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Part 1Basics: Terms and biology
What does PRP activation actually mean?
Platelets are not passive containers for a few growth factors. Their granules contain a complex mixture of signaling proteins, adhesion and matrix proteins, chemokines, and other biologically active components.[1]
α-granules Frequently studied molecules include, for example,
Dense granules contain, among other things,
When platelets encounter suitable activation stimuli, they can change their shape, activate surface receptors, aggregate, and release granule contents. Activation does not "create" these stored factors. Rather, it influences whether, when, and under what conditions stored components are released.
Simultaneously, coagulation processes and the formation of a fibrin matrix can begin. This is precisely why the collective term "activation" should first be broken down into its individual processes.
Activation, degranulation, coagulation, and lysis are not the same.
Four terms are frequently confused in the PRP context. However, they describe different processes – cellular, plasmatic, or as structural cell disruption.
Figure 1 · interactiveFour processes that are frequently confusedThe diagram separates cellular activation, secretion, coagulation, and cell disruption.
Select element in the diagram or tab below
Cellular
Plasma-mediated · possible in parallel
Cell disruption · separate process
Platelet activation
Platelet activation refers to the functional response of an intact cell to a stimulus. Physiological agonists include collagen, thrombin, ADP and thromboxane A₂. The individual agonists engage different receptors and signalling pathways. A common feature of many activation pathways is an increase in intracellular calcium.
Degranulation
Degranulation or secretion is a possible consequence of this activation. In this process, granules fuse with platelet membrane systems and release stored components. Activation and degranulation are therefore closely coupled, but not identical.
Coagulation and fibrin formation
Plasma coagulation can occur in parallel. Here, Thrombin plays a key role: Thrombin cleaves fibrinogen to fibrin. This can change not only the molecular environment but also the physical form of the preparation. A fibrin matrix or gel can form from an initially liquid plasma.
Lysis
These processes are distinct from cell lysis. In lysis, the cell membrane is structurally disrupted. Freeze-thaw cycles, intensive sonication, or other lyse processes can thereby release intracellular components. A platelet lysate is therefore not biologically the same as functionally activated, initially intact platelets.
Activation ≠ Degranulation ≠ Coagulation ≠ Lysis
Why “not activated ex vivo” is more precise than “not activated”
Studies often refer to “activated PRP” and “non-activated PRP.” Biologically, this formulation can be misleading. A PRP preparation to which no activator has been added before application still contains reactive platelets.
Therefore, in this article, we prefer to use the following terms:
before applicationexogenous or ex vivo activated
An activation stimulus was added to the preparation before application.
before applicationnot activated ex vivo
No activator added – the preparation still contains reactive platelets.
after applicationIn situ activation
Activation after application. In the literature, this is sometimes also referred to as “endogenous activation”.
How can PRP be activated after application?
Even without a previously added activator, various potential activation stimuli exist in the tissue.
1
Collagen contact
Fibrillar collagen can trigger activation signals via platelet receptors such as GPVI and integrin α2β1.[3] In the event of injury or puncture, matrix structures that are not normally freely accessible can be exposed.
2
Local thrombin formation
Tissue factor and other components of coagulation can promote the local formation of thrombin after tissue injury. The resulting thrombin can in turn activate platelets and convert fibrinogen to fibrin.
3
Enhancement by activated platelets
Already activated platelets can release ADP and thromboxane, among other mediators, thereby stimulating additional platelets.
These mechanisms make activation after application biologically plausible. However, they do not allow the blanket statement that the target tissue is always completely activated by PRP. Intradermal application into collagen-rich tissue is not biologically equivalent to the situation of a preparation initially distributed freely in the joint space.
In-situ activation is biologically plausible, but neither quantitatively nor between different tissues has it been fully characterized.
Figure 2Ex vivo activation and in-situ activationTwo possible pathways, without ranking either one.
Route Aex vivo
- PRP
- exogenous stimuluse.g. calcium or thrombin
- altered cell responsedepending on the procedure also coagulation/fibrin
Pathway Bin situ
- PRP without ex vivo activatorapplication
- local tissue environmentcollagen · thrombin · other agonists
- possible in situ activation
no ex vivo activation ≠ no biological activation
Part 2Calcium, thrombin and fibrin
What role does calcium play in citrate-containing PRP?
