• Platelets are small cellular fragments that function to promote hemostasis and coagulation upon endothelial cell injury.
• Minor platelet deficiencies usually present as bleeding of mucous membranes, petechiae, and epistaxis; large deficiencies can lead to life-threatening spontaneous hemorrhage.
• Preoperative platelet counts are not typically checked unless there are clinical signs of coagulopathy.
• A low platelet count is a poor indicator of platelet function.
• Tests such as thromboelastograms measure clotting kinetics and aid with assessing function.

• Platelets are small, irregularly shaped cell fragments that lack nuclei. Following endothelial cell injury, subendothelial collagen, von Willebrand factor (vWF), and tissue factor (TF) are exposed and activate the circulating platelets that they contact. Platelet clumping occurs and the coagulation cascade is activated to strengthen the clot.
• Normal values for endogenous platelets:
  • Range: 150,000–400,000/µL
  • Production: ~1 × 10¹¹ produced daily
  • Survival: ~9–10 days
• Platelet donation
  • Pooled platelets: Centrifugation separates plasma and red blood cells from platelets. One platelet concentrate is attained via a "hard spin," followed by a "soft spin" (or vice versa) of a whole blood unit (450 mL). The term "pooled" describes combining concentrates; however, it increases recipient exposure to HLA and infection (usually 6 different donor concentrates to comprise 1 jumbo unit).
  • Plasmapheresis: Extracorporeal collection of blood followed by separation and removal of platelets from red blood cells and plasma; the red blood cells and plasma are returned to the donor. The process lasts ~90 minutes, and yields a single or double plateletpheresis unit (equivalent to 1 or 2 jumbo units of pooled platelets, respectively). The main benefit is the decreased exposure to multiple donors. Today, ~80% of the US platelet supply comes in this manner.
• Platelet storage
  • May be stored for up to ~5 days at room temperature.
  • Unlike other blood products, cannot be cooled to slow down metabolism or reduce bacterial growth; platelet function is temperature sensitive. Temperatures <18°C damage micro-canalicular structures, and induce clustering of platelet receptors. In the recipient's circulation, macrophages will rapidly remove these previously "chilled" platelets.
  • Need to be continuously agitated to reduce bacterial growth
  • Special plastic containers are used to increase the surface-to-volume ratio and allow sufficient gas exchange between the internal volume and external ambient air (hypoxia is detrimental to survival, and metabolism is not slowed as with other transfusion units that can be cooled)
• Platelet transfusion (1,2)
  • Cross-matching the recipient's platelets with those of the donor is unnecessary; however, incompatibility can reduce post-transfusion counts. Platelets possess ABO antigens on their surface and RBCs are present in small amounts (<0.5 mL); when several units are needed, incompatible RBCs may cause hemolysis. Conversely, Type A and B antibodies from the donor's plasma portion may react against the recipient's RBCs when large volumes are administered. If several units are needed, ABO matching is prudent.
  • Rh antigens are not present on the platelet surface; however, RBCs (<0.5 mL) contained in the concentrate may lead to Rh immunization. Avoid incompatibility in women of childbearing years; if unavoidable, Rh Ig should be considered to prevent alloimmunization.
• Normal values for transfused platelets
  • Range:
    • "Platelet concentrate" is the term used for the platelets separated from 1 unit of WB. At most institutions, 6 platelet concentrates = 1 jumbo pack, or ~3 × 10¹¹ platelets.
    • Plateletpheresis donors donate a "single" or "double" unit in a sitting. A single unit or 1 plateletpheresis unit = 3 × 10¹¹ platelets; this is equivalent to 1 jumbo pack or 6 units of platelet concentrate.
  • Survival: ~1–7 days post-transfusion. At day 5, intravascular recovery is ~51%.

