Pathogenesis, Coagulation Screen & Mixing Studies

Question	Answer
1. Define Hemophilia A and Hemophilia B. How are they classified by severity?	- Hemophilia A (Classic Hemophilia): Congenital X-linked recessive deficiency of Factor VIII coagulant activity (FVIII:C) (F8 gene on chromosome Xq28). Accounts for $80-85\%$ of hemophilia cases. - Hemophilia B (Christmas Disease): Congenital X-linked recessive deficiency of Factor IX coagulant activity (FIX:C) (F9 gene on chromosome Xq27). Accounts for $15-20\%$ of cases. - Severity Classification (Based on baseline factor activity): - Severe: $<1.0\%$ ($<0.01\text{ IU/mL}$) — frequent spontaneous bleeds into joints and muscles without trauma. - Moderate: $1.0 - 5.0\%$ ($0.01-0.05\text{ IU/mL}$) — bleeds with minor trauma or minor surgical procedures; spontaneous bleeds rare. - Mild: $>5.0 - 40.0\%$ ($0.05-0.40\text{ IU/mL}$) — bleeds only with major trauma, tooth extractions, or surgery.
2. Contrast the bleeding pattern of Primary Hemostatic Defects from Secondary Hemostatic Defects.
3. What is the typical screening coagulation profile in Hemophilia A?	- Platelet Count: Normal. - Prothrombin Time (PT / INR): Normal (extrinsic and common pathways intact). - Activated Partial Thromboplastin Time (aPTT): Markedly Prolonged (intrinsic pathway defect: Factor VIII, IX, XI, XII deficiency). - Bleeding Time (BT) / PFA-100: Normal. - Thrombin Time (TT): Normal.
4. Explain the 50:50 Plasma Mixing Study (Correction Test) and its clinical significance.	The 50:50 mixing study is the critical laboratory maneuver to differentiate a Factor Deficiency from an Acquired Factor Inhibitor (Antibody): - Patient plasma with a prolonged aPTT is mixed in equal parts ($1:1$) with normal pooled plasma (which contains $100\%$ of all clotting factors). The aPTT is measured immediately and after 2 hours of incubation at $37^\circ\text{C}$. - Correction to Normal: If the aPTT normalizes ($<5\text{ seconds}$ from control), it confirms a True Coagulation Factor Deficiency (Factor VIII, IX, or XI), as even $50\%$ of factor activity is sufficient to yield a normal aPTT. - Failure to Correct: If the aPTT remains prolonged after incubation, it indicates the presence of a circulating Inhibitor / Antibody (e.g., Factor VIII alloantibody or Lupus Anticoagulant) that neutralizes the clotting factors in the normal pooled plasma.
5. VIVA TRAP: Why is Bleeding Time normal in Hemophilia?	Bleeding Time (and PFA-100 closure time) tests primary hemostasis — the initial interaction between the vessel wall and blood platelets leading to platelet adhesion, activation, and formation of the temporary primary hemostatic platelet plug. Because platelet count, platelet function, and von Willebrand factor are completely normal in hemophilia, the primary platelet plug forms normally, resulting in a normal bleeding time. The defect in hemophilia lies in secondary hemostasis (fibrin clot stabilization); hence, the platelet plug fails to be consolidated with fibrin and washes away hours later, causing characteristic delayed bleeding.

