The Viability Window: How Long Does PRP Remain Therapeutically Active After Preparation?

Platelet-rich plasma is drawn from the patient, centrifuged, and prepared — but from the moment the blood enters the collection tube, a biological clock begins. Platelets are metabolically active cells: they consume glucose, respond to mechanical and chemical stimuli, and release their growth factor payload upon activation. The anticoagulant chosen to preserve them, the temperature at which they are stored, and the time elapsed between preparation and injection all meaningfully determine what arrives at the treatment site — and whether what arrives is biologically capable of producing the regenerative effects that justify the procedure. This article examines the evidence on each of these variables, with particular attention to the differential stability of individual growth factors.

Part 1: What Makes PRP Therapeutically Active

The Platelet and Its Payload

Platelets are anucleate cytoplasmic fragments shed from megakaryocytes in the bone marrow, circulating in blood at a concentration of approximately 150,000–400,000 per microlitre. In their resting state, platelets circulate as smooth biconvex discs, maintained in quiescence by endothelial prostacyclin and nitric oxide. Upon vascular injury — or upon exposure to thrombin, collagen, ADP, or the mechanical forces of centrifugation and injection — they undergo shape change, aggregate, and degranulate: releasing the contents of their intracellular storage organelles into the surrounding microenvironment.¹

It is this releasate that constitutes the biological basis of PRP therapy. The therapeutic premise is that concentrating platelets at a wound or treatment site — typically to three to five times the baseline blood concentration — amplifies the local growth factor signal and drives tissue repair, neovascularisation, and cellular proliferation. The relevant content is primarily stored in three types of platelet granules:²

• Alpha (α) granules: The primary secretory organelle, containing the majority of PRP's growth factors — platelet-derived growth factor (PDGF), transforming growth factor-β (TGF-β), vascular endothelial growth factor (VEGF), epidermal growth factor (EGF), fibroblast growth factor (FGF), and insulin-like growth factor-1 (IGF-1). Each platelet contains approximately 50–80 alpha granules.
• Dense (δ) granules: Store ADP, ATP, serotonin, and calcium — mediators of secondary platelet activation rather than growth factor delivery.
• Lysosomes: Contain hydrolytic enzymes; contribute to the degradation of extracellular matrix, relevant to wound remodelling but not the primary therapeutic payload.

Additionally, the plasma fraction of PRP carries circulating growth factors — particularly IGF-1 (which circulates in plasma bound to insulin-like growth factor-binding proteins, or IGFBPs), hepatocyte growth factor (HGF), and stromal cell-derived factor-1α (SDF-1α). These are present regardless of platelet concentration and are included in the PRP preparation by default.³

Why Timing Matters

Unlike pharmaceutical agents with defined shelf lives, PRP is a living biological preparation. Platelets in collected blood begin responding to their new environment immediately — the collection tube, the mechanical force of centrifugation, the temperature of storage, and the pH of the anticoagulant are all stimuli to which platelets are sensitive. Premature activation before injection results in growth factors being released into the collection tube rather than at the treatment site, arriving depleted of their payload. Delayed use after preparation exposes released growth factors to proteolytic degradation and thermal instability. The clinical goal is to deliver activated PRP at the right time: after collection and preparation, but before significant degradation has occurred.⁴

Part 2: Anticoagulant Selection

Why Anticoagulants Matter Beyond Preventing Clotting

The choice of anticoagulant is one of the most consequential and frequently underappreciated decisions in PRP preparation. Different anticoagulants work through different mechanisms, and those mechanisms have direct consequences for platelet viability, activation state, and the growth factor content of the final preparation. The ideal anticoagulant for PRP must prevent premature clotting without activating platelets, without altering platelet morphology, and while maintaining the metabolic conditions that allow platelets to remain viable until injection.⁵

Acid Citrate Dextrose — Formula A (ACD-A)

ACD-A is widely regarded as the optimal anticoagulant for platelet-rich plasma preparation and is the preferred choice in research-grade PRP protocols. It contains three components: trisodium citrate (2.2 g per 100 mL), citric acid (0.8 g per 100 mL), and dextrose (2.45 g per 100 mL), typically added to blood at a 1:6 ratio (one part ACD-A to six parts blood). Its mechanism combines calcium chelation by the citrate–citric acid buffer with a mild acidic pH (approximately 4.5–5.0), which together inhibit the calcium-dependent coagulation cascade and reduce platelet activation. Critically, the dextrose component serves as a metabolic substrate, providing glucose that sustains platelet ATP production during collection and storage — prolonging platelet viability in a way that citrate-only anticoagulants cannot.⁶

Multiple in vitro studies have confirmed that ACD-A produces PRP with higher platelet viability, better preservation of alpha granule morphology, and superior growth factor content compared to other anticoagulants. A study by Mazzucco et al. comparing ACD-A, sodium citrate, and EDTA directly demonstrated that ACD-A produced significantly higher concentrations of TGF-β1 and PDGF-AB in the final PRP preparation, consistent with better preservation of alpha granule integrity prior to use.⁷ The acidic pH of ACD-A, while initially counterintuitive given that acidic conditions can denature proteins, actually reduces metabolic platelet activity during storage — a form of controlled metabolic suppression that reduces premature alpha granule release.

