A 17-year-old girl is brought to the emergency department due to acute abdominal pain, nausea, and vomiting. She has no chronic medical conditions and has had no surgery. The patient has had irregular menstruation and is not sexually active. Family history is significant for deep venous thrombosis following knee replacement in her mother, who takes apixaban. Temperature is 38.3 C (101 F). BMI is 18 kg/m². Physical examination shows right lower quadrant abdominal tenderness. CT scan of the abdomen reveals acute appendicitis, and laparoscopic appendectomy is planned. Preoperative laboratory studies demonstrate a platelet count of 400,000/mm³. Coagulation study results are as follows: Mixing the patient's plasma sample with normal plasma fails to correct the coagulation abnormalities. Which of the following best explains this patient's coagulation study results?

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11ec25c8_d40c_5e63_bc81_913829791149_MD0003_00 This patient's prolonged activated PTT does not correct when mixed with normal plasma. Normal plasma provides coagulation factors; therefore, it corrects prolonged PTT in the setting of coagulation factor deficiency but fails to correct prolonged PTT in the setting of a coagulation inhibitor. One of the most common coagulation inhibitors is lupus anticoagulant (LA), a type of an antiphospholipid antibody. LA binds to the phospholipids used in most PTT tests and prevents them from inducing coagulation (thereby prolonging PTT). This laboratory artifact is not improved with the addition of plasma (mixing test) but is resolved with the addition of phospholipids (which eventually overwhelm all the antibody binding sites). Patients with antiphospholipid antibodies may remain asymptomatic but are at increased risk for antiphospholipid-antibody syndrome, which is associated with thromboembolism and fetal loss. Most cases occur sporadically; this patient's mother's deep venous thrombus likely was related to her orthopedic surgery rather than a clotting disorder. Furthermore, the patient's thrombophilia is likely a byproduct of her infection rather than of a platelet disorder. (Choice B) Most patients with von Willebrand deficiency are asymptomatic, but a minority develop easy bruising or mucocutaneous bleeding. PTT is sometimes prolonged because von Willebrand factor acts as a carrier for clotting factor VIII (degradation is increased in the setting of reduced von Willebrand factor). However, a mixing study would correct PTT. (Choice C) Hemophilia B is an inherited deficiency of clotting factor IX. Although it causes prolonged PTT, it corrects with mixing studies. Furthermore, almost all cases present in early childhood and occur in male children (because it is X-linked recessive). (Choice D) Vitamin K is found in green, leafy vegetables and is required to generate clotting factors VII, IX, X, and prothrombin. Although deficiency is rare in healthy individuals, it usually manifests with prolonged PT because of clotting factor VII deficiency. In severe cases, PTT can be prolonged, but both PT and PTT would correct with mixing studies. (Choice E) Infections can sometimes cause disseminated intravascular coagulation, which is marked by the excessive consumption of clotting factors. However, most patients have thrombocytopenia (due to the formation of intravascular microthrombi) and prolonged PT and PTT. Furthermore, signs of bleeding (eg, oozing at venipuncture sites) and thrombosis are often present. Educational objective: Patients with antiphospholipid antibodies (eg, lupus anticoagulant) have prolonged PTT that fail to correct with a mixing test (combination with normal plasma). Most patients with antiphospholipid antibodies are asymptomatic, but a minority develop antiphospholipid-antibody syndrome (thromboembolism, fetal loss).

A 17-Year-Old Girl Is Brought to the Emergency Department Due