A 32-year-old woman is admitted to the hospital with sudden-onset pleuritic chest pain and… — USMLE Step 3

A $32$ -year-old woman is admitted to the hospital with sudden-onset pleuritic chest pain and dyspnea. Computed tomography angiography of the chest reveals a segmental pulmonary embolism. Her medical history is significant for two spontaneous abortions, one at $12$ weeks and another at $14$ weeks of gestation. She takes no medications and has no prior history of bleeding disorders or anticoagulant use.

Initial laboratory studies reveal the following:

Laboratory Test	Patient Result	Reference Range
Hemoglobin	$12.4\text{ g/dL}$	$12.0 - 16.0\text{ g/dL}$
Platelet count	$135,000/\mu\text{L}$	$150,000 - 400,000/\mu\text{L}$
Prothrombin time (PT)	$12.5\text{ seconds}$	$11 - 15\text{ seconds}$
Activated partial thromboplastin time (aPTT)	$76\text{ seconds}$	$25 - 40\text{ seconds}$

A mixing study is performed using equal parts of the patient's plasma and normal pooled plasma. The repeat aPTT immediately after mixing is $72\text{ seconds}$ , and after a $2$ -hour incubation, it remains $73\text{ seconds}$ .

To further evaluate these findings, a dilute Russell viper venom time (dRVVT) test is performed and found to be significantly prolonged.

Which of the following in vitro laboratory interventions will most likely correct this patient's prolonged dRVVT?

Addition of excess phospholipidsAnswer
B
Addition of factor VIII concentrate
C
Incubation with normal pooled plasma for an additional 2 hours
D
Addition of protamine sulfate
E
Addition of activated protein C

Answer

Addition of excess phospholipids

The correct intervention is the addition of excess phospholipids. The patient's clinical picture of venous thromboembolism and recurrent miscarriages strongly suggests antiphospholipid syndrome (APS). A classic laboratory manifestation of APS is the presence of a lupus anticoagulant, an autoantibody that binds to phospholipid-protein complexes. In vivo, this promotes thrombosis, but in vitro, it depletes the phospholipids necessary for coagulation assays, paradoxically prolonging tests like the aPTT and the dilute Russell viper venom time (dRVVT). Because the prolongation is caused by an antibody (an inhibitor), mixing the patient's plasma with normal plasma fails to correct the clotting time. The definitive diagnostic step in the laboratory is demonstrating that the inhibitor is phospholipid-dependent. By adding a high concentration of excess phospholipids to the dRVVT assay, the autoantibodies are saturated, and the clotting time corrects to normal.

Step-by-Step Solution

Evaluate the clinical presentation alongside the basic laboratory findings.

The patient has a pulmonary embolism, a history of recurrent mid-trimester miscarriages, and mild thrombocytopenia. This triad is highly suspicious for antiphospholipid syndrome (APS).

Recognizing the clinical syndrome contextualizes the paradoxical finding of a prolonged aPTT in a patient with a thrombotic, rather than hemorrhagic, presentation.

Interpret the aPTT mixing study results.

The aPTT is significantly prolonged and fails to correct immediately upon mixing with normal pooled plasma. It also remains prolonged after incubation.

Failure to correct indicates the presence of a coagulation inhibitor (e.g., lupus anticoagulant) rather than a simple factor deficiency.

Analyze the mechanism of the dilute Russell viper venom time (dRVVT).

The dRVVT is prolonged. Russell viper venom directly activates Factor X to Xa, bypassing the intrinsic pathway (Factors XII, XI, IX, VIII).

A prolonged dRVVT proves that the inhibitor targets components at or below the level of Factor X (specifically, the phospholipid-dependent prothrombinase complex), definitively ruling out a Factor VIII inhibitor.

Determine the confirmatory laboratory step for lupus anticoagulant.

Adding excess phospholipids to the in vitro assay will correct the prolonged dRVVT.

Lupus anticoagulants are antibodies that bind and neutralize the phospholipids required for the laboratory test. Flooding the assay with excess phospholipids saturates the antibodies, allowing the coagulation cascade to proceed normally.

Key Concept

Interpretation of specialized coagulation assays (mixing studies and dRVVT) to identify lupus anticoagulant in antiphospholipid syndrome.

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