Platelet Count and Tests of Platelet Function Core Lab Study
Synonym/Acronym
Thrombocytes.
Rationale
The platelet count is used to assist in diagnosing and evaluating treatment for blood disorders such as thrombocytosis and thrombocytopenia and to evaluate preprocedure or preoperative coagulation status; platelet function testing is used to assist in identification of inherited or acquired platelet dysfunction or to evaluate therapeutic response to platelet-inhibiting drugs. Tests of platelet function have largely replaced the bleeding time and clot retraction tests.
A small group of studies in this Manual have been identified as Core Lab Studies. The designation is meant to assist the reader in sorting the basic always need to know laboratory studies from the hundreds of other valuable studies found in the Manuala way to begin putting it all together.
Normal, abnormal, or various combinations of core lab study results can indicate that all is well, reveal a problem that requires further investigation with additional testing, signal a positive response to treatment, or suggest that the health status is as expected for the associated situation and time frame.
Platelet count is part of a CBC, one of the most requested laboratory studies, and is included in the Anemia profile, Complete Blood Count, General Health panel, and Obstetric panel.
Patient Preparation
There are no food, fluid, activity, or medication restrictions unless by medical direction for platelet count. Regarding platelet function tests: Platelet inhibitor drugs will affect the results of platelet function testing. Patients who have been treated with inhibitor drugs should consult the laboratory for the appropriate time frames.
Normal Findings
Method: Automated, computerized, multichannel analyzers for platelet count; closure time endpoint for platelet function screening test; aggregometry for platelet aggregation studies; flow cytometry for evaluation of surface glycoprotein expression.
Note: Platelet counts may decrease slightly with age. *Conventional units. MPV = mean platelet volume. Care must be taken when reviewing platelet counts after a blood product transfusiondocumentation should clearly reflect the time and date of the last transfusion with respect to the collection time of the study. |
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Results should be interpreted with other available clinical and laboratory information. |
Platelet Count
Timely notification to the requesting health-care provider (HCP) of any critical findings and related symptoms is a role expectation of the professional nurse. A listing of these findings varies among facilities.
Consideration may be given to verifying the critical findings before action is taken. Policies vary among facilities and may include requesting immediate recollection and retesting by the laboratory.
Critically low platelet counts can lead to brain bleeds or gastrointestinal hemorrhage, which can be fatal. Some signs and symptoms of decreased platelet count include spontaneous nosebleeds or bleeding from the gums, bruising easily, petechiae, prolonged bleeding from minor cuts and scrapes, heavy or prolonged menstrual bleeding, and bloody urine or stool. Possible interventions for decreased platelet count may include transfusion of platelets or changes in anticoagulant therapy.
An IgA deficiency-mediated platelet transfusion reaction usually occurs within 1 hour of transfusion. A reaction does not occur in all patients with an IgA deficiency and can occur along a spectrum ranging from no symptoms or adverse reaction to full-blown, life-threatening anaphylaxis. (See study titled Blood Typing, Antibody Screen, and Crossmatch Core Lab Study for information regarding transfusion reactions.)
Study type: Blood collected in a lavender-top [EDTA] tube for platelet count; lavender-top [EDTA] or blue-top [3.2% sodium citrate] tube for platelet function test by aggregrometry; lavender-top [EDTA], pink-top [K2EDTA] or yellow-top [ACD Solution B] tube for platelet receptor flow cytometry. The laboratory should be consulted regarding recommended collection containers and order of draw when multiple tubes will be drawn with platelet function studies; related body system: . For the platelet count: the specimen should be mixed gently by inverting the tube 10 times. The specimen should be analyzed within 24 hr when stored at room temperature or within 48 hr if stored at refrigerated temperature. If it is anticipated the specimen will not be analyzed within 24 hr, two blood smears should be made immediately after the venipuncture and submitted with the blood sample. For platelet function studies: the laboratory should be consulted prior to specimen collection for special instructions regarding specimen stability and transportation instructions.
What Are Platelets and Why Are They Important?