Acid Citrate Dextrose — Formula B (ACD-B)

ACD-B contains the same three components as ACD-A but at lower concentrations: trisodium citrate (1.32 g per 100 mL), citric acid (0.48 g per 100 mL), and dextrose (1.47 g per 100 mL). It is used at a higher blood-to-anticoagulant ratio (typically 1:4.5). The practical difference between ACD-A and ACD-B in the context of PRP preparation is modest — both preserve platelet function substantially better than heparin or EDTA — though ACD-A's higher dextrose content and lower pH are generally considered to provide marginally better metabolic support and platelet quiescence during extended storage.⁶ Either formula is clinically appropriate for PRP; the choice is often dictated by kit availability and preparation system design.

Sodium Citrate

Sodium citrate (typically 3.2% or 3.8% trisodium citrate) is the most widely used anticoagulant in standard haematology collection tubes and represents the default anticoagulant in many commercial PRP kits. It works by chelating free calcium ions (Ca²⁺) in the blood, rendering them unavailable to the coagulation cascade and to the calcium-dependent platelet activation pathways. This is effective at preventing clotting and does not directly activate platelets. However, unlike ACD-A, sodium citrate provides no glucose substrate for platelet metabolism, meaning platelets in sodium citrate preparations are dependent entirely on their intrinsic glycogen stores for energy — stores that are finite and exhausted within hours at room temperature.⁵

Sodium citrate is a clinically acceptable anticoagulant for PRP when the preparation will be used within two to four hours, and most commercial single-use PRP systems employ it for this reason. For protocols requiring extended processing times, or where maximal growth factor preservation is a priority, ACD-A is preferable.

Heparin — Not Recommended

Heparin is the most commonly used anticoagulant in clinical medicine generally, but it is poorly suited to PRP preparation. Unlike citrate-based anticoagulants that work by calcium chelation, heparin acts by binding and potentiating antithrombin III, which then irreversibly inhibits thrombin and Factor Xa. The problem for PRP is twofold. First, heparin has been shown to directly activate platelets at concentrations used in clinical collection tubes — a phenomenon well-documented in the heparin-induced thrombocytopenia literature — causing premature degranulation and growth factor release before the preparation is used.⁸ Second, heparin binds to basic fibroblast growth factor (bFGF/FGF-2) — a heparin-binding growth factor — altering its bioavailability and potentially sequestering it within the preparation. Multiple studies have confirmed lower growth factor concentrations in heparin-prepared PRP compared to citrate-based preparations.⁹ Heparin-containing PRP systems should be avoided in clinical practice where growth factor delivery is the therapeutic goal.

EDTA — Contraindicated for PRP

EDTA (ethylenediaminetetraacetic acid) is the anticoagulant used in standard full blood count tubes (the purple-top tube familiar to every clinician). It is an extremely effective calcium chelator — more potent than citrate — and is valued in haematology because it preserves cellular morphology for microscopic examination. This very potency makes it unsuitable for PRP. EDTA chelates not only calcium but also magnesium and zinc, which are required cofactors for numerous platelet membrane proteins and intracellular signalling pathways. In EDTA-anticoagulated blood, platelets undergo rapid morphological change — from smooth discs to activated, pseudopod-bearing forms — within minutes of collection. Studies consistently show that EDTA-prepared PRP contains significantly lower concentrations of PDGF, TGF-β, and VEGF compared to citrate-based preparations, reflecting premature alpha granule release and degradation.¹⁰ EDTA is contraindicated for PRP preparation and should never be used for this purpose, despite the fact that EDTA tubes are ubiquitous in clinical settings.