Calcium is often simply referred to as a “PRP activator.” However, its function is more complex—it can be understood in four steps.
Figure 3 · interactiveRecalcification step by stepSchematic representation, not to scale.
- Ca²⁺
- Citrate
- Fibrin
- Platelet
Step 1 · Initial situationCitrate binds free calcium ions
In citrate-containing PRP, citrate inhibits coagulation, among other things, by binding free calcium ions.
Step 2Recalcification
When calcium ions become available again, calcium-dependent steps of coagulation can occur again. This process is referred to as Recalcification.
Step 3Coagulation, Thrombin and Fibrin
This allows, among other things, coagulation complexes to become functional again, thrombin to form, and fibrinogen to be converted into fibrin.
Step 4Platelet activation
The resulting thrombin can in turn activate the platelets present. In addition, laboratory studies also show direct changes in platelet activation markers after the addition of calcium chloride.[4]
1 / 4
The quantitative significance of these direct and indirect mechanisms can vary depending on the PRP composition and experimental conditions.
In citrate-containing PRP, calcium restores the calcium availability required for coagulation processes and can be associated with platelet activation both directly and via coagulation and local thrombin formation.
Do not equate calcium chloride and calcium gluconate
Calcium chlorideCaCl₂ · provides calcium
≠
Calcium gluconateprovides calcium
Various calcium compounds are used in PRP studies, particularly calcium chloride and calcium gluconate. Both provide calcium, but they are not chemically identical. The amount of available calcium is also not necessarily identical at the same nominal concentration.
Current literature does not permit a general clinical statement that calcium chloride is better than calcium gluconate or vice versa. Laboratory results from one activation protocol should therefore not be extrapolated to another protocol solely on the basis of the common term "calcium activation."
For scientific comparability, the specific calcium compound used should be stated. Specific mixing or dosage protocols are not the subject of this article.
Thrombin: activator and coagulation enzyme at the same time
Thrombin plays a special role in PRP activation. It is a potent platelet agonist and activates, among other things, protease-activated receptors. At the same time, thrombin is a central enzyme in coagulation and cleaves fibrinogen to fibrin.
Figure 4One substance, two roles
as a platelet agonist
- activates protease-activated receptors
- Platelet activation
- secretion
Thrombincan accelerate both
as a coagulation enzyme
- cleaves fibrinogen to fibrin
- Fibrin formation
- gelation
This allows two processes to be accelerated simultaneously: platelet activation and secretion as well as fibrin formation and gelation. In laboratory studies, thrombin therefore typically leads to a comparatively rapid initial reaction. However, this does not automatically mean that faster is better.
Autologous and bovine thrombin
Historical PRP and surgical protocols sometimes used bovine thrombin. Antibody formation against bovine coagulation proteins has been described for bovine thrombin preparations, including possible cross-reactions with human coagulation factors. Autologous and bovine thrombin should therefore not be treated as identical activation methods.
Manufacturing or application instructions for thrombin are not the subject of this article.
Activation also alters the fibrin matrix
One of the most important differences between activation methods does not concern the measured growth factor concentration, but rather the physical structure of the preparation.
Coagulation conditions can influence:
- Rate of fibrin formation
- Fibrin architecture
- Clot retraction
- Mechanical properties
- Local retention
Figure 5From liquid preparation to fibrin matrixSchematic representation of the possible physical change – not to scale or time.
01
Liquid PRP
The preparation is initially liquid.
02
Fibrin formation
With the onset of coagulation, fibrin increasingly forms.
03
Fibrin matrix / Gel
The physical form of the preparation can change significantly.
The question of activation is therefore partly also a question of which physical preparation is being characterized.
Which product form is appropriate from a professional and regulatory perspective depends on the intended purpose, the validated system and the medical context.
Part 3Release, measurement, and delimitation
Activation methods alter release kinetics
In vitro studies show that different activation methods can lead to different release profiles over time.
Cavallo et al. compared, among other things, calcium chloride, thrombin, calcium plus thrombin, and collagen.[2] In the experimental model used, the measured concentrations of various factors increased more progressively over the observation period after calcium chloride. Thrombin, or the combination of calcium and thrombin, led to a greater early increase for several factors.
