Hemophilia A (Factor VIII Deficiency) Workup

Updated: Jul 05, 2023
  • Author: Douglass A Drelich, MD; Chief Editor: Srikanth Nagalla, MD, MS, FACP  more...
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Approach Considerations

Laboratory studies for suspected hemophilia include a complete blood cell count, coagulation studies, and a factor VIII (FVIII) assay. Never delay indicated coagulation correction pending diagnostic testing.

On the hemoglobin/hematocrit assay, expect normal or low values. Expect a normal platelet count. On coagulation studies, the prothrombin time (which assesses the extrinsic coagulation pathway) and thrombin time are normal.

Usually, the activated partial thromboplastin time (aPTT) is prolonged; however, a normal aPTT does not exclude mild or even moderate hemophilia because of the relative insensitivity of the test. The aPTT is significantly prolonged in severe hemophilia.

For FVIII assays, levels are compared with a normal pooled-plasma standard, which is designated as having 100% activity or the equivalent of FVIII U/mL. Normal values are 50-150%. Values in hemophilia are as follows:

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

Aging, pregnancy, oral contraceptive use, and estrogen replacement therapy are associated with increased FVIII levels. Because FVIII is a large molecule that does not cross the placenta, the diagnosis can be made at birth with quantitative assay of cord blood.

Three main FVII assay methods are in use: one-stage and two-stage clotting assays and a two-stage chromogenic method. In approximately one-third of patients with mild hemophilia, FVIII levels on the automated one-stage FVIII assay are significantly higher than—typically, more than double—those of the two-stage coagulation assay. [26, 27]

Those discrepancies, which result from differences in the underlying FVIII mutations in these patients, can result in missed diagnoses or mismanagement due to underestimation of bleeding risk. [26, 28] Given the possibility of discrepant hemophilia A, as this condition is termed,  use of the chromogenic FVIII assay combined with FVIII gene mutation analysis has been recommended for diagnosis of mild hemophilia A. [26]

Differentiation of hemophilia A from von Willebrand disease is possible by observing normal or elevated levels of von Willebrand factor antigen and ristocetin cofactor activity.  A caveat to this is the rare patient with type 2N von Willebrand disease in which the VW antigen and activity levels are normal but binding to FVIII is impaired resulting in short circulating half-life of Factor VIII and low levels.  While type 2N VWD is very rare, it is relevant to exclude this diagnosis as those patients may require therapy wtih combined VWF/FVIII product rather than isolated FVIII concentrate.  Bleeding time is prolonged in patients with von Willebrand disease but conflicting reports of presence of elevation patients with hemophilia as well of lack of standardization and familiarity with proper performance of this assay limits the utility of the bleeding time as a diagnostic strategy.

In patients with an established diagnosis of hemophilia, periodic laboratory evaluations include screening for the presence of FVIII inhibitor and screening for transfusion-related or transmissible diseases such as hepatitis and HIV infection. Screening for infection may be less important in patients who receive only recombinant FVIII concentrate.

Imaging studies for acute bleeds

Early and aggressive imaging is indicated, even with low suspicion for hemorrhage, after coagulation therapy is initiated. Imaging choices are guided by clinical suspicion and the anatomic location of involvement.

Head CT scans without contrast are used to assess for spontaneous or traumatic intracranial hemorrhage. Perform magnetic resonance imaging (MRI) on the head and spinal column for further assessment of spontaneous or traumatic hemorrhage. MRI is also useful in the evaluation of the cartilage, synovium, and joint space.

Ultrasonography is useful in the evaluation of joints affected by acute or chronic effusions. This technique is not helpful for evaluating the bone or cartilage. Special studies such as angiography and nucleotide bleeding scan may be clinically indicated.


Testing for Inhibitors

Laboratory confirmation of a FVIII inhibitor is clinically important when a bleeding episode is not controlled despite infusion of adequate amounts of factor concentrate. For the assay, the aPTT measurement is repeated after incubating the patient's plasma with normal plasma at 37°C for 1-2 hours. If the prolonged aPTT is not corrected, the inhibitor concentration is titrated using the Bethesda method. Ideally, the Nijmegen modification of the Bethesda inhibitor assay should be used to detect an inhibitor if the mixing test result is positive. [14]

By convention, more than 0.6 Bethesda units (BU) is considered a positive result for an inhibitor. Less than 5 BU is considered a low titer of inhibitor, and more than that is a high titer. The distinction is clinically significant, as patients with low-titer inhibitors may respond to higher doses of FVIII concentrate while those with high-titer inhibitors require treatment with agents that bypass FVIII and consideration for induction of immune tolerance.