Part 3: Temperature and Storage Duration

Room Temperature (20–24°C)

Blood bank experience with platelet concentrates — accumulated over decades of transfusion medicine research — provides the most rigorously controlled data on platelet storage under standardised conditions. At 20–24°C with continuous gentle agitation in a dedicated platelet incubator, single-donor platelet units maintain acceptable function for up to five to seven days. However, this is with sterile collection, pH-controlled preservation solutions, gas-permeable bags, and continuous gentle agitation to maintain platelet viability — conditions that bear little resemblance to clinical PRP held in a syringe at room temperature.¹¹

For clinical PRP without these measures, room temperature storage results in metabolic consumption of glucose, progressive acidification of the preparation as lactate accumulates, and — critically — irreversible platelet activation beginning within two to four hours. Once activated, platelets release their alpha granule contents into the surrounding plasma. If the preparation is still in the syringe, those released growth factors are exposed to proteolytic enzymes (including platelet-derived serine proteases), temperature-dependent denaturation, and pH-related instability. The consensus in the PRP literature is that preparations held at room temperature should be injected within four to six hours of centrifugation, with earlier use being preferable.^4,12

Refrigeration (4°C)

Cooling PRP to 4°C dramatically slows platelet metabolism, reduces the rate of spontaneous activation, and extends the viability window for the growth factor content — but not without trade-offs. The landmark study by Murphy and Gardner (1969) established that refrigerated platelets undergo a characteristic shape change from smooth biconvex discs to irregular sphere-like forms with extended pseudopods, a morphological alteration associated with cytoskeletal reorganisation and reduced responsiveness to physiological aggregating stimuli.¹³ Cold-stored platelets have reduced ADP-induced aggregation and altered surface receptor expression compared to room temperature-stored counterparts.

However, more recent work by Hoffmeister et al. demonstrated that cold-stored platelets, despite their altered aggregation response, retain haemostatic efficacy in vivo and may in some contexts be more rapidly cleared from the circulation.¹⁴ For PRP therapy — where the therapeutic goal is growth factor delivery rather than haemostasis — the relevant question is whether growth factor content is preserved under refrigeration, and the answer is generally yes: the proteolytic degradation of released growth factors proceeds significantly more slowly at 4°C than at room temperature. Refrigerated PRP has been shown to retain measurable growth factor activity for 24 to 48 hours, though the evidence supports that earlier use produces superior results.¹⁵

Freezing and Freeze-Thaw Protocols

Freezing PRP at -20°C or -80°C destroys platelet membrane integrity — ice crystal formation during freezing ruptures the platelet plasma membrane, eliminating the cell as a functioning unit. Upon thawing, what remains is a preparation containing released growth factors in plasma but no viable platelets. This "PRP lysate" approach is used in some research and stem cell culture contexts but is not the standard clinical approach for injectable PRP therapy, where intact platelets capable of slow, sustained growth factor release at the treatment site are generally considered superior to a single bolus of released factors.¹⁶

When freezing is unavoidable — or when PRP is being prepared for use in a staged protocol — freezing at -80°C with rapid freeze rates preserves growth factor content significantly better than -20°C storage. A study by Roffi et al. examining the effects of freeze-thaw cycles on PRP growth factor content found that a single freeze-thaw cycle at -80°C resulted in a 20–35% reduction in VEGF concentrations and a 10–20% reduction in TGF-β1, while PDGF-AB was relatively resilient with less than 15% loss after a single cycle.¹⁷ Multiple freeze-thaw cycles produce compounding losses and should be avoided.

Part 4: Differential Stability of Growth Factors

Why Individual Growth Factors Behave Differently

A PRP preparation is not a single entity — it is a complex mixture of proteins, each with distinct structural characteristics, binding partners, storage mechanisms, and sensitivities to temperature, pH, and proteolysis. The clinical implication is that the therapeutic potency of PRP does not decay at a single uniform rate: some factors degrade within hours, others remain measurable for days. Understanding this hierarchy has practical consequences for how quickly PRP should be used and what is lost when storage times are extended.^2,12

PDGF (Platelet-Derived Growth Factor)

PDGF exists as homodimers (AA, BB) and heterodimers (AB), all stored at high concentration in platelet alpha granules. It is the archetypal platelet growth factor and one of the most abundant in any PRP preparation. PDGF is relatively robust: its disulfide-bonded dimeric structure provides resistance to thermal denaturation, and once released, it remains biologically active for several hours at room temperature. Refrigeration at 4°C extends this meaningfully, with studies showing >80% receptor-binding activity retained after 24 hours at 4°C.³ PDGF-BB is the most stable isoform; PDGF-AA slightly less so. For practical purposes, PDGF degradation is not the primary limiting factor in the clinical viability window of PRP held at room temperature within a four-hour window.

