Default one-stage assay for hospitalized hemophilia A patients: A cautionary tale Free

Case Presentation:

A 22-year-old male with past medical history significant for mild hemophilia A (1820 G>A ^Arg531^His in the A2 domain of protein) on on-demand recombinant FVIII (rFVIII) infusions in setting of trauma (without maintenance) presented to the Emergency Department with worsening ambulation and pain due to known right adductor muscle hematoma.

The patient had recently transitioned from pediatric to adult hematology outpatient. His historical Factor VIII activity was 10-20% at diagnosis (12% at age 3 years, 15% at age 6 years) and his maternal grandfather had Factor VIII activity levels in the 20% range. Repeat testing by pediatric hematology indicated increased Factor VIII activity that ranged 39-43% for many years, initially thought to be a lab 2:1 dilution error. As a child he had several traumas requiring rFVIII infusions including right ankle hemarthrosis, thrombosed pseudoaneurysm anterolateral to tibial tuberosity, intramuscular hemorrhage of left vastus intermedius muscle, and right patellar hematoma. Prior to transition from pediatric to adult hematology, the patient suffered a spontaneous 5.8 x 3.3 x 13 cm hematoma in his right adductor muscle requiring rFVIII infusions. His Factor VIII activity level was reported as 51% pre-infusion, 128% immediately post-infusion, and 74% 24-hours post-infusion.

He presented again ten days later to the hospital after feeling a sharp tearing pain to his right inner thigh. Imaging revealed expanded right adductor muscle hematoma now measuring 11cm x 7.0 cm x 18 cm. Patient was admitted for rFVIII infusion. Pre-infusion partial thromboplastin time (PTT) was 41 sec. Initial pre-infusion Factor VIII activity and inhibitor screen failed due to FVIII instability. Repeat activity after rFVIII infusion was reported to be 51%. Factor XIII screen normal and vWF activity (125%) and antigen (141%) normal. Prior testing of Vitamin C levels by pediatric hematologist were normal.

It was learned that the hospital's default assay is the one-stage assay (OSA). Factor VIII activity was reassessed with chromogenic assay (CSA). Levels assessed by CSA were consistently 20-50% lower than levels assessed by OSA with greater discordance at lower Factor VIII levels (OSA 53% when CSA 17%, and OSA 83% when CSA 55%). Patient was discharged with Factor VIII activity level 55% as measured by CSA after rFVIII infusions with outpatient hematology follow up. His corresponding Factor VIII level measured by OSA was 83% at discharge.

Discussion: Hemophilia A phenotype severity correlates with Factor VIII activity level. It is unusual to develop a spontaneous hematoma with Factor VIII activity of 51% given normal levels of Factor VIII activity are between 50-150%. While the patient's ^Arg531^His missense mutation is associated with mild phenotype, his reported Factor VIII activity was incongruent with his worsening spontaneous hematoma.

For years this patient's Factor VIII activity had been measured and treated with on-demand rFVIII infusions using OSA, the institution's default laboratory testing. However, this patient's mutation is associated with discrepancy of OSA and CSA (two-stage assay) results. A two-stage assay such as CSA is known to more accurately reflect his phenotype.

At the time of presentation, Factor VIII activity level was measured as 53% by the lab (on standard OSA), whereas his level was 17% as measured by CSA. More recent testing post-hospitalization measured this patient's Factor VIII activity levels 14-25% on CSA, which is similar to levels measured at patient's diagnosis as a child. Hematologists must be aware of the discordance between OSA and two-stage assays, as this will affect the anticipated severity of disease and number of rFVIII infusions to treat bleeding in hemophilia patients. Close coordination with blood bank and clinical pathology teams is essential for accurate diagnosis and monitoring.

Conclusion:

As patients transition from pediatric to adult hematology, it is important to be aware of default laboratory assay testing. OSA is typically used as default laboratory test at most institutions. For patients with hemophilia and clinically significant bleeding despite reassuring factor VIII activity levels, it is important to evaluate for variation in lab assay performance in addition to excluding additional bleeding diathesis.