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Difficulty: Very hardOrdering Laboratory Tests

A 34-year-old woman presents to the emergency department with a rapidly expanding hematoma on her right thigh and extensive ecchymoses on her arms and torso. She delivered a healthy infant 4 weeks ago via an uncomplicated vaginal delivery. She has no personal or family history of bleeding disorders and had no excessive bleeding during childbirth, prior menses, or dental procedures. Her only medication is a daily prenatal vitamin. Vital signs are within normal limits. Physical examination reveals a 12-cm tense, tender hematoma on the right anterior thigh.

Laboratory evaluation shows:
- Hemoglobin: 10.2 g/dL
- Platelet count: 280,000/mm³
- Prothrombin time (PT): 12 seconds (normal: 11-14 seconds)
- Activated partial thromboplastin time (aPTT): 68 seconds (normal: 25-35 seconds)
- Thrombin time: 14 seconds (normal: 12-14 seconds)
- Fibrinogen: 350 mg/dL (normal: 200-400 mg/dL)

A 1:1 mixing study with normal pooled plasma is performed. The aPTT immediately corrects to 30 seconds. However, after incubating the mixture for 2 hours at 37°C, the aPTT is 62 seconds.

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

  1. Measurement of factor VIII activity and a Bethesda assayAnswer
  2. B
    Dilute Russell viper venom time and hexagonal phase phospholipid neutralization
  3. C
    Measurement of factor VIII, IX, and XI activity levels
  4. D
    Measurement of von Willebrand factor antigen and ristocetin cofactor activity
  5. E
    Measurement of D-dimer and fibrin degradation products

Answer

Measurement of factor VIII activity and a Bethesda assay
The correct answer involves quantifying the factor VIII activity and the inhibitor titer. The patient has acquired hemophilia A, a rare but life-threatening autoimmune disorder caused by autoantibodies against factor VIII. It classically occurs in older adults, patients with underlying autoimmune diseases, or in the postpartum period (usually 1-4 months after delivery). The laboratory hallmark is an isolated prolonged aPTT with a normal PT. In a mixing study, acquired factor VIII inhibitors exhibit a unique time- and temperature-dependent pattern: the aPTT initially corrects (mimicking a factor deficiency) but prolongs again after a 1-to-2-hour incubation at 37°C. The most appropriate next step to confirm the diagnosis is to measure specific factor VIII activity and quantify the inhibitor using a Bethesda assay.

Step-by-Step Solution

1
Analyze the initial coagulation panel results.
The patient has an isolated prolongation of the activated partial thromboplastin time (aPTT) with a normal prothrombin time (PT), thrombin time, and platelet count.
To localize the coagulation defect to the intrinsic pathway (factors VIII, IX, XI, XII) or identify the presence of a circulating inhibitor.
2
Interpret the 1:1 mixing study results.
The immediate correction of the aPTT followed by significant prolongation after a 2-hour incubation at 37°C indicates the presence of a time- and temperature-dependent inhibitor.
To differentiate between a simple factor deficiency (which shows sustained correction) and an immediate or time-dependent inhibitor.
3
Correlate the laboratory findings with the clinical presentation.
A time-dependent inhibitor in a postpartum patient presenting with sudden, severe soft tissue bleeding is highly characteristic of an acquired factor VIII autoantibody (acquired hemophilia A).
To formulate the most likely clinical diagnosis driving the need for specific confirmatory testing.
4
Determine the most appropriate confirmatory laboratory test sequence.
The next step is to measure specific factor VIII activity levels (which will be low) and quantify the inhibitor titer using a Bethesda assay.
To definitively confirm the diagnosis of acquired hemophilia A and guide subsequent immunosuppressive and hemostatic therapies.

Key Concept

Laboratory evaluation and diagnosis of acquired hemophilia A using a mixing study algorithm
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Ordering Laboratory Tests

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