Which bleeding disorder presents with a normal prothrombin time (~11.5 seconds), a markedly prolonged activated partial thromboplastin time (~51 seconds), a normal‑high platelet count (~360 ×10⁹/L), and a decreasing hemoglobin level?

Hemophilia A or Factor VIII Deficiency

The laboratory pattern of normal PT (11.5 seconds), markedly prolonged aPTT (51 seconds), normal platelet count (360,000), and decreased hemoglobin strongly suggests hemophilia A (factor VIII deficiency) or other intrinsic pathway coagulation factor deficiencies. 1, 2

Laboratory Pattern Analysis

The isolated prolongation of aPTT with normal PT indicates a defect in the intrinsic coagulation pathway while the extrinsic and common pathways remain intact. 3

• Normal PT (11.5 seconds) rules out deficiencies in factors VII, X, V, prothrombin, and fibrinogen, as these would prolong the PT 4, 3
• Prolonged aPTT (51 seconds) indicates deficiency or inhibition of factors VIII, IX, XI, XII, or the presence of lupus anticoagulant 1, 3
• Normal platelet count (360,000) excludes thrombocytopenia as the cause of bleeding 4
• Decreased hemoglobin reflects active or recent bleeding, consistent with a coagulopathy causing hemorrhage 4

Most Likely Diagnoses in Order of Probability

Hemophilia A (Factor VIII Deficiency)

This is the most common inherited bleeding disorder with this laboratory pattern. 2

• Factor VIII deficiency causes isolated aPTT prolongation because factor VIII is exclusively part of the intrinsic pathway 2, 3
• The aPTT normalizes with factor VIII replacement therapy 2
• Hemophilia A patients characteristically have normal PT, prolonged aPTT, and normal platelet counts 2
• The decreased hemoglobin reflects bleeding episodes (hemarthroses, muscle hematomas, or other hemorrhages) 2

Hemophilia B (Factor IX Deficiency)

Clinically and laboratory-wise indistinguishable from hemophilia A without specific factor assays. 3

• Also causes isolated aPTT prolongation with normal PT and platelets 3
• Less common than hemophilia A (approximately 1:5 ratio) 3

Factor XI Deficiency

Less common but presents with identical screening test abnormalities. 3

• Typically causes milder bleeding than hemophilia A or B 3
• More common in Ashkenazi Jewish populations 3

Critical Diagnostic Steps

Mixing study is the essential next test to differentiate factor deficiency from an inhibitor:

• If aPTT corrects with 1:1 mixing with normal plasma → factor deficiency (hemophilia A, B, or XI) 3
• If aPTT does not correct → inhibitor present (acquired hemophilia, lupus anticoagulant) 4, 3

Following mixing study, specific factor assays (VIII, IX, XI, XII) definitively identify which factor is deficient. 2, 3

Important Caveats and Pitfalls

Do Not Assume Lupus Anticoagulant

While lupus anticoagulant can prolong aPTT, it typically causes thrombosis, not bleeding. 4

• Lupus anticoagulants usually cause only slight PT prolongation and more prominent aPTT prolongation 4
• These patients paradoxically require anticoagulation due to thrombotic risk, not hemostatic therapy 1
• The presence of decreased hemoglobin from bleeding makes lupus anticoagulant unlikely 4

Factor XII Deficiency Does Not Cause Bleeding

Factor XII deficiency prolongs aPTT but does not cause clinical bleeding. 3

• If factor XII deficiency is identified, look for another cause of the decreased hemoglobin 3

aPTT Reagent Variability

Different aPTT reagents have markedly different sensitivities to factor deficiencies. 1, 5

• An aPTT of 51 seconds may represent different degrees of factor deficiency depending on the laboratory's reagent and reference range 1, 5
• Each laboratory should establish therapeutic ranges specific to their reagent 1

Leukocytosis Can Spuriously Prolong PT

In patients with leukemia, extreme leukocytosis can cause artifactual PT prolongation without true coagulopathy. 6

• However, this pattern shows normal PT, making this artifact irrelevant here 6

Management Implications

For hemophilia A with active bleeding (evidenced by decreased hemoglobin):

• Administer recombinant factor VIII concentrate immediately 2
• Target factor VIII level depends on bleeding severity: 40-50% for minor bleeding, 80-100% for major bleeding 2
• The aPTT will normalize with adequate factor VIII replacement 2
• Do not use FFP as primary therapy—specific factor concentrates are superior 1, 2

If urgent surgery is needed:

• Correct aPTT to <1.5 times normal before high-risk procedures 1
• Factor VIII concentrate dosing: approximately 50 IU/kg achieves ~100% factor VIII level 2
• Monitor aPTT every 6 hours initially during acute management 1