Explainer · August 7, 2026 · 5 min · By Kerensa Molyneux
Leukocyte-Rich vs Leukocyte-Poor PRP: Why the White Cell Question Decides More Than Most Patients Realize
Two injections can both be called PRP and behave like different drugs. Here is what the leukocyte content actually does in tendon versus joint, and why the label on your consent form matters.
Ask ten clinics what platelet-rich plasma is and you will get roughly the same answer: your own blood, spun down, concentrated, reinjected. Ask what kind of PRP they use and the answers scatter. The single biggest fork in that road is leukocyte content, meaning how many white blood cells end up in the final syringe. Leukocyte-rich PRP (LR-PRP) and leukocyte-poor PRP (LP-PRP) are prepared from the same blood draw but have measurably different biology, and the evidence increasingly suggests they belong in different anatomical locations.
The distinction comes from how the centrifuge is run and which layer of the spun sample is harvested. Whole blood separates into red cells at the bottom, a thin buffy coat of white cells and platelets in the middle, and plasma on top. Systems that capture the buffy coat produce LR-PRP, with white cell counts at or above baseline blood levels, sometimes several times higher. Systems that draw only from the plasma layer above the buffy coat produce LP-PRP, with white cells reduced well below baseline. Platelet concentration can be similar in both. The white cells are the variable.
Why white cells change the mechanism. Neutrophils, the most abundant leukocyte in LR-PRP, release matrix metalloproteinases, reactive oxygen species, and pro-inflammatory cytokines such as interleukin-1 beta and tumor necrosis factor alpha. In a chronically degenerated tendon, that inflammatory push may be part of the point. Chronic tendinopathy is often described as a failed healing state, a tissue stuck in disorganized repair without an active inflammatory phase. The working hypothesis behind LR-PRP for tendon is that a controlled inflammatory stimulus restarts the sequence: inflammation, then proliferation, then remodeling. Monocytes in the mix can also transition toward reparative macrophage phenotypes over time, which supports later-stage healing.
Cartilage and synovium respond differently. Chondrocytes and synovial cells exposed to high concentrations of IL-1 beta and TNF alpha upregulate catabolic enzymes that degrade cartilage matrix. Laboratory studies have shown that LR-PRP applied to synovial cells produces more inflammatory gene expression than LP-PRP, and several clinical series report more post-injection pain and swelling with LR-PRP in the knee. This is the mechanistic basis for a pattern many sports medicine physicians now follow: LP-PRP for joints, LR-PRP considered for tendons, though the tendon side of that rule remains genuinely contested.
What the comparative evidence actually shows. For knee osteoarthritis, multiple meta-analyses of randomized trials suggest LP-PRP achieves comparable or better pain and function scores than LR-PRP, with fewer transient inflammatory reactions after injection. The effect sizes for PRP overall in knee osteoarthritis are modest and debated, but within the PRP literature the leukocyte-poor formulations look like the safer intra-articular bet. For lateral epicondylitis, some of the better-known positive trials used leukocyte-rich preparations, which is often cited as support for LR-PRP in tendon. Head-to-head trials directly comparing the two formulations in the same tendon condition are still scarce, and patellar and Achilles tendon data are mixed enough that no formulation has clearly won. Honest summary: the joint recommendation rests on stronger comparative footing than the tendon recommendation.
Red blood cells are a related, quieter issue. Buffy coat harvesting tends to carry more red cells into the final product. Lysed red cells release hemoglobin and iron species that are toxic to chondrocytes in vitro. A red-tinged PRP injected into a joint is not just cosmetic. Preparations described as low in both leukocytes and red cells are generally preferred intra-articularly for this reason.
Questions worth asking before an injection. First, which system is used and does it produce leukocyte-rich or leukocyte-poor PRP. A clinic offering PRP for both knees and elbows with a single fixed protocol is applying one biologic to two different problems. Second, what platelet concentration does the system typically achieve, since very low concentrations may underperform and extremely high concentrations have shown paradoxically inhibitory effects on some cell types in laboratory work. Third, is the preparation activated before injection or allowed to activate in the tissue, another variable that changes growth factor release kinetics.
The myth to retire is that PRP is one therapy with one evidence base. Trials using different formulations are often pooled in public discussion, which flattens real differences and fuels both overclaiming and unfair dismissal. When a headline says PRP works or PRP fails for a condition, the responsible follow-up question is always the same: which PRP. Until preparation reporting becomes standard across studies, and classification systems pushing in that direction do exist, the leukocyte question remains the most practical filter a patient or referring clinician can apply. Same blood, same centrifuge, different medicine.
Related reading: PRP vs PRF: what is the difference?.
Further reading: Histological effects of intra-articular allogeneic leukocyte-rich and leukocyte-poor platelet-rich plasma in healthy rabbit knees: an exploratory study (BMC Res Notes 2026); Leukocyte-Rich Platelet-Rich Plasma Is Predominantly Anti-inflammatory Compared With Leukocyte-Poor Platelet-Rich Plasma in Patients With Mild-Moderate Knee Osteoarthritis: A Prospective, Descriptive Laboratory Study (Am J Sports Med 2023); Parecoxib alleviates the inflammatory effect of leukocyte-rich platelet-rich plasma in normal rabbit tendons (BMC Musculoskelet Disord 2020).