A 28-year-old man with a family history of bleeding disorder presents with recurrent epistaxis, hemarthrosis, and a prolonged aPTT (58 seconds). PT and platelet count are normal. Mixing study does NOT correct the aPTT (remains 56 seconds after 1:1 mix with normal plasma). Which investigation should be performed next to identify the specific inhibitor?

## Clinical Context The patient presents with: - Recurrent epistaxis and hemarthrosis (joint bleeding) - **Prolonged aPTT that does NOT correct on mixing study** — indicates a **circulating inhibitor** - Normal PT and platelet count - Family history of bleeding disorder ## Interpretation of Non-Correcting Mixing Study **Key Point:** A **non-correcting mixing study** (aPTT remains prolonged even after mixing with normal plasma) indicates a **circulating inhibitor** — an antibody or substance that interferes with coagulation factors. ## Differential Diagnosis: Prolonged aPTT with Non-Correcting Mix | Cause | Clinical Setting | Next Investigation | |-------|------------------|-------------------| | **Factor VIII inhibitor (Alloimmunization)** | Hemophilia A patient on repeated transfusions; or acquired hemophilia | **Bethesda assay** | | **Lupus anticoagulant** | SLE, antiphospholipid syndrome, thrombosis | dRVVT (Dilute Russell Viper Venom Time) | | **Factor V inhibitor** | Rare; post-transfusion or post-antibiotic exposure | Factor V inhibitor assay | | **Heparin contamination** | In vitro artifact | Thrombin time (will be prolonged) | ## Why Bethesda Assay is Correct **High-Yield:** In a patient with: 1. Hemophilic bleeding pattern (hemarthrosis, epistaxis) 2. Prolonged aPTT + **non-correcting mix** 3. Family history of bleeding (suggests hemophilia A) **The most likely diagnosis is a Factor VIII inhibitor** (alloimmunization in a hemophilia A patient, or acquired hemophilia). **The Bethesda assay** is the gold-standard test to: - **Detect** the presence of a Factor VIII inhibitor - **Quantify** inhibitor titer in **Bethesda Units (BU)** - 1 BU = amount of inhibitor that reduces Factor VIII activity by 50% in 2 hours at 37°C - Low titer: <5 BU - High titer: ≥5 BU ## Clinical Pearl **Hemarthrosis (bleeding into joints) is pathognomonic for severe factor deficiencies or inhibitors.** In a hemophilia A patient on factor replacement, development of a non-correcting aPTT with hemarthrosis strongly suggests **Factor VIII inhibitor alloimmunization** — a serious complication that requires specialized management (immunotolerance induction, bypassing agents). ## Mnemonic: Inhibitor vs. Deficiency **CORRECTING MIX** = **D**eficiency → Measure **individual factor** levels **NON-CORRECTING MIX** = **I**nhibitor → Perform **Bethesda assay** (for Factor VIII) or **dRVVT** (for lupus anticoagulant)