow, whose primary physiological role is haemostasis: detecting vascular injury, aggregating at wound sites, and initiating the clotting and healing cascades. Their value in regenerative medicine derives from this same function – they carry a dense payload of bioactive proteins that instruct the tissue repair process, packaged in intracellular granules and released on activation.

PRP exploits this biology by concentrating the platelet population and delivering it to a specific tissue site, producing a higher local density of growth factor signal than would occur through normal haematological distribution.

Platelet Biology: Alpha Granules and Dense Granules

Platelets contain two primary granule types relevant to their regenerative function:

Alpha granules are the dominant growth factor store, containing over 300 proteins including the principal regenerative signals: PDGF (all three isoforms: AA, AB, BB), TGF-β1 and TGF-β2, VEGF, EGF, IGF-1, FGF, HGF (hepatocyte growth factor), and CTGF (connective tissue growth factor). These are released on platelet activation by exocytosis and act on target cell surface receptors to initiate downstream signalling cascades governing proliferation, migration, matrix synthesis, and angiogenesis.

Dense granules contain ADP, ATP, serotonin, histamine, and calcium ions – primarily involved in propagating the platelet activation cascade itself rather than tissue repair signalling. Calcium release from dense granules amplifies the local activation signal, recruiting additional platelets to the site and reinforcing the growth factor release event.

On activation – whether by calcium chloride, thrombin, collagen contact, or mechanical disruption – both granule types undergo exocytosis simultaneously. The therapeutic window for PRP’s growth factor availability is consequently narrow: degranulation is largely complete within 15 minutes, and most released growth factors clear from the local tissue environment within hours.

Growth Factor Reference

The Concentration Paradox: More Is Not Strictly More

The relationship between platelet concentration and growth factor efficacy is not linear. At very high concentrations – above approximately 1.5 million platelets per microlitre – receptor saturation effects emerge: target cell surface receptors for PDGF, TGF-β, and EGF become fully occupied, meaning additional growth factor molecules in the local environment have no additional receptor to bind and produce no incremental signal. Some high-concentration PRP studies have demonstrated paradoxically reduced fibroblast proliferation at extreme platelet concentrations compared to moderate concentrations, likely through this mechanism combined with inhibitory feedback from TGF-β at supraphysiological levels.

The practical implication is that optimising PRP preparation means calibrating concentration to an effective range rather than simply maximising it – a nuance obscured by marketing language that treats higher spin speed and higher platelet count as unconditionally superior.

PRP Classification: Why There Is No Universal PRP

The Dohan Ehrenfest classification system, now standard in the platelet concentrate literature, defines four classes of autologous platelet preparations across two axes: leukocyte content (low or high) and fibrin architecture (absent or present):

These four classes behave differently in tissue. P-PRP minimises inflammatory response and is preferred where post-treatment redness must be minimised. L-PRP retains the macrophage and neutrophil fraction for its additional cytokine and growth factor contribution, at the cost of a more pronounced inflammatory response. The PRF classes introduce the fibrin scaffold that converts acute release to sustained release – covered in detail in the PRF entity.

Beyond this classification, preparation variables further differentiate individual protocols: anticoagulant type (sodium citrate vs citrate dextrose vs ACD-A), tube design and separator gel, centrifugation geometry (fixed angle vs horizontal swing-out rotor), and activation method all produce measurably different final compositions. This is why PRP trials that do not specify full preparation methodology are difficult to interpret: the “PRP” in one study may be compositionally distinct from the “PRP” in another.

Activation Methods

PRP must be activated – platelets must degranulate – for growth factors to be released. The activation route affects both the kinetics and completeness of degranulation:

• Exogenous thrombin + calcium chloride: Rapid, complete activation; immediate growth factor release; historically bovine-derived thrombin raised immunogenic concerns, now largely replaced by autologous thrombin or recombinant alternatives
• Calcium chloride alone: Slower activation via the endogenous coagulation cascade; less immediate but more physiologically graded release
• Collagen contact / tissue activation: Natural activation on injection into collagenous tissue; no additive required; release kinetics determined by tissue collagen exposure; the default for many modern injectable PRP protocols
• Mechanical disruption: Physical agitation sufficient to activate platelets; least precise; sometimes used for topical PRP in microneedling applications

The activation-free approach – relying on tissue contact – is preferred for injectable applications at Creative Touch where iPRF has replaced liquid PRP, as it eliminates the additive requirement and respects the autologous principle.

Leukocyte Inclusion: The Ongoing Debate

Whether to include or exclude leukocytes in PRP preparations remains the most contested variable in the field. The argument for inclusion rests on leukocyte-derived cytokines and growth factors – particularly macrophage-derived TGF-β and VEGF – that amplify the platelet signal and provide immunomodulatory benefit. The argument for exclusion rests on evidence that neutrophil-derived reactive oxygen species and matrix metalloproteinases (MMPs) can degrade growth factors and extracellular matrix at the injection site, and that higher leukocyte concentrations correlate with more pronounced post-injection inflammation in some tissue environments.

The resolution is largely context-dependent. For skin rejuvenation and wound healing, L-PRP and leukocyte-containing preparations appear to perform at least as well as leucocyte-poor preparations, and the macrophage contribution to TGF-β signalling appears beneficial in dermal remodelling contexts. For intra-articular injection in joint applications, the evidence more clearly favours leukocyte-poor preparations due to the documented pro-inflammatory effects of neutrophils in synovial tissue.

Biological Variability: PRP Is Only As Good As the Blood It Came From

Because PRP is concentrated from the patient’s own blood, the ceiling on its growth factor payload is set by individual biology. Variables that affect PRP potency include:

• Baseline platelet count and function – thrombocytopenia or qualitative platelet disorders directly reduce growth factor yield; patients on antiplatelet medications (aspirin, clopidogrel) have functionally impaired platelets
• Age – platelet count generally remains adequate with age, but alpha granule content and growth factor concentration do decline modestly; older patients’ PRP retains regenerative capacity but may be quantitatively lower in some growth factors
• Nutritional status – iron, folate, and B12 deficiency affect overall haematopoiesis; severe anaemia reduces plasma volume available for concentration
• Hydration status – dehydration increases blood viscosity and can affect centrifugation behaviour and yield
• Concurrent illness or systemic inflammation – active infection, autoimmune flares, or high circulating inflammatory mediators alter the cytokine environment within which PRP growth factors act

This variability is clinically meaningful. Framing PRP as a pharmaceutical-equivalent product with predictable dosing is inaccurate – it is a biological preparation whose potency varies between patients and, to a lesser extent, between sessions in the same patient.

Clinical Pearl The most common reason for underwhelming PRP outcomes is not the wrong protocol – it is treating the preparation as a pharmaceutical rather than a biological. Patients advised to avoid anti-inflammatory medications (NSAIDs, corticosteroids) for 5–7 days before treatment, to arrive well-hydrated, and to avoid alcohol in the 24 hours prior will consistently produce better-quality PRP than patients who have not been briefed. These variables directly affect platelet function and yield. This pre-treatment optimisation conversation is brief, costs nothing, and has a measurable effect on the preparation quality that follows.