Hemophilia A vs Hemophilia B
How to tell Hemophilia A and Hemophilia B apart when a board question makes them look alike.
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Specific clotting factor assay, low factor VIII versus low factor IX, is the definitive 5-second discriminator.
Side by side
Male patient with deep tissue bleeding and isolated prolonged aPTT has hemophilia A when factor VIII activity is low.
The same bleeding phenotype with low factor IX activity identifies hemophilia B.
X-linked deficiency of factor VIII impairs intrinsic tenase activity and thrombin generation.
X-linked deficiency of factor IX reduces intrinsic pathway amplification despite normal platelet function.
aPTT is prolonged and corrects on mixing study, with factor VIII activity below 40% and severe disease often below 1% activity.
aPTT is prolonged and corrects on mixing study, with factor IX activity below 40% and severe disease often below 1% activity.
The trap
A common mistake is trying to separate these disorders by bleeding pattern alone. Both cause hemarthroses and soft tissue bleeding, so targeted factor assay is required before replacement selection.
The repair move
Next best step: order factor VIII and IX activity assays, then replace the deficient factor immediately during active bleeding. For hemarthrosis, give recombinant factor VIII 25 to 40 IU/kg for hemophilia A or recombinant factor IX 40 to 60 IU/kg for hemophilia B, then recheck factor activity and joint symptoms.
Try a free question
CoreA teenage boy with recurrent hemarthroses has normal platelet count, normal PT, and prolonged aPTT that corrects with mixing. What is the next best step in management?
Show answer and explanation
Answer: A. Order factor VIII and IX activity levels. Corrected mixing study indicates factor deficiency, and factor assays are required to identify hemophilia A versus B before definitive replacement.
Why the other choices are tempting
- B. Wrong because immune thrombocytopenia is not supported by normal platelet count and prolonged aPTT pattern.
- C. Wrong because platelet transfusion does not correct intrinsic factor deficiency.
- D. Wrong because vitamin K affects factors II, VII, IX, and X but does not address congenital VIII deficiency and is not the diagnostic next step.
- E. Wrong because desmopressin may help mild hemophilia A only and should not precede subtype confirmation in this scenario.
Flashcards
Tap a card to flip it.
Which test definitively distinguishes hemophilia A from hemophilia B when both show prolonged aPTT?
Quantitative factor assay identifies the deficient protein: factor VIII for A and factor IX for B.
What does a corrected mixing study indicate in classic hemophilia?
Correction indicates a factor deficiency rather than an inhibitor.
Quick answers
What is the fastest way to tell Hemophilia A from Hemophilia B?
Specific clotting factor assay, low factor VIII versus low factor IX, is the definitive 5-second discriminator.
What is the key clue for Hemophilia A?
Male patient with deep tissue bleeding and isolated prolonged aPTT has hemophilia A when factor VIII activity is low.
What is the key clue for Hemophilia B?
The same bleeding phenotype with low factor IX activity identifies hemophilia B.
What is the common trap with Hemophilia A vs Hemophilia B?
A common mistake is trying to separate these disorders by bleeding pattern alone. Both cause hemarthroses and soft tissue bleeding, so targeted factor assay is required before replacement selection.