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Difficulty: MediumFormulating a Differential Diagnosis

During a specialized hematology clinic visit, a 31-year-old female teacher is evaluated for a recent onset of left-sided calf swelling and erythema. A compressive ultrasound confirms an occlusive thrombus in the left femoral vein. Her past medical history is notable for two first-trimester pregnancy losses and a transient ischemic attack four years ago. She currently takes no daily medications. Physical examination is unremarkable except for the left lower extremity edema. Initial laboratory analysis reveals a normal prothrombin time (PT) but an activated partial thromboplastin time (aPTT) of 62 seconds (reference: 25-40 seconds). When the patient's plasma is mixed in a 1:1 ratio with normal pooled plasma, the aPTT remains prolonged at 58 seconds. Which of the following is the most likely diagnosis?

  1. Antiphospholipid syndromeAnswer
  2. B
    Factor V Leiden thrombophilia
  3. C
    Factor VIII deficiency
  4. D
    Protein C deficiency
  5. E
    von Willebrand disease

Answer

Antiphospholipid syndrome
The correct diagnosis is antiphospholipid syndrome (APS). This patient's clinical history of recurrent miscarriages, transient ischemic attack (arterial thrombosis), and current deep vein thrombosis (venous thrombosis) is highly suspicious for a systemic hypercoagulable state. The laboratory findings clinch the diagnosis: a prolonged aPTT that fails to correct with a 1:1 mixing study indicates the presence of a circulating inhibitor rather than a factor deficiency. In the context of robust clinical thrombosis, this inhibitor is the lupus anticoagulant, which paradoxically prolongs phospholipid-dependent coagulation tests in vitro while promoting severe hypercoagulability in vivo.

Step-by-Step Solution

1
Analyze the patient's clinical history.
The patient has a history of unprovoked arterial (transient ischemic attack) and venous (deep vein thrombosis) thromboses, along with recurrent first-trimester miscarriages.
Identifying the clinical syndrome is the fundamental first step in establishing a differential diagnosis for a hypercoagulable state.
2
Interpret the baseline coagulation laboratory results.
The prothrombin time (PT) is normal, but the activated partial thromboplastin time (aPTT) is significantly prolonged.
This narrows the differential diagnosis exclusively to conditions affecting the intrinsic coagulation pathway or to the presence of an interfering coagulation inhibitor.
3
Evaluate the results of the 1:1 mixing study.
The prolonged aPTT fails to correct upon mixing with normal pooled plasma.
A failure to correct definitively indicates the presence of a circulating coagulation inhibitor rather than a simple coagulation factor deficiency.
4
Synthesize the clinical and laboratory data to formulate the final diagnosis.
The combination of thromboses, recurrent pregnancy loss, a paradoxically prolonged aPTT, and an inhibitor pattern on a mixing study points to the lupus anticoagulant.
These findings fulfill both the clinical and laboratory criteria for diagnosing antiphospholipid syndrome.

Key Concept

Formulating a differential diagnosis for hypercoagulability by integrating clinical history (thrombosis, pregnancy loss) with coagulation studies (mixing study distinguishing factor deficiency from a circulating inhibitor like lupus anticoagulant).
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Formulating a Differential Diagnosis

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