PRP: What Platelets Actually Do.

Platelet-rich plasma sits in an unusual position. It is one of the most requested regenerative treatments, it has been in clinical use for decades, and it remains one of the hardest treatments to make clean general statements about.

The reason is not that the biology is mysterious. It is that PRP is not a single, standardised thing.

The basic idea

PRP is autologous, meaning it is made from your own blood. A sample is drawn and spun in a centrifuge, which separates it by density into red cells, plasma, and a concentrated platelet layer. That platelet concentrate is what gets injected into the target tissue.

Platelets are best known for clotting, but that is only part of what they do. They also carry granules dense with signalling proteins, and release them on activation — among them platelet-derived growth factor, transforming growth factor beta, and vascular endothelial growth factor.

These are the molecules the body uses to co-ordinate tissue repair: recruiting cells to an injury site, stimulating new blood vessel formation, and driving matrix production. The premise of PRP is to deliver a concentrated dose of that signal directly where it is wanted.

Why the evidence is genuinely mixed

Here is the problem that shapes the whole field. Two clinics can both offer “PRP” and produce materially different preparations.

• Platelet concentration varies depending on the spin protocol — sometimes by several-fold.
• Leukocyte content varies. Leukocyte-rich and leukocyte-poor PRP have different inflammatory profiles and appear to suit different tissues.
• Activation method varies, as does injection volume, injection technique, and the number of sessions.

When two trials of “PRP” disagree, they may not have been testing the same intervention at all.

This is a substantial part of why meta-analyses in this area so often conclude that results are heterogeneous and that standardisation is needed.

Where the evidence is strongest

Knee osteoarthritis has the largest body of research. Several randomised trials have reported improvements in pain and function, in some cases sustained for months, though results are not uniform across studies and effect sizes vary.

Androgenetic alopecia has a reasonably encouraging set of smaller trials reporting improvements in hair density, again with variation in protocol.

Tendinopathies such as lateral epicondylitis have a genuinely mixed literature, with some positive trials and some showing no advantage over comparators.

A fair summary is that PRP shows promise in specific, well-selected indications, and is not a general-purpose treatment for pain or ageing.

Regulatory position

PRP occupies a particular regulatory space. Because it is derived from your own blood and subjected to minimal manipulation, it is generally not regulated as a drug. It is important to understand what that means: it indicates a regulatory classification, not that a given PRP treatment has been reviewed and approved as effective for a specific condition.

Marketing that implies otherwise is overstating the position.

Reasonable expectations

Because PRP is autologous, the risk of allergic or immune reaction is low. The realistic risks are those of any injection — discomfort, bruising, a small infection risk — and the meaningful downside is usually cost and time in the event that it does not help.

Response varies between individuals, and it is not currently possible to predict reliably who will respond well. Anyone offering certainty about your result is offering something the evidence cannot support.

The sensible framing is a considered trial in a well-chosen indication, with clear expectations set beforehand about what improvement would look like and when it would be assessed.

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