Two syringes labeled PRP can behave in opposite directions.

LEUKOCYTE-RICH AND LEUKOCYTE-POOR PRP

Two syringes can both be labeled PRP and contain preparations that behave in opposite directions. Which one goes into your joint is a clinical decision, and most clinics never raise it.

The variable almost nobody discusses

“Platelet-rich plasma” names one ingredient and leaves out the rest. The one that matters most after platelet count is the white cell concentration, and preparations divide into two families.

• Leukocyte-rich PRP (LR-PRP) — white cell concentration above that of whole blood. Neutrophils come with it, and they bring proteases, reactive oxygen species and a strong pro-inflammatory signal.
• Leukocyte-poor PRP (LP-PRP) — white cell concentration below whole blood. The platelets and their growth factors are kept; most of the neutrophil load is removed.

This is not a manufacturing detail. It is the difference between deliberately provoking an inflammatory response and deliberately avoiding one, and the right answer depends entirely on what tissue you are injecting.

Inside a joint, we use leukocyte-poor

A synovial joint is a closed space with a lining that reacts. Introduce a concentrated neutrophil load into it and you get exactly what the biology predicts: a catabolic, inflammatory response in a compartment that cannot drain it away. Post-injection flare is more common, and the effect you were trying to produce competes against the one you accidentally caused.

So every intra-articular injection here — knee, hip, shoulder, facet joint — uses a leukocyte-poor preparation. A meta-analysis of randomized trials examining leukocyte concentration and injection number in knee osteoarthritis supported the leukocyte-poor approach against hyaluronic acid, and a randomized trial of three leukocyte-poor injections against saline in mild-to-moderate knee osteoarthritis found in favor of the treatment.

On a tendon, the argument runs the other way

A degenerate tendon is the opposite situation. It is not inflamed — that is the whole point of the tendinopathy argument — it is a stalled, disorganized repair in poorly perfused tissue that has given up. Here a controlled inflammatory stimulus is not a side effect. It is a plausible part of the mechanism: you are trying to restart a process, and inflammation is the opening move of every repair sequence in the body.

It is also extra-articular. There is no synovial compartment to trap the response, so the flare is local, self-limiting and tolerable. That is why leukocyte-rich preparations have the stronger case at the tendon and the enthesis, and why the elbow — where the twelve-month PRP data is at its best — is largely a leukocyte-rich literature.

The honest qualifier: this is not settled everywhere. A multicentre randomized trial in patellar tendinopathy used a leukocyte-poor preparation, and did so deliberately. Tendon site, disease stage and whether a loading program runs alongside all move the answer.

The evidence against our own position, stated plainly

A 2024 analysis in the American Journal of Sports Medicine concluded that leukocytes did not influence the safety or efficacy of PRP injections for knee osteoarthritis. That finding runs against the reasoning above and it belongs on this page rather than in a footnote.

Two things can be true. The mechanistic case for keeping neutrophils out of a closed joint is strong and biologically coherent; the clinical signal at the level of pooled trial data is weaker than that reasoning predicts. Where the mechanism and the meta-analysis disagree and the cost of following the mechanism is nil, we follow the mechanism. Nobody is worse off for having received a preparation with fewer neutrophils in their knee.

Why the literature cannot settle it: half of it does not say what was injected

This is the deeper problem and it explains a great deal about why PRP trials contradict each other. A review of PRP classification systems found that reporting of preparation protocols in clinical trials is highly inconsistent, and that the majority of studies do not provide enough information for the protocol to be reproduced.

Read that again in practical terms. A large share of the published PRP literature does not record the platelet concentration, the leukocyte concentration, the volume, or whether the preparation was activated. Trials are then pooled as though they tested the same thing. They did not. Some of the “PRP does not work” results are studies of preparations nobody would choose, and some of the positive ones cannot be repeated because the recipe was never written down.

That is the single best explanation for the field’s reputation for inconsistency, and it is expanded on why the trials disagree. It is also why a clinic that cannot tell you which preparation it uses is telling you something important.

What we do, and what you may ask us

• Intra-articular — leukocyte-poor. Knee, hip, shoulder, facet.
• Tendon and enthesis — leukocyte-rich, unless the site or stage argues otherwise.
• Near neural tissue — neither. That is where platelet lysate is used instead, because what you want against a nerve is the growth-factor content without cells of any kind.
• Every preparation is image-guided on delivery, because the best-chosen preparation in the wrong plane is a wasted appointment.

Any patient is entitled to ask which preparation is being used on them and why that one. If the answer is vague, that is worth noticing — not because the clinic is dishonest, but because a preparation chosen without reference to the tissue was chosen by the machine in the room rather than by anybody.

What people ask about preparations

Which one is better?

Neither, in the abstract. Leukocyte-poor for joints, leukocyte-rich for most tendon work. A preparation is only better or worse relative to the tissue it is going into, which is why the question of which is superior has no answer and the question of which is right for your problem does.

Will leukocyte-rich PRP hurt more?

Usually a little, and for a day or two longer, at a tendon. That is the expected response rather than a complication, and it is one reason it is not used inside joints where the same reaction has nowhere to go.

How do I know what a clinic is using?

Ask. Any clinic doing this seriously knows which system it runs and what it produces. If nobody can tell you the leukocyte concentration or the platelet multiple, they are running whichever kit the representative sold them.

Does this explain why PRP studies contradict each other?

It is a large part of it. When most trials do not report the preparation in enough detail to reproduce it, pooling them compares treatments that were never the same treatment.

Related reading

• How PRP works
• Why the trials disagree
• Platelet lysate — what we use near a nerve
• PRP for tendonitis
• For physicians
• PRP for knee osteoarthritis

Which is why the number of injections cannot be answered without knowing what is in the syringe.