Platelets are nonnucleated, cytoplasmic, round or oval disks formed by budding off of large, multinucleated cells (megakaryocytes) that move throughout the body via the circulatory system. Platelets have an essential function in coagulation, hemostasis, and blood thrombus formation. The three stages in the process of primary hemostasis include the three As:
Evaluating Platelet Count
Thrombopoiesis or platelet production is reflected by the measurement of the immature platelet fraction (IPF). This parameter can be correlated to the total platelet count in the investigation of platelet disorders. A low platelet count with a low IPF can indicate a disorder of platelet production (e.g., drug toxicity, aplastic anemia, or bone marrow failure of another cause), whereas a low platelet count with an increased IPF might indicate platelet destruction or abnormally high platelet consumption (e.g., mechanical destruction, disseminated intravascular coagulation, idiopathic thrombocytopenic purpura [ITP], thrombotic thrombocytopenic purpura).
Thrombocytosis is an increase in platelet count. In reactive thrombocytosis, the increase is transient and short-lived, and it usually does not pose a health risk. One exception may be reactive thrombocytosis occurring after coronary bypass surgery. This circumstance has been identified as an important risk factor for postoperative infarction and thrombosis. The term thrombocythemia describes platelet increases associated with chronic myeloproliferative disorders.
Thrombocytopenia describes platelet counts of less than 140 × 103/microL. Decreased platelet counts occur whenever the bodys need for platelets exceeds the rate of platelet production; this circumstance will arise if production rate decreases or platelet loss increases. Platelet counts may also be decreased in the presence of autoantibodies as seen in refractoriness to platelet transfusions, posttransfusion purpura, or neonatal alloimmune thrombocytopenia. For additional information regarding platelet antibodies, refer to the study titled, Platelet Antibodies. The severity of bleeding is related to platelet count as well as platelet function. Platelet counts can be within normal limits, but the patient may exhibit signs of internal bleeding; this circumstance usually indicates an anomaly in platelet function.
When abnormal platelet findings are flagged by an automated cell counter, the situation must be investigated; this usually begins with a review of a peripheral blood smear or a platelet estimate. Abnormally large or giant platelets may result in underestimation of automated counts by 30% to 50%. Giant platelets can be as large as a normal red blood cell (RBC). A normal RBC is 788 micrometers, and a normal platelet is 1.53 micrometers. Giant platelets are associated with relatively rare platelet disorders, e.g., inherited disorders such as Bernard-Soulier and May-Hegglin or a condition known as immune thrombocytopenia (previously referred to as immune thrombocytopenic purpura). Underestimation of the platelet count may also be caused by platelet clumping, which can be caused by a traumatic venipuncture or the presence of EDTA-dependent antibodies that interfere with GP IIb/IIIa platelet receptors. In both cases, the problem may be solved by recollecting the specimen. Additionally, in the case of clumping, the re-draw should be performed using a collection container with a different anticoagulant, e.g., sodium citrate (blue top). The laboratorys platelet estimate procedure may include a mathematical calculation to approximate the platelet count from a platelet estimate as well as making an assessment of platelet characteristics that indicate a normal review:
The platelet count may also be affected by administration of heparin that is associated with two types of thrombocytopenia: Type I heparin-induced thrombocytopenia (HIT) is believed to occur as the result of an interaction between heparin and circulating platelets. Type I HIT causes a mild thrombocytopenia soon after administration of heparin, usually occurs in patients who have not been previously received heparin, and resolves in a few days whether the heparin therapy continues or is stopped. Type II HIT, believed to be the result of an immune-mediated response, also occurs in patients who have not been previously treated with heparin, begins later than type I HIT unless the patient is sensitized from a previous exposure, and can result in a 50% or greater decrease in platelet count. Functional assays for HIT II antibodies are available but not routinely requested. The risk of thrombosis is high with type II HIT, and administration of heparin should be stopped in patients who develop type II HIT.
