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Platelets (PLT, thrombocytes) count

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Platelets (Thrombocytes) Overview

Platelets, also known as thrombocytes, are small, irregular-shaped cell fragments that circulate in the blood. They are derived from very large cells in the bone marrow called megakaryocytes.

Platelets play a critical role in hemostasis – the process that stops bleeding. When a blood vessel is injured, platelets adhere to the site of injury, aggregate (clump together) to form a temporary plug, and release factors that initiate and accelerate the blood coagulation cascade, ultimately leading to the formation of a stable fibrin clot.

Measuring platelet count and related parameters is a standard part of the Complete Blood Count (CBC) and is essential for evaluating bleeding and clotting risks.

Normal platelet counts vary slightly with age:

  • Adults: Typically 150–450 x 109/L (or 150,000–450,000/µL)
  • Children (> 1 year): Often similar to adults, e.g., 180–350 x 109/L
  • Infants (< 1 year): May have slightly wider ranges, e.g., 150–400 x 109/L
  • Newborns: Range can be broad, e.g., 100–450 x 109/L
  • Note: Ranges can vary between laboratories. The 150-400/450 x 109/L range is widely accepted for adults.

Deviations from the normal range indicate either an excess (thrombocytosis) or deficiency (thrombocytopenia) of platelets.

Blood is made up of multiple components, including red blood cells, white blood cells, platelets, and plasma.

Thrombocytosis (High Platelet Count)

Thrombocytosis (sometimes called thrombocythemia when referring to primary causes) is defined as a platelet count higher than the upper limit of the normal range (e.g., > 450 x 109/L). It can be broadly categorized as primary or secondary (reactive).

Causes of Thrombocytosis
Category
Common Causes / Associated Conditions
Primary Thrombocythemia
(Due to clonal proliferation of megakaryocytes in bone marrow)
  • Essential Thrombocythemia (ET) - A Myeloproliferative Neoplasm (MPN)
  • Other MPNs: Polycythemia Vera (PV), Primary Myelofibrosis (PMF), Chronic Myeloid Leukemia (CML)
  • Myelodysplastic Syndromes (MDS), especially MDS with isolated del(5q)
  • Rarely, other myeloid neoplasms
Secondary (Reactive) Thrombocytosis
(Response to another condition)
  • Iron Deficiency Anemia (common cause)
  • Inflammation / Infection: Acute or chronic infections (bacterial, viral, fungal), chronic inflammatory diseases (Rheumatoid Arthritis, Inflammatory Bowel Disease like Ulcerative Colitis), tissue damage (surgery, trauma, burns), pancreatitis, acute rheumatic fever, osteomyelitis, tuberculosis.
  • Malignancy: Various solid tumors (lung, ovarian, gastric, colorectal carcinomas), Lymphoma (especially Hodgkin's).
  • Post-Splenectomy: Removal of the spleen leads to reduced platelet sequestration/destruction (can persist for months or longer).
  • Acute Blood Loss / Hemorrhage: Rebound thrombocytosis during recovery.
  • Hemolysis: Acute hemolytic anemia.
  • Post-Surgery Recovery: Often seen within 2 weeks.
  • Medications: Epinephrine, Vincristine (less common).
  • Exercise: Transient increase.

Reactive thrombocytosis is generally much more common than primary thrombocythemia. Platelet counts in reactive thrombocytosis are usually < 1,000 x 109/L, whereas counts can exceed this significantly in primary causes like ET.

Thrombocytopenia (Low Platelet Count)

Thrombocytopenia is defined as a platelet count lower than the normal range (e.g., < 150 x 109/L). It increases the risk of bleeding, with spontaneous bleeding risk rising significantly at counts below 10-20 x 109/L. Causes can be broadly divided into decreased production, increased destruction/consumption, or sequestration.