TGF-β1 and TGF-β2

Transforming growth factor-β is the most abundant growth factor in platelet alpha granules by concentration. It is stored and secreted primarily in a latent form — bound to a latency-associated peptide (LAP) and often further complexed with a large latent TGF-β binding protein (LTBP). This latent complex provides significant structural protection against proteolytic degradation and thermal denaturation. As a result, TGF-β1 is among the most stable growth factors in PRP preparations — retaining bioavailable concentrations for 24 to 48 hours even at room temperature in several studies, and showing good stability through a single freeze-thaw cycle.¹⁸ Activation of the latent TGF-β complex (required for biological activity) occurs in vivo via mechanisms including proteolysis, mechanical force, and reactive oxygen species — all present in the wound microenvironment at the injection site.

VEGF (Vascular Endothelial Growth Factor)

VEGF is among the most labile of the clinically significant PRP growth factors — and this distinction has direct bearing on how quickly PRP should be used. VEGF is stored in platelet alpha granules and released upon activation. Its 165-amino-acid isoform (VEGF-165, the dominant form in platelets) is sensitive to pH shifts, oxidative stress, and proteolytic degradation by matrix metalloproteinases and serine proteases — enzymes present in the platelet releasate itself. Studies using ELISA quantification have shown VEGF concentrations in PRP declining by 20–40% within four hours at room temperature, and by up to 60% within eight hours.¹⁹ Refrigeration at 4°C substantially slows this degradation. The practical implication: for protocols where neovascularisation and angiogenic stimulation (i.e. VEGF-mediated effects) are the therapeutic priority, prompt use of freshly prepared PRP within two to four hours is more important than for fibrous tissue repair applications where TGF-β and PDGF predominate.

IGF-1 (Insulin-Like Growth Factor-1)

IGF-1 in PRP derives from both platelet alpha granules and the plasma component of the preparation. In plasma, the vast majority of IGF-1 (>99%) circulates bound to one of six insulin-like growth factor-binding proteins (IGFBP-1 through IGFBP-6), with IGFBP-3 and its acid-labile subunit forming the predominant ternary complex. This binding dramatically extends the half-life of IGF-1 compared to the free form: unbound IGF-1 has a plasma half-life of minutes, while IGFBP-3-bound IGF-1 has a half-life of 12–15 hours.²⁰ In a PRP preparation, the IGFBP-bound fraction represents the majority of the IGF-1 pool and provides significant buffering against rapid degradation. Studies by Amable et al. showed IGF-1 concentrations in PRP preparations remaining relatively stable compared to VEGF and EGF over comparable storage intervals at 4°C.³ However, the biological interpretation is complex, because IGFBP binding both stabilises and inhibits IGF-1 receptor binding — the net bioavailability depends on local protease activity that cleaves the binding protein, liberating free IGF-1 at the treatment site.

EGF (Epidermal Growth Factor)

EGF is stored in platelet alpha granules and released upon activation. It is a small, heat-stable protein under physiological conditions, but is susceptible to proteolytic degradation by metalloproteinases and serine proteases present in the PRP releasate. Studies quantifying EGF in PRP over time show a pattern intermediate between the relative stability of TGF-β and the lability of VEGF: approximately 15–30% loss within four hours at room temperature, with refrigeration providing meaningful preservation for up to 24 hours.¹² EGF is particularly relevant to skin rejuvenation applications given its role in keratinocyte migration and epidermal proliferation.

bFGF (Basic Fibroblast Growth Factor / FGF-2)

Basic FGF is a heparin-binding growth factor — its interaction with cell surface heparan sulphate proteoglycans (HSPGs) is required for receptor binding and signal transduction. This heparin-binding property has two important implications for PRP stability. First, it means that any heparin present in the preparation (as an anticoagulant or a contaminant) will directly compete with cellular HSPGs for bFGF binding, reducing its bioavailability at the treatment site — providing one more reason to avoid heparin-based anticoagulants. Second, bFGF is particularly susceptible to proteolytic degradation by thrombin and plasmin, both of which may be generated in activated PRP preparations. Studies show measurable bFGF decline within two to four hours at room temperature, making it among the more time-sensitive of the major PRP growth factors.⁹

HGF (Hepatocyte Growth Factor)

HGF circulates primarily in the plasma phase of blood (produced mainly by hepatocytes and stromal cells), with a smaller contribution from platelet granules. It is a multifunctional growth factor with roles in angiogenesis, epithelial repair, and anti-inflammatory signalling — increasingly recognised as relevant to hair follicle regeneration and skin healing. HGF is less stable than PDGF or TGF-β, showing sensitivity to both temperature and proteolytic degradation. Its plasma half-life in vivo is short (approximately 3–5 minutes due to rapid hepatic clearance), though concentrations in PRP in vitro are maintained for longer in the absence of clearance mechanisms. Practically, HGF contributes to the early-phase biological activity of freshly prepared PRP but is not a primary target for extended storage protocols.²¹

Part 5: Comparative Stability Summary

The following table summarises the differential stability of key PRP growth factors across storage conditions, drawing from published in vitro quantification studies. Values represent approximate retention of measured growth factor concentration relative to immediate post-preparation levels.

