Platelet Count and Tests of Platelet Function


Core Lab Study


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 Manual—a 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.

AgePlatelet Count*SI Units (Conventional Units × 1)MPV (fL)IPF (%)
Birth150–300 × 103/microL150–300 × 109/L8.1–12.21.6–7.1
6–23 mo200–450 × 103/microL200–450 × 103/microL8.1–12.21.7–4.8
2–5 yr150–400 × 103/microL150–400 × 103/microL8.1–12.21.3–3.9
6–17 yr150–450 × 103/microL150–450 × 103/microL8.1–12.21.3–6.7
Adult, older adult150–450 × 103/microL150–450 × 109/L8.1–12.21.1–11.1
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 transfusion—documentation should clearly reflect the time and date of the last transfusion with respect to the collection time of the study.
Summary: Tests of Platelet Function
Study TypeInterpretation of ResultsComments
Platelet Function-Closure Time (screening test)Normal Closure Time: variable, based on agonist and method, e.g., Collagen/ADP 60–130 sec by one method and 77–133 sec by another; Collagen/Epinephrine 80–180 sec by one method and 98–185 sec for anotherThe screening test is used to identify abnormal bleeding tendency. Closure times can be used to distinguish between acquired (e.g., inhibitory drugs) and hereditary platelet defects
ADP = adenosine diphosphate
Summary Tests of Platelet Function: Aggregation Studies
Normal Platelet Aggregation and ATP Release Range (varies by laboratory/method): Two representative examples
AgonistExample #1 Aggregation (%)Example #1 ATP Release (nmol)Example #2 Aggregation (%)Example #2 ATP Release (nmol)
ADP45–940.19–1.4567–97Less than 2.15
Arachidonic42–790.04–1.172–94Less than 1.27
Epinephrine48–93Greater than 0.5267–880.19–2.63
Ristocetin (High)30–100N/A73–104N/A
Ristocetin (Low)Less than 5N/AN/AN/A
Platelet aggregation testing often requires a coagulation consult with a pathologist prior to scheduling specimen collection. The consultation includes completion of a patient history form, specifically detailing patient/family history of abnormal bleeding, and the patient’s complete medication history. Normal aggregation response varies by agonist and method. (Note: abnormal aggregation results may include interpretation by a pathologist.)
Summary Continued Tests of Platelet Function: Flow Cytometry
Study TypeInterpretation of ResultsComments
Platelet surface glycoprotein (GP) receptor expression. Common receptors marked for evaluation include GPIa (CD49b); GPIb (CD42b); GPIIb (CD41); GPIIIa (CD61); GPVI (CD-NA); GPIX (CD42a)Platelet surface glycoprotein expression = Normal (Some laboratories report Greater Than or Equal to 70.0%) Abnormal findings may be supplemented with an interpretation by a pathologist.Flow cytometry is used to identify hereditary platelet disorders by known patterns of significantly decreased receptor expression, e.g., low levels of GPIIb and GPIIIa expression are associated with Glanzmann’s thrombasthenia; low levels of GPIX and GPIb are associated with Bernard-Soulier syndrome
Note: platelet receptors can also be evaluated using electron microscopy.
Tests to Evaluate the Effectiveness of Antiplatelet Therapy
Study TypeInterpretation of ResultsComments
VerifyNow® Platelet Reactivity to AspirinMeasured in Aspirin Reactivity Units (ARU): Less than 550 ARU = consistent with aspirin-induced inhibition of platelet function; Greater than or equal to 550 ARU = platelet dysfunction related to aspirin has not been detectedIdentifies platelet dysfunction due to the effects of aspirin
Verify Now® P2Y12 Inhibitors (e.g., clopidogrel (Plavix), prasugrel (Effient), ticagrelor (Brilinta), and ticlopidine (Ticlid).Measured in Platelet Reactivity Units (PRU): Baseline (prior to start of drug administration): 194–418 PRU Therapeutic Effect: Less than 194 PRURecommendations for special circumstances:
  • Long-term clopidogrel therapy Equal to or Less than 180 PRU = adequate response
  • Equal to or Greater than 208 PRU with pending cardiac surgery = may proceed
  • Less than 208 PRU with pending surgery = delay and retest in 12–24 hr
Results should be interpreted with other available clinical and laboratory information.

Critical Findings and Potential Interventions
Platelet Count

  • Less than 30 × 103/microL (SI: Less than 30 × 109/L)
  • Greater than 1,000 × 103/microL (SI: Greater than 1,000 × 109/L)

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.)

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