Evaluating Platelet Size and Shape
Platelet size, reflected by mean platelet volume (MPV), and cellular age are inversely related; that is, younger platelets tend to be larger. An increase in MPV indicates an increase in platelet turnover. Therefore, in a healthy patient, the platelet count and MPV have an inverse relationship. Abnormal platelet size may also indicate the presence of a disorder. MPV and platelet distribution width are both increased in ITP. MPV is also increased in May-Hegglin anomaly, Bernard-Soulier syndrome, myeloproliferative disorders, hyperthyroidism, and pre-eclampsia. MPV is decreased in Wiskott-Aldrich syndrome, septic thrombocytopenia, and hypersplenism. Platelet size and shape may be evaluated by manually performing a platelet estimate as described in the previous section subtitled, Evaluating Platelet Count.
Platelet Function Tests
Platelet dysfunction can be inherited or acquired. Common causes of acquired platelet dysfunction include medications such as aspirin, clopidogrel, and nonsteroidal anti-inflammatory drugs or systemic disorders such as myeloproliferative neoplasms, systemic lupus erythematosus, and uremia. A combination of tests that evaluate platelet function (adhesion, aggregation, flow cytometry) and genetic testing can be used to identify the presence and type of dysfunction and to differentiate between inherited and acquired dysfunction. The bleeding time and clot retraction tests have largely been replaced by
A variety of laboratory methods and specimen types are employed to evaluate platelet aggregation; some methods require platelet-rich plasma, while others are performed using whole blood specimens. Many of the aggregation systems use natural platelet agonists and high shear flow conditions to simulate the normal physiological environment of circulating platelets. Platelet aggregation can be measured in a patient specimen by time to closure or from aggregometers using optical or impedance detection:
Tests That Measure Response to Therapeutics, e.g., Aspirin, P2Y12 Inhibitors, and Glycoprotein (GP) IIb/IIIa Inhibitors
Antiplatelet drugs prevent platelets from clumping and forming clots. These tests are mainly used to identify patients who are nonresponders to drug therapy and therefore may not be good candidates to undertake a specific or combined approach to antiplatelet therapy. The use of platelet function testing to guide the dosing or withdrawal (e.g., establishing time frames for withholding before surgery) of therapeutics is not generally recommended at this time by the American Heart Association (AHA) or the Choosing Wisely Campaign (an initiative of the American Board of Internal Medicine Foundation) due to the lack of strongly supportive data that clearly identify its clinical utility in this regard. According to current AHA guidelines, platelet function testing may be considered for guidance on the timing of cardiac surgery in some patients (e.g., those who have recently received P2Y12 receptor inhibitors or who have ongoing dual antiplatelet therapy). There are current AHA recommendations for the timing of withdrawal from aspirin; guidelines may vary by provider and facility.
Currently, there are three classes of antiplatelet agents: aspirin (acetylsalicylic acid), P2Y12 inhibitors, and GP IIb/IIIa inhibitors.
Aspirin blocks production of thromboxane A2, thereby blocking platelet aggregation. The test uses an agonist to activate platelets in the patient sample. Platelet function is measured by the amount of activated platelet GP IIb/IIIa receptors that bind to fibrinogen-coated microparticles. If aspirin has had the expected antiplatelet effect, aggregation will be reduced.
Thienopyridines are a class of P2Y12 inhibitors (e.g., clopidrogrel [Plavix]), prasugrel (Effient), ticlopidine (Ticlid), and ticagrelor (Brilinta®) whose antiplatelet effect is achieved by inhibiting ADP-mediated platelet activation. P2Y12 inhibitors irreversibly inhibit the binding of ADP to the P2Y12 receptor on the platelet surface, which has the effect of also disrupting platelet degranulation and inhibiting other receptors down the line.
The GP IIb/IIIa inhibitors block platelet aggregation by preventing fibrinogen and vWF from binding to the IIb/IIIa platelet receptor, which disrupts the eventual formation of a stable clot.
Genetic Testing
Genetic test methods to identify drug-related defects in platelet function are mainly used to identify patients who are nonresponders to drug therapy and therefore may not be good candidates to undertake a specific or combined approach to antiplatelet therapy. The use of genetic testing to guide the dosing of antiplatelet therapeutics is not recommended at this time by the American Heart Association (AHA) or the Choosing Wisely Campaign due to the lack of strongly supportive data that clearly identify its clinical utility in this regard.
