A 72-year-old man is evaluated in the emergency department for a massive hematoma on his r… — USMLE Step 3

A 72-year-old man is evaluated in the emergency department for a massive hematoma on his right thigh that developed after a minor mechanical trauma. He has no prior personal or family history of bleeding disorders and has undergone multiple surgical procedures, including a cholecystectomy and prostatectomy, without excessive hemorrhage. His medical history includes hypertension, for which he takes lisinopril. Vital signs are within normal limits.

Laboratory studies show:
- Hemoglobin: $9.2 \text{ g/dL}$
- Platelet count: $250,000/\mu\text{L}$
- Prothrombin time (PT): $12 \text{ seconds}$ (Normal: $11-14 \text{ seconds}$ )
- Activated partial thromboplastin time (aPTT): $78 \text{ seconds}$ (Normal: $25-35 \text{ seconds}$ )

A mixing study is performed using normal pooled plasma. Immediately after mixing, the patient's aPTT corrects to $32 \text{ seconds}$ . However, after incubating the mixture for $2 \text{ hours}$ at $37^\circ\text{C}$ , the aPTT prolongs again to $68 \text{ seconds}$ .

Which of the following is the most likely primary mechanism underlying this patient's bleeding diathesis?

Autoantibody-mediated neutralization of coagulation factor VIIIAnswer
B
Non-neutralizing autoantibodies directed against phospholipid-binding proteins
C
Inherited deficiency of intrinsic pathway contact activation factors
D
Systemic consumption of plasma coagulation factors and platelets
E
Immune-mediated destruction of platelets by anti-glycoprotein IIb/IIIa antibodies

Answer

Autoantibody-mediated neutralization of coagulation factor VIII

The correct mechanism is autoantibody-mediated neutralization of coagulation factor VIII, which describes Acquired Hemophilia A. This condition typically presents in older adults or postpartum women as severe, sudden-onset bleeding without a prior bleeding history. The laboratory hallmark is an isolated prolonged aPTT. Because the factor VIII autoantibody's neutralizing activity is time- and temperature-dependent, a standard 1:1 mixing study with normal plasma may immediately correct the aPTT (as the normal plasma initially provides enough factor VIII). However, after incubating the mixture for 1 to 2 hours at

37^\circ\text{C}

, the autoantibody has time to bind and neutralize the added factor VIII, causing the aPTT to prolong once again. This kinetic profile distinguishes it from simple factor deficiencies and most lupus anticoagulants.

Step-by-Step Solution

Analyze the clinical presentation.

An elderly patient with no prior bleeding history presents with a severe, spontaneous or minor-trauma-induced hemorrhage (massive hematoma).

This presentation strongly suggests an acquired bleeding diathesis rather than an inherited disorder (which would have manifested during prior surgeries).

Interpret the baseline laboratory values.

The patient has an isolated prolonged aPTT (78 seconds) with a normal PT and normal platelet count.

An isolated prolonged aPTT narrows the differential diagnosis to an intrinsic pathway defect, which can be either a factor deficiency (Factors VIII, IX, XI, XII) or the presence of an inhibitor (Lupus anticoagulant or specific factor autoantibody).

Interpret the kinetics of the mixing study.

The aPTT corrects immediately upon mixing but prolongs again after 2 hours of incubation at

37^\circ\text{C}

Factor deficiencies correct immediately and remain corrected after incubation. Lupus anticoagulants typically fail to correct immediately. A time- and temperature-dependent inhibitor (initial correction followed by prolongation after incubation) is the classic hallmark of an acquired autoantibody against factor VIII (Acquired Hemophilia A).

Key Concept

Acquired Hemophilia A (Factor VIII inhibitor) diagnosis and mixing study interpretation

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