Figure 6 · interactiveTwo temporal patternsExemplary course from an in vitro model – not original measurement curves, no absolute amounts, and no clinical superiority claim.
↑ Measured concentration (schematic)
Time after activation →
shaded: early observation period
Other studies, however, show that activator dose, PRP composition, and experimental conditions can also alter the observed kinetics.
Activation methods can produce different release profiles. Therefore, no general quality ranking such as "slow = better" or "fast = worse" can be derived.
More measured growth factor does not automatically mean more effect.
Activation studies often measure individual growth factors using ELISA or similar methods. However, a higher measured concentration of a molecule does not automatically mean greater biological activity, longer availability in tissue, stronger receptor activation, a more favorable biological response, or better clinical outcomes.
Figure 7 · interactiveThree levels, two gapsSelect a ≠ to see why the next step does not follow automatically.
LaboratoryMeasured value
BiologyBioactivity
ClinicalClinical effect
Measured value → Bioactivity
An assay measures the concentration of a molecule – not necessarily its biologically active fraction at the time of measurement. A clear example is TGF-β1 (see below).
Bioactivity → Clinical effect
The molecular form, temporal availability, receptor interaction, and the rest of the preparation also play a role. A biological response in the laboratory is not yet proof of a better clinical outcome.
TGF-β1
Example TGF-β1
TGF-β1 can be secreted as a biologically latent complex and requires further activation steps outside the platelet. An assay can determine total TGF-β1 without automatically capturing the biologically active fraction at the time of measurement.
Measured value ≠ Bioactivity ≠ Clinical effect
This principle is central to the interpretation of preclinical PRP studies.
Activation or Lysis?
Some studies compare classical activators with freeze-thaw cycles, ultrasound, or similar methods. A terminological distinction should be made.
Comparison: Functional Activation and Lysis
| Feature | Functional activation | Lysis |
|---|---|---|
| Starting point | Initially intact platelet | Platelet is structurally damaged |
| Process | The platelet reacts to a stimulus | The membrane is damaged or ruptured |
| Release | Secretion of stored granule contents | Intracellular components are released through cell disruption |
| Examples | Collagen, thrombin, recalcification | Freeze-thaw cycles, intensive sonication or ultrasound |
| Lysis also releases cellular components – but the underlying process is different. | ||
For study comparison and reporting, it should therefore be clear whether intact platelets were activated or platelets were lysed.
Physical activation methods: separate area of evidence
In addition to calcium and thrombin, various physical methods have also been investigated, for example, light, electric fields, mechanical stimuli, and temperature-related methods. The mechanisms and clinical evidence levels differ considerably.
They should therefore not automatically be treated as equivalent variants of the same “PRP activation”.
Part 4Clinical evidence
Overview of the evidence: Where are comparative data available?
The data situation regarding the activation question differs significantly depending on the indication – both in scope and in the type of evidence.
Knee osteoarthritis1 direct RCT · 1 meta-analysis · 1 network meta-analysis · 1 meta-regressionInconsistentDirect comparison without demonstrable additional benefit; indirect analyses with individual signals favoring activated preparations.To section
Androgenetic alopecia1 randomized split-head studydirect signalGreater increase in hair density on the activated side from month four – single study with limited follow-up.To section
Alopecia areata1 comparative studyno differenceNo statistically significant difference between the groups after four applications.To section
Aesthetic dermatologyno robust head-to-head comparisonnot directly investigatedIn situ activation in collagen-rich dermal tissue is mechanistically plausible – not directly compared clinically.To section
What does the clinical evidence show for knee osteoarthritis?
Knee osteoarthritis currently provides the most extensive clinical data on the activation question. This is also where it becomes clear why the type of evidence must be considered.
Figure 8 · interactiveDirect and indirect evidence on activationDifferent study designs answer different questions. Select evidence source:
The most direct comparison: triple-blind RCT 2024
For the specific question of an additional exogenous activator, a randomized triple-blind trial published in 2024 is particularly relevant.[7] 91 patients with early knee osteoarthritis received either PRP without added activator or the same PRP with calcium gluconate.
