Severe deficiency of VWF-cleaving protease (ADAMTS13) activity defines a distinct population of thrombotic microangiopathy patients.

Background: Severe deficiency of ADAMTS13 activity is a biologic risk factor for thrombotic microangiopathy (TMA). It was hypothesized that severe ADAMTS13 deficiency is associated with a distinct TMA subpopulation.
Study Design and Methods: ADAMTS13 activity before treatment was determined retrospectively in 107 adult TMA patients treated with plasma exchange. Patients were not clinically categorized, but divided between severely deficient (n = 50) and nonseverely deficient (n = 57) ADAMTS13 activity. Laboratory and clinical factors before treatment were compared between the groups.
Results: Median PLT counts were 44,000 per micro L in nonseverely deficient ADAMTS13 patients and 13,000 per micro L in severely deficient ADAMTS13 patients (p < 0.001). Median serum creatinine levels were 2.7 mg per dL in nonseverely deficient patients and 1.2 mg per dL in severely deficient patients (p < 0.001). In surviving patients, median plasma exchange procedures were 9 in nonseverely deficient patients and 14.5 in severely deficient patients (p < 0.01). Rates of relapse following remission were 4 of 47 in nonseverely deficient patients and 16 of 46 in severely deficient patients (p < 0.01). Among analyzed factors only mortality rates were not significantly different.
Conclusion: In a heterogeneous population of TMA patients treated with plasma exchange, ADAMTS13 activity defined two subpopulations with distinct clinical and laboratory features. These results suggest that TMA with severe ADAMTS13 deficiency is a distinct pathologic process.