5.1 Platelets

Why Platelets Matter on the SBB Exam

Platelet components carry the highest transfusion-transmitted bacterial infection risk of any routine blood product because platelets are stored at room temperature instead of refrigerated or frozen. The SBB exam expects specialist command of platelet manufacturing methods, storage requirements, the bacterial risk mitigation strategies AABB and FDA require, and the corrected count increment (CCI) used to investigate platelet transfusion refractoriness.

Whole Blood-Derived (Random Donor) Platelets

Whole blood-derived platelets, commonly called random donor platelets (RDP), are prepared from a unit of whole blood using one of two methods: the platelet-rich plasma (PRP) method (a light centrifugation spin isolates platelet-rich plasma, followed by a hard spin that concentrates the platelets) or the buffy coat method (a hard spin first isolates the buffy coat layer, which is then pooled with buffy coats from other units and processed into a platelet concentrate). AABB Standards require a minimum of 5.5 x 10^10 platelets per RDP unit, suspended in roughly 50-70 mL of plasma, with a pH of at least 6.2 at outdate. Because a single RDP unit does not provide a therapeutic adult dose, 4 to 6 units are pooled to reach a typical adult dose of about 3 x 10^11 platelets.

Apheresis (Single-Donor) Platelets

Apheresis platelets, also called single-donor platelets (SDP), are collected from one donor using an automated cell separator that continuously withdraws whole blood, separates the platelet layer by centrifugation, and returns the remaining red cells and plasma to the donor. AABB Standards require a minimum of 3.0 x 10^11 platelets per apheresis unit - roughly equivalent to a pool of 4 to 6 RDP units - collected from a single donor. Because one apheresis collection replaces multiple RDP donor exposures, SDPs are preferred whenever exposure reduction matters clinically: HLA-matched or crossmatch-compatible platelet support for alloimmunized patients, pediatric and neonatal transfusion, and chronically transfused oncology patients. Leukoreduced apheresis platelets (fewer than 5.0 x 10^6 residual white blood cells per unit) further reduce febrile nonhemolytic reactions, HLA alloimmunization, and CMV transmission risk, and are considered a "CMV-safe" equivalent to CMV-seronegative components.

Storage Conditions and the Rationale Behind Them

Platelet concentrates are stored at 20-24C with continuous gentle agitation in gas-permeable plastic containers. Agitation and gas-permeable plastic are both essential: platelets are metabolically active and continue aerobic respiration during storage, so the bag must allow oxygen in and carbon dioxide out, and continuous horizontal or flat-bed agitation prevents platelets from aggregating into clumps that would otherwise reduce the viable dose. The standard shelf life is 5 days from collection. FDA guidance permits extending platelet dating to 7 days when the blood center pairs conventional storage with an approved bacterial risk mitigation strategy, discussed below.

Bacterial Contamination: The Dominant Infectious Risk

Because platelets are the only routinely transfused component stored at room temperature, bacterial contamination introduced at phlebotomy (typically skin flora) can proliferate throughout the storage period in a way that is not possible in refrigerated red cells or frozen plasma. Historically, bacterial sepsis from platelet transfusion has exceeded the combined risk of transfusion-transmitted HIV, hepatitis B, and hepatitis C. Four complementary strategies address this risk:

ABO/Rh Considerations for Platelet Selection

Because RDP units carry substantial donor plasma, ABO-identical platelets are preferred, particularly for infants and small-volume recipients, to avoid passive transfer of high-titer anti-A or anti-B between donor and recipient. While platelets themselves do not express Rh antigens, RDP units contain a small volume of residual red cells; an RhD-negative recipient, especially a female of childbearing potential, who receives RhD-positive platelets should be evaluated for Rh immune globulin (RhIG) to prevent Rh alloimmunization.

Assessing Platelet Transfusion Effectiveness: The Corrected Count Increment

The corrected count increment (CCI) adjusts the platelet count rise for body size and dose, allowing comparison across patients and transfusions:

CCI = (post-transfusion count - pretransfusion count, per uL) x body surface area (m^2) / number of platelets transfused (x10^11)

A 1-hour CCI below roughly 5,000-7,500 signals platelet refractoriness - failure to achieve the expected increment. The workup distinguishes immune refractoriness (most often HLA class I alloantibodies from prior transfusion or pregnancy, occasionally platelet-specific [HPA] antibodies) from nonimmune consumption (fever, sepsis, DIC, splenomegaly, amphotericin B). Immune refractoriness is managed with HLA-matched or platelet-crossmatch-compatible apheresis units; nonimmune causes require treating the underlying consumptive process rather than escalating platelet matching.

Worked Example

A 1.8 m^2 patient's platelet count rises from 10,000/uL to 34,000/uL one hour after receiving one apheresis unit containing 3.5 x 10^11 platelets. CCI = (34,000 - 10,000) x 1.8 / 3.5 = 24,000 x 1.8 / 3.5 = 43,200 / 3.5, or approximately 12,340. This CCI is well above the refractoriness threshold, indicating a normal transfusion response.

Common Traps

• Confusing the RDP minimum (5.5 x 10^10/unit) with the apheresis minimum (3.0 x 10^11/unit) - they differ by roughly an order of magnitude because one apheresis unit replaces several pooled RDP units.
• Assuming refrigeration would be safer for platelets - cold storage activates platelets and shortens their circulating lifespan after transfusion, which is why conventional inventory is stored at room temperature despite the bacterial growth tradeoff.
• Forgetting that a low CCI requires distinguishing immune from nonimmune causes before ordering HLA-matched platelets, which are a limited resource.