Causes of Thrombocytopenia
Category
Common Causes / Associated Conditions
I. Decreased Platelet Production (Bone Marrow Issues)
1. Acquired:
  • Aplastic Anemia (Idiopathic hematopoietic hypoplasia)
  • Bone Marrow Infiltration: Leukemia (acute/chronic), Lymphoma, Multiple Myeloma, Metastatic Cancer, Myelofibrosis, Osteomyelosclerosis
  • Myelodysplastic Syndromes (MDS)
  • Megaloblastic Anemia (Severe Vitamin B12 or Folate deficiency)
  • Medications/Toxins: Chemotherapy, radiation therapy, certain antibiotics (chloramphenicol, sulfonamides, linezolid), thiazide diuretics, alcohol, chemicals (benzene, insecticides), heparin (via HIT)
  • Infections: Viral infections (HIV, Hepatitis C, EBV, parvovirus B19, dengue), severe bacterial sepsis, miliary tuberculosis, certain parasitic infections (malaria).
  • Liver Disease (Severe): Decreased thrombopoietin production.
  • Paroxysmal Nocturnal Hemoglobinuria (PNH)
2. Hereditary (Congenital): Fanconi Anemia, Wiskott-Aldrich Syndrome, May-Hegglin Anomaly, Bernard-Soulier Syndrome, Thrombocytopenia-Absent Radius (TAR) syndrome, Congenital Amegakaryocytic Thrombocytopenia.

II. Increased Platelet Destruction/Consumption
1. Immune-Mediated:
  • Autoimmune:
    • Primary: Immune Thrombocytopenia (ITP, Idiopathic Thrombocytopenic Purpura).
    • Secondary: Associated with SLE, Antiphospholipid Syndrome, Rheumatoid Arthritis, Chronic Lymphocytic Leukemia (CLL), Lymphomas, HIV, Hepatitis C, H. pylori infection.
  • Alloimmune:
    • Neonatal Alloimmune Thrombocytopenia (NAIT): Maternal antibodies against fetal platelet antigens.
    • Post-Transfusion Purpura (PTP): Antibodies develop after blood/platelet transfusion.
  • Drug-Induced Immune Thrombocytopenia (DITP): Many drugs can act as haptens (e.g., heparin - HIT, quinine, quinidine, sulfonamides, vancomycin, rifampin).
  • Virus-induced (sometimes immune component).
2. Non-Immune Consumption:
  • Thrombotic Microangiopathies (TMA): Thrombotic Thrombocytopenic Purpura (TTP), Hemolytic Uremic Syndrome (HUS).
  • Disseminated Intravascular Coagulation (DIC) - associated with sepsis, trauma, malignancy, obstetric complications.
  • Mechanical Destruction: Artificial heart valves, extracorporeal circuits (ECMO, dialysis), giant hemangiomas (Kasabach-Merritt syndrome).
  • Severe Infections / Sepsis (consumption).
III. Sequestration
Splenic Sequestration (Hypersplenism) Cirrhosis with portal hypertension, lymphomas, leukemias (e.g., hairy cell), storage diseases (Gaucher), infections (malaria, kala-azar, tuberculosis), sarcoidosis.
IV. Dilutional Thrombocytopenia
Massive Transfusion / Fluid Resuscitation Dilution of platelet concentration with large volumes of IV fluids or platelet-poor blood products.

Mean Platelet Volume (MPV)

Mean Platelet Volume (MPV) measures the average size of platelets in the blood sample, typically expressed in femtoliters (fL).

  • Normal Range: Usually around 7.4 to 11.5 fL (ranges vary).
  • Interpretation: MPV reflects platelet production rate and kinetics. Younger, newly released platelets are generally larger than older ones.
    • Increased MPV: Suggests increased platelet production, often seen when the bone marrow compensates for peripheral platelet destruction or consumption. Common causes include:
      • Idiopathic Thrombocytopenic Purpura (ITP)
      • Disseminated Intravascular Coagulation (DIC)
      • Myeloproliferative Neoplasms (ET, PV, PMF)
      • Recovery from chemotherapy-induced thrombocytopenia
      • Hyperthyroidism
      • Atherosclerosis / Cardiovascular risk factors (sometimes)
      • Diabetes Mellitus
      • Smoking
      • Bernard-Soulier syndrome (large platelets)
    • Decreased MPV: Suggests impaired platelet production or production of smaller platelets. Causes include:
      • Aplastic anemia
      • Bone marrow suppression (e.g., chemotherapy)
      • Wiskott-Aldrich syndrome (small platelets)
      • Some chronic inflammatory conditions
      • Hypersplenism (older, smaller platelets may dominate circulation)
      • After splenectomy (splenic pool of large platelets removed - can be variable)
  • Clinical Use: MPV is most useful when interpreted alongside the platelet count, helping to differentiate causes of thrombocytopenia (high MPV suggests destruction, low MPV suggests production defect).

