The term 'intrinsic' implies that no extrinsic factor such as tissue factor or thrombin is added to the blood, other than a contact with nonendothelial 'surface'. The clinical test of this pathway is the activated partial thromboplastin time (APTT), also known as the kaolin-cephalin clotting time (KCCT) because kaolin (microparticulated clay) is added as a standard 'surface' and cephalin (brain phospholipid extract) as a substitute for platelet phospholipid. The normal range of the APTT is about 30-50 seconds; prolongations are observed in deficiencies of factors XII (or its cofactors, prekallikrein or HMWK), XI, IX (or its cofactor, factor VIII), X (or its cofactor, factor V), prothrombin (factor II), or fibrinogen (factor I) (see Table 6.1). The test is used to exclude the common congenital hemophilias (deficiencies of factors VIII, IX, or XI; see Table 6.2), and to monitor heparin treatment (see box). Hemophilias caused by factor VIII or IX deficiency occur in about 1 in 10000 males; inheritance is X-linked recessive, transmitted by carrier females. Treatment is usually with factor VIII or IX concentrates.
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