Both groups showed improvements over six months in WOMAC and VAS. However, there was no statistically significant differencebetween the groups. Patient satisfaction was also nearly identical.
With this specific PRP protocol and this population, no additional clinical benefit of the exogenous activator was demonstrable.
- Design
- randomized, triple-blind
- Scope
- 91 patients, early knee osteoarthritis
- Comparison
- PRP without activator vs. the same PRP with calcium gluconate
- Time period · Endpoints
- 6 months · WOMAC, VAS, satisfaction
- Interpretation
- Very direct question, but only one protocol
2023 Meta-analysis: Activation signal in cross-study comparison
A meta-analysis of 14 randomized placebo-controlled trials with a total of 1,292 patients showed a more favorable pooled result for studies with exogenously activated PRP.[6] Calcium chloride was the most frequently reported activator.
These results are relevant, but answer a slightly different question than the direct RCT. The analysis primarily compares different studies and PRP protocols. Therefore, in addition to activation, platelet concentration, leukocyte profile, volume, preparation, number of applications, and patient population can vary simultaneously.
- Design
- Meta-analysis of placebo-controlled RCTs
- Scope
- 14 RCTs · 1,292 patients
- Most frequent activator
- Calcium chloride
- Interpretation
- Relevant indirect signal – activation not isolated from other PRP characteristics
Network meta-analysis 2026: Benefits in individual outcomes
A systematic review published in 2026 included 56 RCTs with 5,251 patients; 53 RCTs with 5,031 participants were included in the network meta-analysis.[9] With activated PRP, benefits were found after twelve months in individual KOOS domains, while no significant activation effect was found for numerous other pain and functional endpoints.
When evaluating the sources, it should be taken into account that all authors reported affiliations with the BTI Biotechnology Institute or the PRGF-Endoret technology. This alone does not invalidate the analysis, but increases the importance of contextualizing it with independent data.
- Design
- systematic review with network meta-analysis
- Scope
- 56 RCTs · 5,251 patients (NMA: 53 RCTs · 5,031)
- Activation outcome
- Benefits in individual KOOS domains after 12 months; No significant effect on numerous other endpoints
- Note
- All authors' stated connections to BTI/PRGF-Endoret
Meta-regression 2026: interesting CaCl₂ signal, but no causality
A systematic review published in August 2026 including a meta-regression examined 20 placebo-controlled RCTs.[10] CaCl₂ activation was associated with greater improvements in pain and function after six months in the meta-regression. However, after twelve months, none of the moderators examined explained the differences in outcome.
The authors themselves rate the evidence as very low. Due to intercorrelated PRP characteristics and incomplete reporting, the independent contribution of individual characteristics cannot be reliably determined. The results are hypothesis-generating and not definitive proof for a specific PRP formulation.
- Design
- systematic review with meta-regression
- Scope
- 20 placebo-controlled RCTs
- Activation outcome
- CaCl₂ at 6 months associated with greater improvement; at 12 months no explanatory moderator
- Certainty of evidence
- very low (authors' assessment)
Direct, indirect, and meta-regressive evidence are not equivalent proofs of causality.
Activation status could be a clinically relevant PRP parameter. However, the available direct and indirect data are insufficient to consider a general exogenous activation strategy superior.
What do we know about androgenetic alopecia?
For androgenetic alopecia, an interesting direct comparison is available. A randomized double-blind split-head study investigated 80 patients.[11] Each participant received activated PRP on one side of the scalp and PRP not activated ex vivo on the other.
- Design
- randomized, double-blind, split-head
- Scope
- 80 patients
- Comparison
- activated vs. not activated ex vivo
- Result
- both sides improved; greater increase in hair density on the activated side
Both sides improved. From month four onward, the activated side showed a statistically greater increase in hair density; a difference in hair thickness was also observed at the end of follow-up.
The study therefore provides a direct activation signal. However, it is a single study with limited follow-up, and statistical differences should not automatically be interpreted as large clinical effects.
Alopecia areata: no significant difference in activation
A study in alopecia areata compared activated and non-ex vivo activated PRP.[12] After four applications, there was no statistically significant difference between the groups.
