Two syringes labeled PRP can behave in opposite directions.

LEUKOCYTE-RICH AND LEUKOCYTE-POOR PRP

Leukocyte-rich PRP carries more white cells than whole blood. It sends a strong inflammatory signal. Leukocyte-poor PRP keeps the platelets and removes most of the neutrophils (the main white cells). Inside a joint we use leukocyte-poor. On a degenerative tendon, leukocyte-rich has the stronger case.

Two syringes can both be labeled PRP and hold preparations that behave in opposite ways. Which one goes into your joint is a clinical decision. Most clinics never bring it up.

The variable almost nobody discusses

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

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

This is not a factory detail. It is the difference between stirring up inflammation on purpose and avoiding it on purpose. 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. Its lining reacts. Put a concentrated neutrophil load into it and you get just what the biology predicts. You get a catabolic (tissue-breaking), inflammatory response in a space that cannot drain it away. Flares after the injection are more common. And the effect you wanted competes with the one you caused by accident.

So every injection into a joint here — knee, hip, shoulder, facet joint — uses a leukocyte-poor preparation. One meta-analysis of randomized trials looked at knee osteoarthritis. It studied white cell level and the number of shots. It backed the leukocyte-poor approach over hyaluronic acid. A randomized trial in mild-to-moderate knee osteoarthritis tested three leukocyte-poor shots against saline. It found in favor of the treatment.

On a tendon, the argument runs the other way

A degenerative tendon is the opposite case. It is not inflamed — that is the whole point of the tendinopathy argument. It is a stalled, messy repair in tissue with poor blood flow that has given up. Here a controlled inflammatory push is not a side effect. It is a plausible part of how it works. You are trying to restart a process, and inflammation is the opening move of every repair sequence in the body.

It is also outside the joint (extra-articular). There is no closed joint space to trap the response. So the flare stays local, settles on its own and is tolerable. That is why leukocyte-rich preparations have the stronger case at the tendon and the enthesis (where tendon meets bone). It is also why the elbow — where the twelve-month PRP data is at its best — is mostly a leukocyte-rich literature.

The honest qualifier: this is not settled everywhere. A multicenter randomized trial in patellar tendinopathy used a leukocyte-poor prep, and did so on purpose. Which tendon, what 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 looked at PRP shots for knee osteoarthritis. It concluded that leukocytes did not influence the safety or efficacy (how well it works). That finding runs against the reasoning above. It belongs on this page, not in a footnote.

Two things can be true. The biology case for keeping neutrophils out of a closed joint is strong. It makes sense. But in pooled trial data, the clinical signal is weaker than that reasoning predicts. When the mechanism and the meta-analysis disagree, and following the mechanism costs nothing, we follow the mechanism. Nobody is worse off for getting a prep 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. It explains a lot about why PRP trials contradict each other. A review looked at the systems used to classify PRP. It 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 research does not record the platelet level, the leukocyte level, the volume, or whether the preparation was activated. Trials are then pooled as if they tested the same thing. They did not. Some of the “PRP does not work” results are studies of preps nobody would choose. Some of the positive ones cannot be repeated, because the recipe was never written down.

That is the single best reason the field has a name for being inconsistent. It is expanded on in why the trials disagree. It is also why a clinic that cannot tell you which prep 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 nerve tissue — neither. That is where platelet lysate is used instead, because against a nerve you want the growth-factor content without cells of any kind.
• Every preparation is image-guided on delivery, because the best-chosen preparation in the wrong layer is a wasted visit.

Every patient has a right to ask which prep is being used and why that one. If the answer is vague, take note. That is not because the clinic is dishonest. It is because a prep picked without regard to the tissue was picked by the machine in the room, not 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 prep is only better or worse compared with the tissue it goes into. That 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, not a complication. 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 that takes this seriously knows which system it runs and what it makes. If nobody can tell you the leukocyte level or the platelet multiple, they are running whatever kit the sales rep sold them.

Does this explain why PRP studies contradict each other?

It is a large part of it. Most trials do not describe the prep in enough detail to repeat it. So 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.