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

Abstract

Essentials Severe ADAMTS-13 deficiency is key to thrombotic thrombocytopenic purpura (TTP) diagnosis. PLASMIC score predicts ADAMTS-13 deficiency in suspected TTP with high discrimination. PLASMIC score is more generalizable with fewer missing data than alternative clinical scores. PLASMIC score identifies a subgroup of patients lacking significant response to plasma exchange.

Summary: 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.

Keywords: plasma exchange; platelet count; purpura, thrombotic thrombocytopenic; thrombotic microangiopathies; validation studies.

Figure 1

Patient selection criteria for PLASMIC score validation

Figure 2

Overall survival by PLASMIC score risk and plasma exchange (PEX) In the high-risk for TTP group (PLASMIC score 6–7), treatment with PEX led to significantly longer survival (log rank P-value <0.01). In the low-intermediate risk for TTP group (PLASMIC score 0–5), treatment with PEX did not lead to a difference in survival (log rank P-value 0.50) and both treated and untreated groups had worse prognosis.