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Difficulty: Very hardOrdering Laboratory Tests

A 26-year-old woman is evaluated in the hematology clinic prior to a scheduled elective cholecystectomy due to an incidentally discovered prolonged activated partial thromboplastin time (aPTT) on routine preoperative laboratory screening. She reports a history of heavy menstrual bleeding necessitating the use of oral contraceptives. She also states she experienced prolonged bleeding following a wisdom tooth extraction 5 years ago. She has no history of deep venous thrombosis, joint swelling, or spontaneous bruising. Her family history is unremarkable for known bleeding disorders. Physical examination is within normal limits with no petechiae or ecchymoses.

Laboratory results:
Prothrombin time (PT): 11 seconds (Normal: 11-15 sec)
Activated partial thromboplastin time (aPTT): 48 seconds (Normal: 25-35 sec)
Platelet count: 280,000/μL280,000/\mu L

A 1:11:1 mixing study with normal pooled plasma is performed. The patient's aPTT corrects to 29 seconds immediately, and remains 30 seconds after a 2-hour incubation.

Which of the following is the most appropriate next step in laboratory evaluation?

  1. von Willebrand factor antigen, ristocetin cofactor activity, and Factor VIII coagulant activityAnswer
  2. B
    Dilute Russell viper venom time and hexagonal phase phospholipid neutralization assay
  3. C
    Factor VIII inhibitor assay (Bethesda assay)
  4. D
    Factor IX, Factor XI, and Factor XII coagulant activity assays
  5. E
    Platelet aggregometry and flow cytometry for surface glycoproteins

Answer

The most appropriate next step is to order von Willebrand factor antigen, ristocetin cofactor activity, and Factor VIII coagulant activity.
The correct answer identifies the diagnostic panel for von Willebrand disease (vWD). The patient's clinical presentation (menorrhagia, prolonged bleeding after tooth extraction) is characteristic of a defect in primary hemostasis. Her laboratory evaluation shows an isolated prolonged aPTT with a normal platelet count. The 1:1 mixing study corrects immediately and remains corrected after a 2-hour incubation, confirming a coagulation factor deficiency rather than an inhibitor (which would either fail to correct immediately, or correct initially but prolong after incubation, as seen with Factor VIII autoantibodies). The combination of mucosal bleeding, an isolated prolonged aPTT, and full correction on a mixing study is highly suggestive of vWD. Von Willebrand factor is necessary for platelet adhesion and serves as a carrier protein for Factor VIII, protecting it from rapid degradation. A deficiency in vWF leads to a concomitant Factor VIII deficiency, prolonging the aPTT. Therefore, the most appropriate diagnostic next step is a comprehensive vWD panel.

Step-by-Step Solution

1
Analyze the bleeding history.
The patient has a history of heavy menses and dental bleeding, indicating a primary hemostatic defect (mucosal bleeding).
This helps differentiate between platelet/vWF disorders and coagulation factor deficiencies, which typically cause deep tissue or joint bleeding.
2
Evaluate the baseline coagulation profile.
PT is normal, but aPTT is isolated and prolonged.
An isolated prolonged aPTT points to an abnormality in the intrinsic coagulation pathway (Factors VIII, IX, XI, XII) or von Willebrand disease.
3
Interpret the mixing study results.
The immediate correction and sustained correction at 2 hours confirm a factor deficiency, ruling out both immediate and time-dependent inhibitors.
Immediate inhibitors (like lupus anticoagulant) fail to correct immediately. Time-dependent inhibitors (like acquired FVIII autoantibodies) correct immediately but prolong after incubation.
4
Synthesize clinical and laboratory findings to determine the most likely diagnosis.
A young woman with mucosal bleeding and a confirmed factor deficiency (likely Factor VIII secondary to low vWF) should be tested for von Willebrand disease.
vWD perfectly explains both the clinical phenotype of primary hemostatic bleeding and the prolonged aPTT, making a comprehensive vWD panel the most appropriate targeted workup.

Key Concept

Evaluation of an isolated prolonged aPTT using a mixing study with 2-hour incubation to distinguish factor deficiencies from time-dependent inhibitors, and sequencing diagnostic testing based on clinical presentation.
Estimated Time:1m 30s
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Ordering Laboratory Tests

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