External validation of the PLASMIC score: a clinical prediction tool for thrombotic thrombocytopenic purpura diagnosis and treatment

Background: The PLASMIC score was recently published to distinguish patients with severe ADAMTS-13 deficiency from those without for early identification of thrombotic thrombocytopenia purpura (TTP). Objective: We performed an independent external validation of the PLASMIC score for clinical prediction of severe ADAMTS-13 deficiency. Patients/Methods: We studied an independent cohort of 112 consecutive hospitalized patients with suspected thrombotic microangiopathy and appropriate ADAMTS-13 testing (including 21 patients with TTP diagnosis). Results: The PLASMIC score model predicted severe ADAMTS-13 deficiency with a c statistic of 0.94 (0.88-0.98). When dichotomized at high (score 6-7) vs. low-intermediate risk (score 0-5), the model predicted severe ADAMTS-13 deficiency with positive predictive value of 72%, negative predictive value of 98%, sensitivity of 90% and specificity of 92%. In the low-intermediate risk group (score 0-5) there was no significant improvement in overall survival associated with plasma exchange. Conclusions: The PLASMIC score model had excellent applicability, discrimination and calibration for predicting severe ADAMTS-13 deficiency. The clinical algorithm allowed identification of a subgroup of patients who lacked a significant response to empiric treatment.