Caution is warranted when obtaining blood samples for coagulation assays from heparinized central lines because of the effect of heparin contamination on all coagulation test results. The excess heparin causes false-positive results and/or higher inhibitor titer values than are actually present in the patient, because heparin is also an inhibitor of coagulation.

One study found significant heparin contamination in 45% of all specimens obtained through implanted venous access devices. These researchers suggested that all blood samples obtained from such devices, which are usually flushed with heparin, should be treated with heparinase before performing an inhibitor assay. [29]


Carrier Testing and Fetal Testing

Screening for carrier status can be performed by measuring the ratio of FVIII coagulant activity to the concentration of von Willebrand factor (vWF) antigen. A ratio that is less than 0.7 suggests carrier status.

The International Society on Thrombosis and Haemostasis has proposed nomenclature for the diagnosis and management of hemophilia carriers. [30] For hemophilia A, the nomenclature uses FVIII levels and clinical characteristics to distinguish the following categories for women/girls:

  • Mild hemophila A: FVIII > 0.05 and < 0.40 IU/mL
  • Moderate hemophilia A: FVIII 0.01-0.05 IU/mL
  • Severe hemophilia A: FVIII < 0.01 IU/ml, respectively
  • Symptomatic carriage: FVIII ≥0.40 IU/mL with a bleeding phenotype
  • Asymptomatic carriage: FVIII ≥0.40 IU/mL without a bleeding phenotype

Direct genetic testing for known gene mutation is a more accurate screening technique. Linkage analysis by restriction fragment length polymorphism (RFLP) in multiple family members can be used. Direct mutation analysis is available in several laboratories for unknown FVIII mutations. Inversion of the FVIII gene can be detected by Southern blot.

For prenatal testing, carriers whose mutation has been identified can have chorionic villus sampling at approximately 10-12 weeks' gestation or amniocentesis at 16-20 weeks' gestation to obtain fetal cells for DNA analysis or for linkage studies. If DNA analysis cannot be performed, then fetal blood obtained by fetoscopy at approximately 20 weeks' gestation can be assayed for factor VIII level.

All of those procedures carry a risk ranging from a low of 0.5% for maternal-fetal complications to a high of 1-6% for fetal death from fetoscopy. These procedures should be undertaken only after patients receive intense genetic and obstetric counseling. Genetic counseling before the woman becomes pregnant is ideal and may help couples make informed decisions before conception.

Noninvasive prenatal diagnosis using quantitative digital polymerase chain reaction testing of free fetal DNA in the maternal circulation has been reported. However, this technique remains a research tool. [31]

If the fetus is a female, the couple may elect to carry the pregnancy to term because carriers rarely have bleeding problems. If the fetus is a severely affected male, the couple must make a decision about continuing the pregnancy to term. With pregnancies that will be carried to term, prenatal diagnosis allows for planning of delivery so as to minimize the risk of intracranial hemorrhage (eg, avoidance of vacuum devices). [31]



Radiography for joint assessment is of limited value in acute hemarthrosis. Evidence of chronic degenerative joint disease may be visible on radiographs in patients who have been untreated or inadequately treated or in those with recurrent joint hemorrhages. In these patients, radiographs may show synovial hypertrophy, hemosiderin deposition, fibrosis, and damage to cartilage that progresses with subchondral bone cyst formation.

Hemophilic arthropathy evolves through 5 stages, starting as an intra-articular and periarticular edema due to acute hemorrhage and progressing to advanced erosion of the cartilage with loss of the joint space, joint fusion, and fibrosis of the joint capsules. [9] See the image below. For discussion of the 5-stage Arnold-Hilgartner classification of hemophilic arthropathy, see Imaging in Musculoskeletal Complications of Hemophilia.

Photograph of a hemophilic knee at surgery, with s Photograph of a hemophilic knee at surgery, with synovial proliferation caused by repeated bleeding; synovectomy was required.