Next-generation sequencing is used to identify more than 30 genetic variants associated with significant defects in platelet function. Drugs such as clopidogrel (Plavix), abciximab (ReoPro), eptifibatide (Integrilin), and tirofiban block platelet receptor sites and inhibit platelet function. Aspirin also can affect platelet function by the irreversible inactivation of a crucial cyclooxygenase enzyme. Medications such as clopidogrel and aspirin are prescribed to prevent heart attack, stroke, and blockage of coronary stents. Studies have confirmed that up to 30% of patients receiving these medications may be nonresponsive.
The metabolism of many commonly prescribed medications is driven by the cytochrome P450 (CYP450) family of enzymes. Genetic variants can alter enzymatic activity that results in a spectrum of effects ranging from the total absence of drug metabolism to ultrafast metabolism. Impaired drug metabolism can prevent the intended therapeutic effect or even lead to serious adverse drug reactions. Poor metabolizers are at increased risk for drug-induced adverse effects due to accumulation of drug in the blood, while ultra-rapid metabolizers require a higher than normal dosage because the drug is metabolized over a shorter duration than intended. Other genetic phenotypes used to report CYP450 results are intermediate metabolizer and extensive metabolizer. CYP2C19 is a gene in the CYP450 family that metabolizes drugs such as clopidogrel. Genetic testing can be performed on blood samples submitted to a laboratory. Testing for the most common genetic variants of CYP2C19 is used to predict altered enzyme activity and anticipate the most effective therapeutic plan. The test method commonly used is polymerase chain reaction. Counseling and informed written consent are generally required for genetic testing.
Knowledge of genetics assists in identifying those who may benefit from additional education, risk assessment, and counseling. Genetics is the study and identification of genes, genetic mutations, and inheritance. For example, genetics provides some insight into the likelihood of inheriting a tendency to abnormally metabolize drugs. Some conditions are the result of mutations involving a single gene, whereas other conditions may involve multiple genes and/or multiple chromosomes. Further information regarding inheritance of genes can be found in the study titled Genetic Testing.
Platelet Transfusion ReactionRelated to the Therapeutic Use of Platelets in Some IgA-Deficient Individuals
IgA is primarily involved in immune health maintenance of the mucosa that lines the gastrointestinal, genitourinary, and respiratory systems. IgA deficiency is the most common deficiency of the five main immunoglobulin classes (IgA, IgD, IgE, IgG, and IgM). An IgA deficiency-mediated transfusion reaction is a relatively rare type of anaphylaxis. It has been observed when an IgA-deficient recipient is transfused with a platelet product containing anti-IgA antibodies. The reaction usually occurs within 1 hour of transfusion. A reaction does not occur in all patients with an IgA deficiency and can occur along a spectrum ranging from no symptoms or adverse reaction to full-blown, life-threatening anaphylaxis. Commonly transfused blood products, such as platelets, contain small amounts of plasma. Antibodies to IgA and other immunoglobulins are present in the plasma unless they are sourced from
Platelet Count
Platelet Function Tests
Factors That May Alter the Results of the Study
Platelet Count
Other Considerations
Platelet Function Tests
Platelet Count
Increased In
Conditions that involve inflammation activate and increase the number of circulating platelets:
Platelet Function:
Decreased In
Platelet Count
Conditions that are a result of megakaryocytic hypoproliferation:
Conditions that are a result of ineffective thrombopoiesis:
Conditions that are a result of bone marrow replacement:
Conditions that are a result of increased destruction, loss, or consumption:
Conditions that are a result of increased destruction as a result of an immune reaction:
Conditions that are a result of increased destruction as a result of an immune reaction secondary to infection:
Conditions that are a result of increased destruction as a result of other causes:
Platelet Function
Acquired
Inherited
Potential Nursing Problems: Assessment & Nursing Diagnosis
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Before the Study: Planning and Implementation
Teaching the Patient What to Expect
After the Study: Implementation & Evaluation Potential Nursing Actions
Avoiding Complications
Treatment Considerations
Bleeding
Confusion
Pain
Tissue Perfusion
Safety Considerations
Protection
Nutritional Considerations
Clinical Judgement
Follow-Up and Desired Outcomes