Part 6: The Effect of Activation Timing on Growth Factor Release

Resting vs Activated PRP — Implications for Storage

The discussion above concerns storage of PRP prior to activation — that is, PRP in which platelets remain intact in their granule-loaded resting state, preserved by anticoagulant. This is the standard clinical scenario. However, some protocols involve pre-activating PRP using thrombin, calcium chloride, or mechanical means prior to injection, converting the preparation from a cellular suspension to a platelet-poor growth factor releasate. The storage implications differ substantially.⁴

In resting PRP, growth factors are sequestered within platelet granules — protected from the extracellular environment and from the proteases present in plasma. The rate of spontaneous release from resting platelets at room temperature is relatively slow in the presence of a good anticoagulant, and the four-to-six-hour viability window for room temperature resting PRP reflects this protection. Once activated, however, all alpha granule contents are released into solution simultaneously — and from that point, the exposed growth factors are subject to the full degradation kinetics described above. Activated PRP releasate should therefore be used within one to two hours, with refrigeration between preparation and injection if any delay is anticipated.¹²

The 70% Release Threshold

Studies quantifying growth factor kinetics after platelet activation with thrombin and calcium chloride consistently show that approximately 70–80% of alpha granule content is released within the first hour post-activation, with the remaining 20–30% released more slowly over the subsequent one to two hours.²³ This means that a PRP preparation activated immediately prior to injection delivers most of its growth factor payload in the first hour of tissue contact — which may be the most relevant timeframe for initiating regenerative signalling in the target tissue.

Part 7: Practical Clinical Recommendations

Anticoagulant of Choice

For clinical PRP preparation, ACD-A or ACD-B are the preferred anticoagulants, followed by sodium citrate as an acceptable alternative for same-day use. Heparin should be avoided entirely; EDTA is contraindicated. When using a commercial PRP kit system, verify which anticoagulant is employed — this information is available in the product's IFU and is an important quality variable that is often overlooked in clinical practice.^6,7

Time from Preparation to Injection

The preponderance of evidence supports the following clinical windows:

• Ideal: Injection within two hours of centrifugation, at room temperature, without pre-activation. This window ensures maximal viability of all growth factors including the more labile VEGF and bFGF.
• Acceptable: Injection within four to six hours at room temperature. PDGF and TGF-β remain substantially preserved; VEGF and bFGF will have declined by 25–55%.
• Extended (refrigerated): If the preparation cannot be used immediately, refrigeration at 4°C and use within 24 hours preserves the majority of growth factor content for most factors. The platelet shape change associated with cold storage is an acceptable trade-off for this extension.
• Not recommended: Frozen storage unless preparing a PRP lysate under a specific protocol designed for this purpose, or unless the preparation can be frozen at -80°C and used after a single thaw cycle only.

Implications for Scalp PRP with PRP from Previous Draw

Some scalp PRP protocols involve using PRP prepared from a prior session — either refrigerated or frozen — at a subsequent treatment appointment. If this approach is used, the above data indicate that refrigerated PRP should be used within 24 hours of preparation, and frozen PRP should be frozen at -80°C, thawed once only, and used promptly after thawing. The growth factor content of frozen-and-thawed PRP is lower than fresh preparations, particularly for VEGF, bFGF, and HGF, and practitioners should set expectations accordingly when using stored PRP compared to freshly prepared material.¹⁷

Conclusion

PRP is a biological product with a finite therapeutic lifespan that begins at the moment of collection. Anticoagulant selection — with ACD-A at the top of the evidence hierarchy — establishes the conditions under which platelets are preserved. Storage temperature determines the rate of platelet activation and the rate of growth factor degradation in the releasate. And time, inexorably, degrades even the best-preserved preparation, with VEGF and bFGF showing the steepest declines while TGF-β1 and PDGF prove more resilient.

The practical synthesis is straightforward: use an ACD-based anticoagulant wherever possible, prepare and inject within two to four hours at room temperature, refrigerate only if a delay is unavoidable, and avoid freezing unless specifically preparing a lysate product. A freshly prepared, optimally anticoagulated PRP injected promptly is biologically different — and clinically superior — to the same preparation used six hours later. This is not a minor consideration: in a treatment whose therapeutic mechanism depends entirely on growth factor delivery, protecting that delivery from the moment of collection is the foundation of clinical efficacy.

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