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Difficulty: HardInterpreting Laboratory Results

A 34-year-old woman presents to the clinic for evaluation of left calf pain and swelling that began two days ago. Venous duplex ultrasonography confirms a deep vein thrombosis. Her medical history is significant for an unprovoked pulmonary embolism 3 years ago and one pregnancy that ended in fetal demise at 14 weeks gestation. She currently takes no medications. Vital signs are within normal limits.

Laboratory results:

TestResultReference Range
White blood cell count6,500/mm36,500/\text{mm}^34,500−11,000/mm34,500-11,000/\text{mm}^3
Hemoglobin12.8 g/dL12.8 \text{ g/dL}12.0−15.5 g/dL12.0-15.5 \text{ g/dL}
Platelet count115,000/mm3115,000/\text{mm}^3150,000−400,000/mm3150,000-400,000/\text{mm}^3
Prothrombin time (PT)13 seconds13 \text{ seconds}11−15 seconds11-15 \text{ seconds}
Activated partial thromboplastin time (aPTT)68 seconds68 \text{ seconds}25−40 seconds25-40 \text{ seconds}

A mixing study is performed, in which the patient's plasma is combined 1:1 with pooled normal plasma. The immediate aPTT of the mixture is 64 seconds64 \text{ seconds}, and the aPTT after a 2-hour incubation remains 65 seconds65 \text{ seconds}.

Based on the interpretation of these laboratory results, which of the following is the most likely explanation for the patient's prolonged aPTT?

  1. In vitro assay interference by autoantibodies targeting phospholipid-protein complexesAnswer
  2. B
    Acquired neutralizing autoantibodies directed against coagulation Factor VIII
  3. C
    Inherited deficiency of an intrinsic pathway coagulation factor
  4. D
    Systemic activation of the coagulation cascade leading to factor consumption
  5. E
    Impaired hepatic synthesis of vitamin K-dependent coagulation factors

Answer

In vitro assay interference by autoantibodies targeting phospholipid-protein complexes
The patient's clinical presentation of unprovoked DVT, previous pulmonary embolism, and mid-trimester fetal demise is classic for antiphospholipid syndrome (APS). A hallmark laboratory finding in APS is a prolonged activated partial thromboplastin time (aPTT) that fails to correct when the patient's plasma is mixed with normal plasma. This failure to correct indicates the presence of a circulating inhibitor rather than a coagulation factor deficiency. In APS, the inhibitor is the lupus anticoagulant, an autoantibody that binds to phospholipid-protein complexes. Because the aPTT assay relies on phospholipids to catalyze the coagulation cascade, these antibodies interfere with the assay in vitro, leading to an artificially prolonged aPTT. Paradoxically, in vivo, these antibodies create a highly prothrombotic state. The patient's mild thrombocytopenia is also a common associated manifestation of APS.

Step-by-Step Solution

1
Identify the patient's clinical syndrome based on her history.
The patient has a history of unprovoked DVT, pulmonary embolism, and mid-trimester fetal loss, indicating a highly hypercoagulable state.
Establishing the clinical phenotype is necessary to interpret the laboratory abnormalities in the correct context.
2
Interpret the basic coagulation panel.
The patient has an isolated prolongation of the aPTT with a normal PT and mild thrombocytopenia.
Isolating the specific pathway involved narrows the differential diagnosis.
3
Analyze the mixing study results.
The prolonged aPTT fails to correct when mixed 1:1 with normal plasma.
A failure to correct distinguishes the presence of a circulating inhibitor (antibody) from a simple coagulation factor deficiency.
4
Synthesize the laboratory and clinical findings to determine the underlying mechanism.
An inhibitor causing isolated aPTT prolongation in a patient with severe thrombosis and fetal loss is characteristic of the lupus anticoagulant seen in antiphospholipid syndrome.
Lupus anticoagulant binds to assay phospholipids in vitro (delaying clot formation) but promotes thrombosis in vivo.

Key Concept

Interpretation of mixing studies to differentiate factor deficiencies from circulating inhibitors, specifically identifying lupus anticoagulant in antiphospholipid syndrome.
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Interpreting Laboratory Results

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