Screening laboratory studies for bleeding patients include a platelet count, prothrombin time, partial thromboplastin time, fibrinogen, and bleeding time or other screening test of platelet function. Many laboratories have adopted the platelet function analyzer (PFA) to replace the bleeding time as a screening test for platelet function abnormalities and
Figure 151-2 Simplified pathways of blood coagulation. The area inside the solid black line is the intrinsic pathway measured by the activated partial thromboplastin time (APTT). The area inside the green line is the extrinsic pathway, measured by the prothrombin time (PT). The area encompassed by both lines is the common pathway. AT-III, Antithrombin III; F, factor; HMWK, high-molecular-weight kininogen; P-C/S, protein C/S; PL, phospholipid; TFPI, tissue factor pathway inhibitor. (Modified from Scott JP, Montgomery RR: Hemorrhagic and thrombotic diseases. In Kliegman RM, Behrman RE, Jenson HB, Stanton BF, editors: Nelson Textbook of Pediatrics, ed 18, Philadelphia, 2007, Saunders, p 2061.)
Prothrombin time Extrinsic and common pathway <12 sec beyond neonate; Defect in vitamin K–dependent factors; 12–18 sec in term neonate hemorrhagic disease of newborn, malabsorption, liver disease, DIC, oral anticoagulants, ingestion of rat poison
Thrombin time Fibrinogen to fibrin conversion 10–15 sec beyond neonate; Fibrin split products, DIC, 12–17 sec in term neonate hypofibrinogenemia, heparin, uremia
Bleeding time Hemostasis, capillary and platelet 3–7 min beyond neonate Platelet dysfunction, thrombocytopenia, von function
Willebrand disease, aspirin
Platelet count Platelet number 150,000–450,000/mL Thrombocytopenia differential diagnosis (see Fig. 151-4) von Willebrand disease. The PFA has variable sensitivity and specificity for common bleeding disorders. No single laboratory test can screen for all bleeding disorders. The findings on screening tests for bleeding vary with the specific disorder (Table 151-2).
From Scott JP: Bleeding and thrombosis. In Kliegman RM, editor: Practical Strategies in Pediatric Diagnosis and Therapy, Philadelphia, 1996, WB Saunders.
Disorders of Platelets
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Platelet counts less than 150,000/mm3 constitute thrombocytopenia. Mucocutaneous bleeding is the hallmark of platelet disorders, including thrombocytopenia. The risk of bleeding correlates imperfectly with the platelet count. Children with platelet counts greater than 80,000/mm3 are able to withstand all but the most extreme hemostatic challenges, such as surgery or major trauma. Children with platelet counts less than 20,000/mm3 are at risk for spontaneous bleeding. These generalizations are modified by factors such as the age of the platelets (young, large platelets usually function better than old ones) and the presence of inhibitors of platelet function, such as antibodies, drugs (especially aspirin), fibrin degradation products, and toxins formed in the presence of hepatic or renal disease. The size of platelets is routinely measured as the mean platelet volume (MPV). The etiology of thrombocytopenia (Fig. 151-4) may be organized into three mechanisms: 1.
2.
3.