What is the recommended approach to diagnose hemophilia in children?

Diagnosing Hemophilia in Children

The diagnosis of hemophilia in children begins with measuring activated partial thromboplastin time (aPTT), followed by specific factor VIII and IX assays when aPTT is prolonged, and mixing studies to distinguish factor deficiency from inhibitors. 1

Initial Laboratory Screening

• Obtain aPTT and PT as the first-line screening tests when hemophilia is suspected 1
• A prolonged aPTT with normal PT suggests deficiency in the intrinsic pathway (factors VIII, IX, XI, or XII) 1
• Normal PT helps exclude extrinsic pathway defects and narrows the differential 1

Critical pitfall: Do not stop at aPTT alone—mild hemophilia can be missed if specific factor levels are not measured when clinical suspicion exists 1

Mixing Studies to Differentiate Deficiency from Inhibitors

When aPTT is prolonged, perform mixing studies immediately:

• Mix patient plasma 1:1 with normal plasma and measure aPTT both immediately and after 1-2 hour incubation at 37°C 2, 1, 3
• Immediate correction (normalized aPTT) indicates factor deficiency rather than an inhibitor 1, 3
• Failure to correct or prolongation after incubation suggests presence of an inhibitor (acquired hemophilia or lupus anticoagulant) 2, 1
• Calculate the Rosner index: values <11% support factor deficiency, while ≥11% indicates inhibitor presence 4, 3

Important caveat: Immediate correction does not completely exclude acquired hemophilia A in children with active bleeding—proceed with inhibitor workup if clinical presentation is suggestive 2, 3

Specific Factor Assays

Once mixing studies suggest factor deficiency:

• Measure factor VIII and factor IX levels using functional coagulation assays (one-stage clotting or chromogenic assays) 1, 5
• Factor VIII deficiency confirms hemophilia A 1, 5
• Factor IX deficiency confirms hemophilia B 1, 5
• Also measure factors XI and XII to complete intrinsic pathway evaluation 2, 1

Severity classification based on factor levels: 5

• Severe: <1% factor activity
• Moderate: 1-5% factor activity
• Mild: >5% factor activity

Distinguishing Hemophilia A from von Willebrand Disease

When factor VIII is isolated and low:

• Measure von Willebrand factor (VWF) antigen and VWF activity (VWF:RCo or VWF:GPIbM) 3
• Low VWF with low factor VIII indicates von Willebrand disease rather than hemophilia A 3
• Normal VWF with isolated low factor VIII confirms hemophilia A 3

Testing pitfall: VWF levels can be falsely elevated by stress, exercise, pregnancy, or inflammation—repeat testing may be necessary if initial results are borderline 1, 3

Inhibitor Testing When Indicated

If mixing studies fail to correct or clinical bleeding is severe despite treatment:

• Perform Bethesda assay to quantify factor VIII or IX inhibitor titers 1, 4
• This distinguishes acquired hemophilia (autoantibodies) from congenital hemophilia 4
• Acquired hemophilia in children is rare (0.045 per million) but can cause life-threatening bleeding 6, 7

Key clinical context: Acquired hemophilia may present without bleeding initially—elderly patients and postpartum women are highest risk, but pediatric cases do occur and require immediate investigation 3, 6, 7

Excluding Confounding Factors

Before finalizing the diagnosis:

• Check thrombin time or review medication history to exclude heparin contamination, which can prolong aPTT 4, 3
• If patient is on warfarin, defer testing until INR <1.5 or 1-2 weeks after discontinuation 4, 3
• Perform lupus anticoagulant testing even when mixing studies correct, as both conditions can coexist 3

Laboratory artifact warning: If all intrinsic factors appear decreased, this may be an in vitro artifact from inhibitor depleting factor VIII in substrate plasma—repeat assays at higher serial dilutions 1, 3

Diagnostic Algorithm Summary

1. Prolonged aPTT detected → Proceed to mixing study
2. Mixing study corrects immediately → Measure factors VIII, IX, XI, XII
3. Isolated low factor VIII → Measure VWF to distinguish hemophilia A from von Willebrand disease
4. Isolated low factor IX → Confirms hemophilia B
5. Mixing study fails to correct → Perform Bethesda assay and lupus anticoagulant testing
6. Classify severity based on factor level (<1%, 1-5%, >5%)

Special consideration for neonates: Maternal transplacental transfer of autoantibodies can rarely cause acquired hemophilia in newborns, presenting with clinically significant bleeding 7