A possible activation signal in one indication is not a general PRP rule.
What do we know in aesthetic dermatology?
For skin rejuvenation and other aesthetic dermatological applications, the activation question has not been sufficiently directly investigated. Reviews describe relevant clinical signals for PRP, but also considerable heterogeneity in manufacturing and application protocols.[14]
Activated and non-ex vivo activated preparations are used in different studies, without a robust general head-to-head comparison. Mechanistically, in-situ activation in collagen-rich dermal tissue is plausible.
Mechanistic plausibility does not replace direct clinical comparative evidence.
Part 5Classification and Documentation
Activation is part of PRP characterization
Activation status is not a minor procedural detail. The DEPA classification explicitly includes activation as a separate dimension: Dose – Efficiency – Purity – Activation.[5] International consensus and reporting proposals also mention the activation method as a relevant parameter.[8]
DDose
EEfficiency
PPurity
AActivation
Two otherwise similar preparations can differ due to different activation conditions with regard to secretion profile, coagulation, fibrin formation, physical form, and release time.
Activation is therefore a characterizing feature, but not a quality grade.
How should the activation status be documented?
The statement “activated PRP” alone is not scientifically informative. For studies and product characterization, the following nine pieces of information are useful, if relevant and available:
Scientific documentation
Which information improves comparability?
Depending on the research and reporting context, the following information may be particularly helpful:
Activation statusexogenously activated, not ex vivo activated, or not reported
Activation methode.g. calcium, thrombin or other method
Activator usede.g. specific calcium compound or thrombin type
Time of administrationActivation relative to application
Physical formliquid preparation or fibrin/gel formation
Platelet doseif available in the study protocol
Cell profilee.g. leukocyte and erythrocyte data
VolumeFinal or applied volume, if relevant
Manufacturing processSystem and essential processing steps
This list is a scientific guideline and not a binding clinical minimum standard. For a more complete PRP profile, the PRP Classification & Reporting Calculator is available.
This does not mean that every practice must routinely measure all parameters. However, the mere designation "activated" or "not activated" is not sufficient for a reliable scientific comparison.
What can be deduced from the current state of research – and what cannot?
✓Well-founded
- Platelet activation and coagulation are different processes.
- Degranulation is a possible consequence of platelet activation.
- Calcium can restore coagulation in citrate-containing PRP.
- Thrombin is both a platelet agonist and a coagulation enzyme.
- Activation methods can generate different release kinetics.
- Activation conditions can alter the fibrin matrix and physical form.
- Platelets not activated ex vivo can encounter local activation stimuli after application.
- The extent of in situ activation is likely tissue-specific.
- Activation status is part of PRP characterization.
- Activation and lysis should be documented separately.
✕Not generally justifiable
- Activated PRP is generally more effective.
- PRP not activated ex vivo is biologically inactive.
- Calcium activation is generally required for PRP injections.
- Calcium chloride is generally clinically superior to calcium gluconate, or vice versa.
- Slower growth factor release is generally more favorable.
- A higher measured growth factor level indicates a stronger clinical effect.
- Thrombin activation is generally better or worse.
- In situ activation is sufficient in every indication.
- Freeze-thaw and functional platelet activation are the same process.
Conclusion: Activation is a process characteristic, not a quality seal.
PRP activation cannot be reduced to the question of whether calcium or thrombin has been added. Platelet activation, degranulation, coagulation, fibrin formation, and cell lysis are different processes. Depending on the procedure, they can be coupled in various ways.
Calcium can restore coagulation capacity in citrate-containing PRP and thus, among other things, enable local thrombin and fibrin formation. Thrombin acts simultaneously as a potent platelet agonist and as a central coagulation enzyme. Platelets applied without an exogenous activator can, in turn, encounter physiological activation stimuli after application.
The temporal release of individual molecules can also differ between activation methods. However, this does not automatically imply a clinical ranking.
More measured growth factor does not automatically mean more bioactivity or a better clinical effect.
The activation status should therefore be documented as an independent feature of the PRP protocol and interpreted together with platelet dose, cell profile, volume, and manufacturing process.
Further reading
Characterizing PRP protocols in a comprehensible way
The activation status is only one dimension of a complete PRP profile.
