U01.11.045 Antiphospholipid syndrome

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Learning Objectives

Differentiate between primary and secondary Antiphospholipid Syndrome (APS). Identify the clinical hallmarks of thrombosis and recurrent pregnancy loss, and master the laboratory paradoxes, such as the prolonged PTT and false-positive VDRL.

1. Pathogenesis & Clinical Criteria

Antiphospholipid Syndrome (APS) is a systemic autoimmune disorder characterized by a “hypercoagulable state.” It can be a primary disorder or secondary to other autoimmune conditions, most commonly SLE.

  • Clinical History: Diagnosis requires at least one clinical event:
    • Thrombosis: Any history of arterial or venous thromboembolism (e.g., DVT, PE, Stroke).
    • Obstetric Complications: Recurrent miscarriages (abortions) or unexplained fetal death.
  • Treatment: Requires long-term systemic anticoagulation (e.g., Warfarin) to prevent further clotting events.

2. Laboratory Findings & Paradoxes

APS is well known in medical board exams for laboratory findings that appear contradictory to the patient’s clinical state.

Lab Finding Explanation / High-Yield Note
Lupus Anticoagulant Causes a prolonged PTT in vitro. Paradoxically, the patient is pro-thrombotic (clotting) in vivo. The PTT does not correct with a 1:1 mixing study.
Anticardiolipin Antibodies It can cause a false-positive VDRL/RPR (syphilis screen) because the test uses cardiolipin as an antigen.
Anti-\beta_2 Glycoprotein I One of the three required diagnostic antibodies for APS.

Activity: Interpreting the Mixing Study – APS vs. Factor Deficiency

High-Yield Mnemonics:

  • The APS Paradox: “Long PTT in the lab, but Big Clots in the patient.”
  • False Syphilis: “If the VDRL is positive but the FTA-ABS is negative in a woman with clots, think APS.”
  • Secondary APS: Always screen for SLE in a young female with new-onset thrombosis.

Activity