Symptoms to Diagnosis

An unusual cause of bruising

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A 61-year-old woman presented to our hematology clinic for evaluation of multiple episodes of bruising. The first episode occurred 8 months earlier, when she developed a large bruise after water skiing. Two months before coming to us, she went to her local emergency room because of new bruising and was found to have a prolonged activated partial thromboplastin time (aPTT) of 60 seconds (reference range 23.3–34.9), but she underwent no further testing at that time.

At presentation to our clinic, she reported having no fevers, night sweats, unintentional weight loss, swollen lymph nodes, joint pain, rashes, mouth sores, nosebleeds, or blood in the urine or stool. Her history was notable only for hypothyroidism, which was diagnosed in the previous year. Her medications included levothyroxine, vitamin D3, and vitamin C. She had been taking a baby aspirin daily for the past 10 years but had stopped 1 month earlier because of the bruising.

Table 1. Our patient's complete blood cell count results
On examination, she had a single small hematoma on her right thigh. She had no ecchymoses, petechiae, or adenopathy, and her spleen was nonpalpable.

Ten years earlier she had been evaluated for a possible transient ischemic attack; laboratory results at that time included a normal aPTT of 25.1 seconds and a normal factor VIII level of 153% (reference range 50%–173%).

Table 2. Our patient's coagulation test results
Laboratory testing at our clinic showed a normal complete blood cell count (Table 1); the coagulation factor assay confirmed that her aPTT was elevated (prolonged), but other values were normal (Table 2).
Table 3. Differential diagnosis associated with coagulation assay results
Causes of an isolated prolonged aPTT include medications (eg, heparin), inherited factor deficiencies, acquired inhibitors of coagulation factors, and inherited or acquired von Willebrand disease. Lupus anticoagulant can prolong the aPTT but is usually associated with thrombosis rather than bleeding.1 The differential diagnoses for this and other patterns of coagulation assay abnormalities are listed in Table 3.

EVALUATION FOR AN ISOLATED PROLONGED aPTT

1. What is the appropriate next test to evaluate this patient’s prolonged aPTT?

  • Lupus anticoagulant panel
  • Coagulation factor levels
  • Mixing studies
  • Bethesda assay

Mixing studies

Once a prolonged aPTT is confirmed, the appropriate next step is a mixing study. This involves mixing the patient’s plasma with pooled normal plasma in a 1-to-1 ratio, then repeating the aPTT test immediately, and again after 1 hour of incubation at 37°C. If the patient does not have enough of one of the coagulation factors, the aPTT immediately returns to the normal range when plasma is mixed with the pooled plasma because the pooled plasma contains the factor that is lacking. If this happens, then factor assays should be performed to identify the deficient factor.1

Various antibodies that inhibit coagulation factors can also affect the aPTT. There are 2 general types: immediate-acting and delayed.

With an immediate-acting inhibitor, the aPTT does not correct into the normal range with initial mixing. Immediate-acting inhibitors are often seen together with lupus anticoagulants, which are nonspecific phospholipid antibodies. If an immediate-acting inhibitor is detected, further testing should focus on evaluation for lupus anticoagulant, including phospholipid-dependency studies.

With a delayed inhibitor, the aPTT initially comes down, but subsequently goes back up after incubation. Acquired factor VIII inhibitor is a classic delayed-type inhibitor and is also the most common factor inhibitor.1 If a delayed-acting inhibitor is found, specific intrinsic factor levels should be measured (factors VIII, IX, XI, and XII),2 and testing should also be done for lupus anticoagulant, as these inhibitors may occur together.

Bethesda assay

Table 4. Our patient's mixing study results
If factor levels are decreased, a Bethesda assay should be performed to differentiate a specific factor inhibitor from a lupus anticoagulant. In the case of a factor VIII inhibitor, serial dilutions of patient plasma are incubated at 37°C with pooled normal plasma for 2 hours, then factor VIII activity is measured. The reciprocal dilution of patient plasma that results in 50% of factor VIII activity in the control plasma is 1 Bethesda unit (BU). The stronger the inhibitor in the patient’s sample, the more dilutions are required to measure factor VIII activity, and thus the higher the Bethesda titer.3

Case continued: Results of mixing and Bethesda studies

Table 5. Further studies
Results of the mixing study (Table 4) showed an initial correction of the aPTT in a 1-to-1 mix, but after 1 hour of incubation, the aPTT was again prolonged at 42 seconds (reference range < 37.3). Further testing revealed very low levels of factor VIII (< 1%), and the presence of a factor VIII inhibitor, quantified at 5.8 BU (reference range < 0.5) (Table 5). Additional coagulation tests, including von Willebrand factor testing and a lupus anticoagulation panel, were negative.

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