A 28-year-old man with a family history of bleeding disorders presents with spontaneous hemarthrosis and muscle hematomas. Prothrombin time (PT) is normal, but activated partial thromboplastin time (aPTT) is prolonged. Platelet count and bleeding time are normal. Which investigation is most appropriate to confirm the diagnosis?

## Investigation of Choice for Prolonged aPTT with Normal PT ### Clinical Presentation This patient has: - Spontaneous hemarthrosis and muscle hematomas (deep tissue bleeding) - Normal PT (extrinsic pathway intact) - Prolonged aPTT (intrinsic pathway defect) - Normal platelet count and bleeding time (primary hemostasis intact) - Family history of bleeding (suggests inherited coagulopathy) This pattern is classic for **Factor VIII, IX, XI, or XII deficiency** (intrinsic pathway factors). ### Why Individual Factor Assays Are Correct **Key Point:** Individual factor assays (VIII, IX, XI, XII levels) are the definitive investigation to identify which intrinsic pathway factor is deficient and confirm the specific diagnosis (hemophilia A, hemophilia B, Factor XI deficiency, or Factor XII deficiency). **High-Yield:** The combination of: - Normal PT (rules out factors II, V, VII, X, fibrinogen) - Prolonged aPTT (indicates deficiency in VIII, IX, XI, or XII) - Normal platelet count and bleeding time (rules out platelet disorders) ...narrows the diagnosis to intrinsic pathway factors. Quantitative assays are needed to identify which factor and severity. **Mnemonic:** **PT = Extrinsic (II, V, VII, X, fibrinogen)**; **aPTT = Intrinsic (VIII, IX, XI, XII) + Common (II, V, X, fibrinogen)** ### Diagnostic Algorithm for Coagulation Defects ```mermaid flowchart TD A[Bleeding disorder]:::outcome --> B{PT & aPTT?}:::decision B -->|Both prolonged| C[Common pathway defect]:::outcome B -->|PT normal, aPTT prolonged| D[Intrinsic pathway defect]:::decision B -->|PT prolonged, aPTT normal| E[Extrinsic pathway defect]:::outcome D --> F[Factor assays: VIII, IX, XI, XII]:::action F --> G[Identify deficient factor]:::outcome C --> H[Fibrinogen, PT, aPTT, TT]:::action ``` ### Why Other Investigations Are Inferior | Investigation | What It Tests | Why Not First-Line | |---|---|---| | Thrombin time (TT) | Fibrinogen level and function | Normal in intrinsic pathway defects; only abnormal if fibrinogen is low or dysfunctional | | Fibrinogen level | Quantitative fibrinogen | Normal in this patient (PT normal implies adequate fibrinogen); does not identify intrinsic factor deficiency | | D-dimer and FDP | Fibrinolysis and DIC | Useful for DIC, thrombosis, or fibrinolytic disorders; not indicated for isolated aPTT prolongation with normal PT | **Clinical Pearl:** Mixing study (patient plasma + normal plasma) can be done first to confirm a factor deficiency (corrects aPTT) vs. inhibitor (does not correct). However, once a factor deficiency is suspected, direct factor assays are the gold standard. **Warning:** Do not confuse aPTT prolongation with Factor V deficiency—Factor V is in the common pathway, so both PT and aPTT would be prolonged.