Clinical Pathology: Hematology & Coagulation

• Hemophilia A is caused by a deficiency in factor VIII.

• Hemophilia A is inherited in an X-linked recessive manner so that males are affected more than females. Females can develop hemophilia A by inheriting two affected alleles, having Turner syndrome (XO), or having skewed lyonization.

• Factor deficiencies associated with an elevated activated partial thromboplastin time (aPTT) but a normal prothrombin time (PT) includes factors VIII, IX, XI, and XII, prekallikrein, and high-molecular-weight kininogen deficiencies. Deficiencies in prekallikrein, factor XII, and high-molecular-weight kininogen are not associated with an increased risk for bleeding.

• An elevated partial thromboplastin time (PTT) caused by a factor deficiency can be corrected in vitro by mixing patient plasma with pooled normal plasma (mixing study) if an inhibitor is not present.

• Hemophilia A is classified into severe (<1% activity), moderate (1% to 5%), and mild (>5% to 30%).

Severe hemophilia is associated with spontaneous bleeding.

Moderate hemophilia is associated with bleeding with minor trauma or surgery.

Mild hemophilia usually requires a greater degree of trauma or invasive procedures to cause bleeding.

• Clinical manifestations include hemarthrosis, deep soft tissue hematomas, hematuria, intracranial hemorrhage, and gastrointestinal bleeding.

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