Factor Replacement, Prophylaxis & WFH 2020 Guidelines

Question	Answer
6. State the exact mathematical formulas for calculating Factor VIII and Factor IX replacement doses.	- Factor VIII Calculation (Hemophilia A): $$\text{Required Factor VIII Units} = \text{Body Weight (kg)} \times \text{Desired Factor Rise (\%)} \times 0.5$$ (Rule of Thumb: $1\text{ Unit/kg}$ of Factor VIII raises plasma level by $2\%$. In vivo recovery is $100\%$; volume of distribution is intravascular). - Factor IX Calculation (Hemophilia B): $$\text{Required Factor IX Units} = \text{Body Weight (kg)} \times \text{Desired Factor Rise (\%)} \times 1.0$$ (Rule of Thumb: $1\text{ Unit/kg}$ of Factor IX raises plasma level by $1\%$. In vivo recovery is only $50\%$; volume of distribution is extravascular).
7. What are the target factor levels and duration for different bleeding sites in Hemophilia A?
8. What is a "Target Joint" in Hemophilia, and how is it defined?	A Target Joint is defined according to the World Federation of Hemophilia (WFH) as a single joint (most commonly the knee, elbow, or ankle) that has experienced $\ge 3$ spontaneous bleeds into the same joint within a consecutive 6-month period. - Recurrent intra-articular bleeding creates a vicious cycle: free iron/hemosiderin causes chronic synovitis, synovial hypertrophy, and neoangiogenesis with fragile friable capillaries that re-bleed with minimal movement, ultimately causing end-stage Chronic Hemophilic Arthropathy.
9. Contrast Primary Prophylaxis from Secondary Prophylaxis in Hemophilia (WFH 2020 Guidelines).	- Primary Prophylaxis (Standard of Care): Regular, continuous factor replacement initiated before the age of 3 years and prior to the occurrence of the second joint bleed (or documented joint disease). - Regimen: Factor VIII concentrate at $20-40\text{ U/kg}$ IV every 48 hours (or 3 times weekly) to maintain a trough factor level consistently $>1-3\%$. Prevents joint damage entirely. - Secondary Prophylaxis: Regular continuous factor replacement initiated after $\ge 2$ joint bleeds, or after the emergence of a target joint. - Tertiary Prophylaxis: Prophylaxis initiated after documented radiological arthropathy to halt progression.
10. What is Emicizumab (Hemlibra) and how has it revolutionized Hemophilia A care?	Emicizumab is a recombinant, humanized, bispecific monoclonal antibody that bridges activated Factor IX (FIXa) and Factor X (FX), effectively mimicking the cofactor function of missing Factor VIII. - Route of Administration: Subcutaneous injection (eliminates the need for difficult venous access in toddlers). - Dosing Schedule: Loading dose of $3.0\text{ mg/kg}$ once weekly for 4 weeks, followed by maintenance of $1.5\text{ mg/kg}$ once weekly (or $3.0\text{ mg/kg}$ every 2 weeks or $6.0\text{ mg/kg}$ every 4 weeks). - VIVA Breakthrough: Emicizumab does NOT induce Factor VIII inhibitors and is equally effective in patients WITH or WITHOUT Factor VIII inhibitors!
11. What are Factor VIII Inhibitors, and how are they managed?	- Inhibitors: Neutralizing IgG alloantibodies against infused Factor VIII, developing in $25-30\%$ of severe hemophilia A patients (usually within the first 20–50 factor exposure days). Quantified by the Bethesda Assay (1 Bethesda Unit / BU neutralizes $50\%$ of factor activity in normal plasma). - Management: 1) Low-Titer Inhibitors ($<5\text{ BU}$): Can overcome with high-dose Factor VIII infusions. 2) High-Titer Inhibitors ($\ge 5\text{ BU}$): Factor VIII is ineffective. Use Bypassing Agents: Recombinant Activated Factor VII (rFVIIa / NovoSeven, $90\text{ mcg/kg}$ q2-3h) OR Activated Prothrombin Complex Concentrate (aPCC / FEIBA, $50-100\text{ U/kg}$). 3) Definitive Eradication: Immune Tolerance Induction (ITI) — daily high-dose factor infusions ($100-200\text{ U/kg/day}$) for months to induce immune tolerance.
12. VIVA TRAP: Can intramuscular (IM) injections or NSAIDs be given to a child with Hemophilia?	ABSOLUTELY CONTRAINDICATED! 1) Intramuscular Injections: Strictly prohibited because they cause massive, deep intramuscular hematomas with compartment syndrome. All routine vaccines must be given subcutaneously using a fine 25-gauge needle, followed by firm digital pressure for at least 5 minutes. 2) Aspirin and NSAIDs (Ibuprofen, Diclofenac): Strictly contraindicated because they inhibit cyclooxygenase-1 (COX-1), blocking platelet thromboxane A2 synthesis and disabling the patient's only remaining hemostatic defense (primary hemostasis). For analgesia, use Oral Paracetamol or opioids (Tramadol/Codeine).