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Introduction ⬇

Platelets (thrombocytes) are the smallest of the formed elements in the blood. These cells are nonnucleated, round or oval, flattened, disk-shaped structures. Platelet activity is necessary for blood clotting, vascular integrity and vasoconstriction, and the adhesion and aggregation activity that occurs during the formation of platelet plugs that occlude (plug) breaks in small vessels. Thrombocyte development takes place primarily in the bone marrow. The lifespan of a platelet is about 7.5 days. Normally, two-thirds of all the body platelets are found in the circulating blood and one-third in the spleen.

The platelet count is of value for assessing bleeding disorders that occur with thrombocytopenia, uremia, liver disease, or malignancies and for monitoring the course of disease associated with bone marrow failure. This test is indicated when the estimated platelet count (on a blood smear) appears abnormal. It is also part of a coagulation profile or workup.

The MPV is sometimes ordered in conjunction with a platelet count. The MPV indicates the uniformity of size of the platelet population. It is used for the differential diagnosis of thrombocytopenia.

Normal Findings ⬆ ⬇

Platelet count:

MPV:

Procedure ⬆ ⬇

  1. Obtain a 7-mL venous whole blood sample in a lavender-topped tube (with EDTA). Label the specimen with the patient’s name, date and time of collection, and test(s) ordered.

  2. Count the platelets by phase microscopy or by an automated counting instrument. The MPV is also calculated by many instruments at the time of the platelet count.

  3. Make a blood smear and note the size, shape, and clumping of the platelets.

  4. Place the specimen in a biohazard bag.

Clinical Implications ⬆ ⬇

  1. Abnormally increased numbers of platelets (thrombocythemia, thrombocytosis) occur in:

    1. Essential thrombocythemia

    2. Chronic myelogenous and granulocytic leukemia, myeloproliferative diseases

    3. Polycythemia vera and primary thrombocytosis

    4. Splenectomy

    5. Iron-deficiency anemia

    6. Asphyxiation

    7. RA and other collagen diseases, SLE

    8. Rapid blood regeneration caused by acute blood loss, hemolytic anemia

    9. Acute infections, inflammatory diseases

    10. Hodgkin disease, lymphomas, malignancies

    11. Chronic pancreatitis, tuberculosis, inflammatory bowel disease

    12. Kidney disease

    13. Recovery from bone marrow suppression (thrombocytopenia)

  2. Abnormally decreased numbers of platelets (thrombocytopenia) occur in:

    1. Idiopathic thrombocytopenic purpura, neonatal purpura

    2. Pernicious, aplastic, and hemolytic anemias

    3. Massive blood transfusion (dilution effect)

    4. Viral, bacterial, and rickettsial infections

    5. Heart failure, congenital heart disease

    6. Thrombopoietin deficiency

    7. During cancer chemotherapy and radiation, exposure to dichlorodiphenyltrichloroethane and other chemicals

    8. HIV infection

    9. Lesions involving the bone marrow (e.g., leukemias, carcinomas, myelofibrosis)

    10. DIC and thrombotic thrombocytopenic purpura

    11. Inherited syndromes such as Bernard–Soulier syndrome (autosomal recessive coagulopathy), May–Hegglin anomaly, Wiskott–Aldrich syndrome, Fanconi syndrome

    12. Toxemia of pregnancy, eclampsia

    13. Alcohol toxicity, ethanol abuse

    14. Hypersplenism

    15. Renal insufficiency

    16. Antiplatelet antibodies

  3. Increased MPV is observed in:

    1. Idiopathic thrombocytopenic purpura (autoimmune)

    2. Thrombocytopenia caused by sepsis

    3. Prosthetic heart valve

    4. Massive hemorrhage

    5. Myeloproliferative disorders

    6. Acute and CML

    7. Splenectomy

    8. Vasculitis

    9. Megaloblastic anemia

  4. Decreased MPV occurs in Wiskott–Aldrich syndrome.

Clinical Alert

  1. In 50% of patients who exhibit unexpected platelet increases, a malignancy is found.

  2. In patients with an extremely elevated platelet count (>1000 × 103/mm3 or >1000 × 109/L) as a result of a myeloproliferative disorder, assess for bleeding caused by abnormal platelet function.

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain test purpose and procedure.

  2. Avoid strenuous exercise before blood is drawn.

  3. Note what medications and what treatments the patient is receiving.

  4. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Review test results; report and record findings. Modify the nursing care plan as needed. Counsel the patient regarding abnormal findings; explain the need for possible follow-up testing and treatment. Observe for signs and symptoms of gastrointestinal bleeding, hemolysis, hematuria, petechiae, vaginal bleeding, epistaxis, and bleeding from the gums. When hemorrhage is apparent, use emergency measures to control bleeding and notify the attending healthcare 
provider.

  2. Use platelet transfusions if the platelet count is less than 20 × 103/mm3 (less than 20 × 109/L) or if there is a specific bleeding lesion. One unit of platelet concentrate raises the count by 15 × 103/mm3 (15 × 109/L).

  3. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors ⬆

  1. Platelet counts normally increase at high altitudes; after strenuous exercise, trauma, or excitement; and in winter.

  2. Platelet counts normally decrease before menstruation and during pregnancy.

  3. Clumping of platelets may cause falsely lowered results.

  4. Oral contraceptives cause a slight increase.

  5. see Appendix E for drugs that affect test outcomes.