Hemophilia Calculators

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Hemophilia is an X-linked recessive bleeding disorder caused by a deficiency or dysfunction of specific blood clotting factors. Hemophilia A (the most common form, ~1 in 5,000 male births) results from deficiency of factor VIII; hemophilia B (Christmas disease, ~1 in 25,000 male births) from deficiency of factor IX. Both prevent the formation of a stable fibrin clot, causing prolonged bleeding after injury and spontaneous hemorrhage into joints (hemarthrosis) and muscles. Treatment involves replacement therapy with factor concentrates or, increasingly, non-factor therapies (emicizumab). Gene therapy has recently achieved long-term hemostasis correction in clinical trials.

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Genetics of Hemophilia

Both hemophilia A and B are caused by mutations on the X chromosome (factor VIII gene at Xq28; factor IX gene at Xq27). X-linked recessive pattern: males (XY) with one mutant X are affected. Females (XX) with one mutant X are usually unaffected carriers with ~50% factor levels. Carrier females can occasionally be symptomatic ('manifesting carriers') if their normal X is preferentially inactivated. Daughters of affected males are obligate carriers; sons are unaffected (receive Y from father). New mutations (de novo) account for ~30% of hemophilia A cases.

Severity Classification

Based on residual factor activity: Severe: < 1% factor activity; spontaneous bleeding into joints and muscles; most significant morbidity. Moderate: 1–5%; bleeding with minor trauma. Mild: 5–40%; bleeding with significant trauma or surgery; often undiagnosed until a bleeding challenge.

Treatment

  • Standard half-life (SHL) factor concentrates: IV infusions 2–3×/week (prophylaxis) or on demand for bleeding
  • Extended half-life (EHL) factors: modified with PEGylation or Fc fusion; once or twice weekly dosing
  • Emicizumab (Hemlibra): bispecific antibody mimicking factor VIII; subcutaneous; weekly/biweekly; works for hemophilia A with or without inhibitors
  • Gene therapy: AAV vectors delivering factor VIII or IX cDNA; achieving near-normal factor levels for multiple years in clinical trials

Glossary

Hemophilia A
An X-linked recessive bleeding disorder caused by factor VIII deficiency; ~1 in 5,000 males; severity based on residual factor activity (<1% severe; 1–5% moderate; 5–40% mild).
Emicizumab (Hemlibra)
A bispecific antibody mimicking factor VIII activity; subcutaneous injection for hemophilia A prophylaxis; effective with or without inhibitors; replaced frequent IV factor infusions for many patients.
Hemarthrosis
Bleeding into joints; the hallmark complication of severe hemophilia; most commonly affects knees, elbows, and ankles; repeated episodes cause synovitis, cartilage destruction, and hemophilic arthropathy.

Frequently Asked Questions

Hemophilia is an inherited bleeding disorder where blood cannot clot normally. Hemophilia A: deficiency of factor VIII; most common; caused by mutations (including large inversions) in the F8 gene on chromosome Xq28. Hemophilia B: deficiency of factor IX; caused by mutations in F9 gene on Xq27. Both are X-linked recessive — primarily affect males. The coagulation defect: in the intrinsic pathway, factor VIII (with factor IX) activates factor X → factor Xa → with factor Va converts prothrombin to thrombin → thrombin converts fibrinogen to fibrin. Without adequate FVIII or FIX, this amplification step fails → weak or no clot.

Hemophilia A and B follow X-linked recessive inheritance. X-linked pattern: the mutation is on the X chromosome. Males (XY): have only one X → one mutant copy = affected. Females (XX): need two mutant copies to be affected; one mutant = carrier (usually unaffected with ~50% factor levels). Transmission: affected father → all daughters are obligate carriers (receive mutant X from father); carrier mother → 50% of sons affected; 50% of daughters are carriers. De novo mutations (no family history) account for ~30% of hemophilia A cases. Daughters of affected males are never affected (receive Y from father, not mutant X); sons of affected males are unaffected (receive Y from father).

Clinical features depend on severity: Severe hemophilia (< 1% factor): spontaneous hemarthrosis (joint bleeding → synovitis → joint destruction, 'hemophilic arthropathy'); muscle hematomas (iliopsoas hemorrhage); intracranial hemorrhage (~10% lifetime risk). Moderate (1–5%): bleeding with minor trauma or mild surgery. Mild (5–40%): significant bleeding only with major trauma, surgery, or dental extraction; often undiagnosed until late. Distinctive features: bleeding into joints (knees, elbows, ankles most common); deep tissue bleeds (distinguishing from platelet disorders which cause mucocutaneous bleeding — petechiae, gingival). PT (prothrombin time) is NORMAL (extrinsic pathway intact); PTT (partial thromboplastin time) is prolonged (intrinsic pathway defective).

Prophylactic (preventive) factor replacement is the standard of care for severe hemophilia: Standard half-life (SHL) products: recombinant FVIII or FIX; IV infusions 3×/week (FVIII) or 2×/week (FIX). Extended half-life (EHL) products: modified with PEGylation, Fc fusion, or albumin fusion; once or twice weekly dosing; improved adherence. Non-factor therapy — emicizumab (Hemlibra): a bispecific antibody bridging FIXa and FX, mimicking FVIII function; subcutaneous injection weekly/biweekly/monthly; eliminates need for IV access; effective even with inhibitors (neutralizing antibodies against factor concentrates). Gene therapy: AAV5-FIX (Hemgenix) — FDA approved 2022 for hemophilia B; single IV infusion; sustained FIX levels for 3+ years in trials; $3.5M per dose — most expensive approved drug at approval.