Indications for Testing
Hemophilia testing is warranted in individuals with spontaneous bleeding (particularly into joints, muscles, and soft tissues) or prolonged bleeding that is suggestive of a coagulation disorder. Common bleeding manifestations include epistaxis, hemarthroses, muscle hemorrhage, hematomas, and menorrhagia in women. Individuals with a family history of hemophilia and those who have experienced acute or recent-onset bleeding should be tested.
Laboratory Testing
Initial testing for a coagulation disorder involves a CBC with platelet count, and PT and PTT tests.
Interpretation of Initial Tests
| |
Hemophilia A or Ba,b |
VWD |
Platelet Defect |
|---|
| Platelet count |
Normal |
Normal or reduced |
Normal or reduced |
| PT |
Normal |
Normal |
Normal |
| PTT |
Prolongedc |
Normal or prolongedc |
Normal |
| aThe same pattern can occur in deficiencies of FXI, FXII, prekallikrein, or high molecular weight kininogen.
bThrombin clotting time and fibrinogen activity will also be normal in hemophilia A and B.
cPTT results are affected by method of analysis used by lab. In addition, the PTT may yield a normal result in mild cases of hemophilia and in female hemophilia carriers.
VWD, won Willebrand disease
Source: Srivastava
|
Mixing Studies
A PTT that corrects with a mixing study suggests a factor deficiency, whereas a PTT that does not correct with a mixing study suggests that an inhibitor is present. Incubated mixing studies (in which a mixture of patient plasma and normal pooled plasma is incubated for 1-2 hours at 37°C before testing) are often necessary to detect factor VIII inhibitors, which can develop in patients with hemophilia A. (See Monitoring below.)
Factor VIII and IX Assays
Factor assays are used to confirm the diagnosis of hemophilia by demonstrating FVIII or FIX deficiency. Normal factor levels do not rule out carrier status; definitive determination of carrier status relies on genetic testing. In infants with an FVIII level at the lower end of normal, testing should be repeated at about 6 months of age. In neonates who may be mildly affected with hemophilia B, the FIX activity may need to be remeasured 3-6 months after birth for a definitive result.
Consider confirmation of low factor activity using a chromogenic assay. A number of interfering substances (eg, heparin, lupus anticoagulants) can interfere with the first-line clot-based factor assays. Clinically significant discrepancies have also been observed between clot-based and chromogenic assay results for some forms of hemophilia.
von Willebrand Factor Assay
Since von Willebrand factor (VWF) is a carrier protein for FVIII, von Willebrand disease (VWD) should be ruled out in patients with decreased FVIII levels. The VWF level will be normal in patients with hemophilia. A rare subtype of VWD (type 2N) can demonstrate low FVIII activity with normal VWF levels and can resemble hemophilia A. Specialized coagulation or genetic testing can be used to distinguish these disorders.
Bethesda Assays
The Bethesda assay is used as a reflex test to help distinguish between factor inhibitors and factor deficiencies that are not due to an inhibitor, to titer inhibitors, and to monitor treated patients with hemophilia for the development of an inhibitor. See Monitoring below.
Genetic Testing
In patients with a family history of hemophilia, individual patient risk should be calculated by a clinical geneticist based on laboratory results and family history. Genetic testing can confirm the presence of the causative F8 or F9 gene variant in affected individuals, allows for targeted testing of documented familial mutations, and can determine carrier status in girls or women at risk.
In Utero Testing
In utero genetic testing (third-trimester amniocentesis) can be performed to determine whether hemophilia is present in male fetuses. At birth, uncontaminated cord blood can be used to establish diagnosis if testing was not performed previously. Testing should include FVIII and FIX activity in addition to PTT. However, interpretation of both PTT and FIX activity is difficult in neonates, so repeat testing at several months of age or genetic testing may be necessary to establish the diagnosis.
Monitoring
Trough-Level Testing
Trough levels (last dose of factor infused plus time since last infusion, for correct interpretation) should be measured regularly to monitor factor concentrates. Trough-level tests should be performed in conjunction with inhibitor testing if breakthrough bleeds have occurred. Confirm which type of factor assay (clot-based versus chromogenic) is appropriate for the factor replacement product being used (review of the package insert may provide helpful guidance in appropriate assay selection).
Inhibitor Testing
Individuals receiving treatment for hemophilia are at risk for the development of inhibitors. Monitoring is required because clinical signs do not necessarily accompany inhibitor development.
Some groups recommend a washout period of 72 hours (ie, stopping factor treatment) before an inhibitor assay is performed, but heat treatment of specimens makes this unnecessary, so it is important to know the laboratory protocol that will be used.
Factor concentrates are monitored by measuring factor levels before and after infusion. Reduced half-life of the infused clotting factor or lower than expected recovery may suggest inhibitor presence.
In mild or moderately affected patients, inhibitor testing should be performed annually, and also in the following circumstances :
- When a patient does not respond as expected to factor concentrate replacement
- Before and after changing factor products
- 5-7 days before elective invasive procedures
- Approximately 3 weeks after intensive treatment (>5 exposure days) or surgery
- After every concentrate exposure in patients with increased risk of inhibitor formation because of a particular mutation
In patients with severe hemophilia A and B, inhibitor screening should be performed on every third concentrate exposure day or every 3 months until 20 exposure days have been reached. After that, inhibitor testing should be performed every 3-6 months until 150 exposure days are reached. (In many patients with severe hemophilia, prophylaxis has been established by the 20th exposure day and then trough levels are checked approximately every 3-6 months; if FVIII/FIX is <1 IU/dL, inhibitor testing should be performed. )
In hemophilia B, inhibitor testing is unnecessary after 150 exposure days, unless there is clinical suspicion that an inhibitor is present.
Comorbidity Screening
Screening for hemophilia-specific comorbidities has become more important because patients with hemophilia are living longer. Screening might include blood count, blood coagulation tests, iron status, viral screening (hepatitis, HIV), and liver/kidney function tests.