PRP Classification & Reporting Calculator →Frequently Asked Questions about PRP Activation
What does PRP activation mean?
PRP activation refers to the functional response of the contained platelets to an activation stimulus. This can include changes in shape, receptor activation, aggregation, and granule secretion. Coagulation and fibrin formation can occur simultaneously but are independent processes.
Does PRP need to be activated before application?
Current clinical evidence does not support a general requirement for exogenous pre-activation. Platelets that are not activated ex vivo can respond to local activation stimuli after application. However, whether and to what extent this occurs depends, among other things, on the target tissue and the specific PRP protocol.
What does in-situ activation mean?
This refers to activation after application by local biological stimuli. The term "endogenous activation" is sometimes used in the literature. Possible activation stimuli include collagen contact and locally generated thrombin.
What does calcium do in citrate-containing PRP?
Citrate binds calcium and thereby inhibits calcium-dependent steps in coagulation. When calcium becomes available again, coagulation processes can resume. This can involve the formation of thrombin and fibrin, among other things. Direct changes in platelet activation markers after calcium addition have also been described.
What is the difference between calcium chloride and calcium gluconate?
Both compounds provide calcium but are not chemically identical. Studies use different preparations and protocols. A general clinical superiority of one calcium compound over the other has not yet been demonstrated.
What does thrombin do in PRP?
Thrombin is both a potent platelet agonist and a central enzyme in coagulation. It can activate platelets and cleave fibrinogen to fibrin. This allows it to influence secretion and fibrin formation simultaneously.
Is unactivated PRP biologically inactive?
No. The more precise term is "not activated ex vivo." After application, local agonists can trigger activation. However, how quickly and completely this process occurs is not directly quantified for every target tissue.
Does more growth factor after activation mean a better effect?
No. A higher measured concentration does not prove greater bioactivity or a better clinical effect. The molecular form, temporal availability, receptor interaction, and the rest of the preparation also play a role.
Is freeze-thaw a PRP activation?
Freeze-thaw cycles can structurally disrupt the membrane of platelets. This cell disruption is more accurately described as lysis and should not be equated with the functional activation of intact platelets.
Selected scientific sources
- Blair P, Flaumenhaft R. Platelet α-granules: basic biology and clinical correlates.
- Cavallo C et al. Platelet-Rich Plasma: The Choice of Activation Method Affects the Release of Bioactive Molecules. 2016.
- Harrison S et al. Platelet activation by collagen provides sustained release of anabolic cytokines. 2011.
- Toyoda T et al. Direct activation of platelets by addition of CaCl₂ leads coagulation of platelet-rich plasma. 2018.
- Magalon J et al. DEPA classification: a proposal for standardising PRP use and a retrospective application of available devices. 2016. PMID 27900152.
- Simental-Mendía M et al. Activated vs. non-activated PRP in knee osteoarthritis. Clin Rheumatol. 2023. PMID 36502442.
- Rai S et al. Triple-blinded randomized trial of PRP with vs. without exogenous activator. 2024. PMID 38948365.
- Hurley ET et al. Expert consensus on PRP characterization and reporting. PMID 37625660.
- Anitua E et al. PRP composition and knee osteoarthritis: systematic review and network meta-analysis. 2026. PMID 42015328.
- Delgado D et al. PRP characteristics and short-term outcomes in knee osteoarthritis. 2026. PMID 42605960.
- Singh SK et al. Split-head randomized study of activator use in androgenetic alopecia. PMID 37067143.
- Nouh AH et al. Activated vs. non-activated PRP in alopecia areata. 2023. PMID 36643192.
- Anitua E et al. PRP in the management of alopecia: systematic review and meta-analysis. 2025. PMID 40944844.
- Asubiaro J, Avajah F. Platelet-Rich Plasma in Aesthetic Dermatology: Current Evidence and Future Directions. 2024. PMID 39268288.
For professional users only. This article provides a scientific overview of PRP activation methods and the current literature. It does not constitute a diagnostic, therapeutic, dosing, mixing or treatment recommendation. The intended purpose and instructions for use of the medical devices used, as well as professional assessment in the individual medical context, remain decisive.