• Platelets are produced by invaginations from the surface of megakaryocytes in the marrow of trabecular bone.
• Inactive platelets circulate in the blood for ~10 days as smooth, disk-shaped cells and are degraded via phagocytosis in the spleen and by Kupffer cells in the liver

• Thrombocytopenia: A low platelet count can result from:
  • Increased destruction
  • Decreased production
  • Dilutional effects
  • Pseudothrombocytopenia
• Thrombocytosis (thrombocythemia): Elevated platelet counts are due to:
  • Essential causes (e.g., myeloproliferative diseases)
  • Reactive causes (e.g., hyposplenism, splenectomy, acute phase reactant)
  • Iatrogenic causes
• Infections
  • Storage of platelets at higher temperatures increases the risk of bacterial growth.
  • Frequently, infection is introduced by a needle entering the vein through nonsterile skin; it can also occur, albeit rarely, from donor bacteremia or occult colon cancer.
  • Mortality from infection is high, particularly in immunocompromised patients with cancer or blood disorders.
  • Donor pathogens include HIV, HBV, HCV, CMV.
• Febrile transfusion reactions: Secondary to leukocytes and soluble cytokines; may be avoided by transfusing leukocyte-reduced platelets.
• Acute hemolytic reactions: Type A and B antibodies from the donor's plasma may react against the recipient's RBCs when large numbers of ABO-incompatible units are given.
• Transfusion-associated graft versus host disease (TA-GVHD): Describes the abnormal immune response of T-lymphocytes attacking the host tissue; the risk is increased in immunocompromised patients. It is uniformly fatal, and there are minimal to no proven treatment options. Prevention consists of gamma-irradiating donor platelets prior to transfusions.
• Transfusion related lung injury (TRALI): To reduce the incidence, storage may be performed in additive solutions instead of plasma.

• Quantitative: for most patients, a minimum of 50,000–100,000/µL is adequate to avoid bleeding during procedures and surgeries. See table for specific procedures and suggestions.
• Qualitative: Platelet dysfunction and microvascular bleeding can occur despite adequate platelet counts. Uremia, aplastic anemia, chemotherapy, irradiation, ITP, and DIC can impair proper platelet function. Functional tests include the thromboelastogram (increased R time, decreased MA suggest platelet dysfunction).
• Transfusion parameters
  • In a 70 kg patient, 1 × 10¹¹ platelets increase platelet counts ~10,000/µL.
  • 1 unit of pooled platelets (~5–10 concentrates) contains ~3 × 10¹¹ and can increase recipient platelet counts by ~30–60,000/µL. This is roughly equivalent to dosing 1 platelet concentrate/10kg
  • 1 plateletpheresis unit contains ~3 × 10¹¹ platelets, and can increase counts ~30–60,000/µL (equivalent to a jumbo unit of pooled platelets)
• Transfusion considerations (2)
  • In children, consider ordering specific amounts from the blood bank to decrease waste (e.g., 20–25 mL).
  • "Volume reduce" by removing plasma to decrease the volume load in children or fluid overloaded patients (dialysis patients, congestive heart disease). However, this can result in a loss of 15–55% of platelets; alternatively, a smaller dose (with smaller volume) may be appropriate.
  • Inpatients or chronic conditions: Frequent transfusions of a lower dose of platelets may reduce the total amount of platelets needed, whereas larger doses extend the time needed between transfusions and may be more convenient.
  • Lack of response may be due to patient factors such as HLA antibodies, splenomegaly, DIC, amphotericin administration, and fever, as well as long storage time. Consider HLA matching, cross-matching, or "fresh platelets" (<48 hours after collection).
  • If donor-incompatible platelets are administered, consider washing or removing plasma and resuspension with saline. This serves to decrease donor plasma antibodies and incompatible RBCs.
• Massive transfusion/"damage control resuscitation." 1:1:1 transfusion of pRBC:FFP:platelets reduces risk of thrombocytopenia that can occur from the initial loss, followed by dilution from component therapy (FFP, PRBC devoid of platelets).
• "Swirling" is a visual effect from diffraction when platelets are manually re-suspended and held up to a strong light. This indicates that the platelets are high quality and discoid and can be easily performed at bedside. Old platelets lack "swirling" and have a spherical morphology that lacks diffraction.

Recommended platelet levels for procedures and surgeries:

• Fresh Frozen Plasma
• Thrombocytopenia
• Transfusion Related Lung Injury (TRALI)
• Von Willebrand's Disease
• Transfusion Infection

Yun Rose Li , BS

Nina Singh-Radcliff , MD