Platelet Count

Platelet Count

CPT Test code: 85049

Related Information:
Specimen: Whole blood. Peripheral blood smears prepared at the time of collection may also be useful, particularly when platelet aggregation is a problem.
Volume: Tube fill capacity
Minimum Volume: 0.5 mL
Container: Lavender-top (EDTA) tube.
Special Instructions: For citrated platelet counts, order 115345.
Collection: Invert tube immediately 8 to 10 times once tube is filled at time of collection.
Storage Instructions: Maintain specimen at room temperature.
Causes for Rejection: Hemolysis; clotted specimen; tube not filled with minimum volume; improper labeling; transfer tubes with whole blood; specimen diluted or contaminated with IV fluid; specimen received with plasma removed; specimen collected in any anticoagulant other than EDTA)
Use: Evaluate, diagnose, and/or follow up bleeding disorders, drug-induced thrombocytopenia, idiopathic thrombocytopenia purpura (acute or chronic), disseminated intravascular coagulation, leukemia states, chemotherapeutic management of malignant disease states; investigate purpura, petechiae; evaluate response to platelet transfusions, steroids, or other therapy
Limitations: Clumping may cause false low count.1 Platelet satellitism around neutrophils will cause a pseudothrombocytopenia. RBC or WBC fragments including fragmented fragile leukemic cells and neutrophil pseudoplatelets2 may cause falsely elevated counts.
Additional Information: The platelet, of growing practical clinical importance in hemostatic considerations and a variety of medical/surgical processes is also fundamental to etiologic considerations of arteriosclerotic3 and malignant disease.4 Order 115345 for citrated platelet count.

Careful estimate of platelet number from stained peripheral blood smear can provide useful information. A variety of factors affect the distribution of platelets on a peripheral blood smear, and thus platelet estimates lack precision. Capillary blood platelet counts (c.f. to venous blood counts) may be significantly underestimated. Platelets are often clumped on smears obtained from capillary blood, contributing to imprecision. A small whole blood clot or very small fibrin clots in the EDTA anticoagulated specimen will usually be associated with clumping of platelets on the slide, and with a false low platelet count.

Quantitative platelet disorders have varied etiology. Thrombocytopenia may have an immunologic basis, the result of production deficiency due to the effect of drugs or physical agents, abnormal platelet pooling or increased destruction (eg, sequestration by large vascular tumor), or result from a variety of probably nonimmunologic mechanisms (eg, hypersplenism). Decreases may occur after bleeding, transfusion, infections, or relating to defective production of or regulation by thrombopoietin.

Drugs and chemicals associated with thrombocytopenia, often on an immune mediated basis5 or as the result of marrow suppression, include quinidine, quinine, heparin, gold salts, sulfas, rifampicin, ASA, digitoxin, apronal, chlorothiazides, chlorpropamide, meprobamate, antihistamines, chloramphenicol, penicillin, DDT, benzol, a variety of other industrial organic chemicals, diphenylhydantoin, PAS, hydrochlorothiazide, phenylbutazone, and a variety of antineoplastic chemotherapeutic agents. ASA acts by acetylating cyclo-oxygenase.

Thrombocytosis is less common, but likewise varied in etiology: physiologic (eg, postpartum, or after exercise); myeloproliferative syndromes (eg, thrombocythemia, some cases of chronic myelogenous leukemia, myelofibrosis with myeloid metaplasia); rebound following thrombocytopenia, marrow regenerative activity after bleeding episode, hemophilia, iron deficiency; asplenism, infections, inflammatory or malignant disease, especially carcinomatosis. Oral contraceptives may cause slight increase.

Congenital causes of thrombocytopenia include Wiskott-Aldrich syndrome, May-Hegglin anomaly, thrombocytopenia with absent radius, and Bernard-Soulier syndrome. See table.

Inherited Abnormalities of Platelet Production (Characterized by Thrombocytopenia)
Condition Inheritance Abnormality Therapy
Adapted from Penner J. Blood Coagulation Laboratory Manual. University of Michigan Medical School; Sep, 1979.
May-Hegglin Autosomal Dominant Severe thrombocytopenia Platelet replacement
Wiskott-Aldrich Sex-linked Severe thrombocytopenia with small platelets Possibly splenectomy
Congenital thrombopoietin deficiency ? Autorecessive Severe thrombocytopenia Plasma transfusion
Thrombocytopenia with absent radius Autorecessive Moderate thrombocytopenia Platelet replacement
Abnormalities of Platelet Function, Familial Transmission, Autorecessive
Thrombasthenia Absent clot retraction, absent aggregation, mild thrombocytopenia Platelet replacement, steroids
Bernard-Soulier syndrome Giant platelets, absent Ristocetin® aggregation Platelet replacement
Platelet storage pool disease Absent aggregation with collagen, mild thrombocytopenia, absent dense granules with decreased platelet serotonin Splenectomy, platelet replacement
Hermansky-Pudlak syndrome Aggregation abnormal with epinephrine and collagen, decreased dense granules and absent ADP stores Platelet replacement
Release reaction abnormalities Absent second wave aggregation with epinephrine and collagen, absent PF-3 release, varied inheritance Platelet replacement
Footnotes: 1. Solanki DL, Blackburn BC. Spurious leukocytosis and thrombocytopenia. A dual phenomenon caused by clumping of platelets in vitro. JAMA. 1983 Nov 11; 250(18):2514-2515.PubMed 6632146

2. Merz B. Newly identified particle may explain spurious platelet count. JAMA. 1983 Jun 17; 249(23):3146-3147. PubMed 6854834

3. Halushka PV, Knapp DR, Grimm L. Prostaglandins, thromboxanes, and platelet function. Curr Top Hematol.1979; 2:75-143.

4. Doolittle RF, Hunkapiller MW, Hood LE, et al. Simian sarcoma virus onc gene, v-sis, is derived from the gene (or genes) encoding a platelet-derived growth factor. Science. 1983 Jul 15, 221(4607):275-277. PubMed 6304883

5. Moss RA. Drug-induced immune thrombocytopenia. Am J Hematol. 1980; 9(4):439-446.PubMed 6452056