Platelet Distribution Width (PDW)

Platelet Distribution Width (PDW) measures the variation in platelet size (similar to RDW for red blood cells), reflecting platelet anisocytosis. It is usually reported as a percentage (%) or in femtoliters (fL).

  • Normal Range: Typically around 9-14% or 9-17 fL (highly method-dependent).
  • Increased PDW: Indicates greater variability in platelet size. Often seen when there is a mix of old and new (larger) platelets, such as during active platelet production or destruction. Causes overlap significantly with causes of increased MPV, including:
    • Myeloproliferative neoplasms
    • Immune Thrombocytopenia (ITP)
    • Megaloblastic anemia
    • Sepsis
    • Some cancers
    • Presence of platelet aggregates, fragmented RBCs (microerythrocytes), or giant platelets can falsely increase PDW readings on some analyzers.
  • Decreased PDW: Less common, may suggest more uniform platelet size.
  • Clinical Use: Less commonly used than MPV, but provides complementary information about platelet size heterogeneity.

Plateletcrit (PCT)

Plateletcrit (PCT), also known as thrombocrit, represents the volume proportion of platelets in the total blood volume, analogous to hematocrit for red blood cells. It is calculated as: PCT (%) = Platelet Count (x 109/L) × MPV (fL) / 10,000.

  • Normal Range: Typically around 0.15–0.40%.
  • Interpretation: Reflects the total platelet mass. It generally parallels the platelet count but also incorporates platelet size (MPV). Low PCT indicates thrombocytopenia or small platelets; high PCT indicates thrombocytosis or large platelets.
  • Clinical Use: Not as widely used clinically as the platelet count or MPV, but may provide additional information in some settings.

The Platelet Test Procedure (Part of CBC)

  • Test Name: Complete Blood Count (CBC).
  • Sample Type: Whole blood collected in an anticoagulant tube (usually EDTA - purple top).
  • Preparation: No fasting or special preparation is generally required.
  • Collection: Standard venipuncture. Proper mixing of the tube after collection is important to prevent platelet clumping.
  • Analysis: Performed using automated hematology analyzers which count platelets and measure indices. Platelet clumping in the EDTA tube can sometimes cause a falsely low automated count (pseudothrombocytopenia); examination of a blood smear or collection in a different anticoagulant (e.g., citrate - blue top) may be needed for confirmation.

References

  1. National Heart, Lung, and Blood Institute (NHLBI). (n.d.). Platelet Count. NIH. Retrieved from https://www.nhlbi.nih.gov/health/platelet-count
  2. Lab Tests Online. (n.d.). Platelet Count. Retrieved from https://labtestsonline.org/tests/platelet-count
  3. Mayo Clinic Staff. (n.d.). Low platelet count (thrombocytopenia). Mayo Clinic Patient Care & Health Information. Retrieved from https://www.mayoclinic.org/diseases-conditions/thrombocytopenia/symptoms-causes/syc-20378293
  4. Mayo Clinic Staff. (n.d.). Thrombocytosis. Mayo Clinic Patient Care & Health Information. Retrieved from https://www.mayoclinic.org/diseases-conditions/thrombocytosis/symptoms-causes/syc-20378315
  5. Bain, B. J., Bates, I., & Laffan, M. A. (Eds.). (2016). *Dacie and Lewis Practical Haematology* (12th ed.). Elsevier. (Chapters on Platelets and CBC).
  6. Latger-Cannard, V., Bensoussan, D., & Lecompte, T. (2011). Platelet count and mean platelet volume. *eJIFCC*, 22(1), ifcc2011vol22no1pp031-ifcc2011vol22no1pp041.