A 28-year-old man with a family history of bleeding disorder presents with spontaneous hemarthrosis and muscle hematomas. Laboratory tests show normal PT, prolonged aPTT, and normal bleeding time. Mixing study corrects the prolonged aPTT. Which of the following is NOT consistent with a deficiency of a contact factor (Factor XII, prekallikrein, or high-molecular-weight kininogen)?

Explanation

## Contact Factor Deficiencies: Laboratory & Clinical Paradox **Key Point:** Contact factor deficiencies (Factor XII, prekallikrein, HMWK) are characterized by a striking **laboratory-clinical dissociation**: markedly prolonged aPTT but **minimal or no clinical bleeding** — the opposite of what the lab would predict. ### The Contact Factor Paradox | Feature | Finding | |---------|----------| | **aPTT** | Markedly prolonged (often >100 sec) | | **PT** | Normal | | **Bleeding time** | Normal | | **Mixing study** | Corrects (indicates factor deficiency, not inhibitor) | | **Clinical bleeding** | **Absent or minimal** — no spontaneous bleeding, no hemarthrosis, no muscle hematomas | | **Inheritance** | Autosomal recessive | **High-Yield:** This is a **high-yield NEET PG trap**. Students often assume that prolonged aPTT = bleeding risk. Contact factor deficiencies prove this wrong. ### Why Contact Factors Don't Cause Bleeding Contact factors (Factor XII, prekallikrein, HMWK) are part of the **intrinsic pathway** and the **contact activation system** (kallikrein-kinin system). However: 1. **In vivo**, the **extrinsic pathway** (Tissue Factor + Factor VII) is the primary initiator of coagulation 2. Contact factors are important **in vitro** (in the aPTT test) but **not essential in vivo** 3. Patients with contact factor deficiency have normal hemostasis because the extrinsic pathway is intact 4. No spontaneous bleeding, no hemarthrosis, no surgical bleeding ### Clinical Pearl Contact factor deficiency is often discovered **incidentally** during routine preoperative screening when a prolonged aPTT is found in an asymptomatic patient. The key diagnostic clue is the **absence of clinical bleeding despite severe laboratory abnormality**. ### Comparison with True Factor Deficiencies | Factor | aPTT | PT | Bleeding | Mixing | |--------|------|----|---------|---------| | **Factor VIII (Hemophilia A)** | Prolonged | Normal | **Severe** (hemarthrosis, hematomas) | Corrects | | **Factor XII (Contact deficiency)** | **Markedly prolonged** | Normal | **Absent/minimal** | Corrects | | **Factor V** | Prolonged | Prolonged | Severe | Corrects | | **Lupus anticoagulant** | Prolonged | Normal | Thrombosis (not bleeding) | **Does NOT correct** | ### Why Option 0 Is Incorrect "Severe clinical bleeding manifestations despite prolonged aPTT" is **NOT consistent** with contact factor deficiency. In fact, contact factor deficiency is defined by the **absence** of clinical bleeding despite a severely prolonged aPTT. Severe bleeding (hemarthrosis, muscle hematomas) would suggest Factor VIII or IX deficiency (hemophilia), not a contact factor deficiency. ### Why Other Options Are Correct - **Option 1:** Normal PT and normal bleeding time are hallmarks of contact factor deficiency (intrinsic pathway only). - **Option 2:** Correction on mixing study indicates a factor deficiency (not an inhibitor like LA). - **Option 3:** Contact factor deficiencies follow autosomal recessive inheritance (unlike X-linked hemophilia).