BACKGROUND: Rapid quantification of ADAMTS13 activity in plasma is essential for establishing a diagnosis of thrombotic thrombocytopenic purpura (TTP); a rare, but potentially lethal disorder. The current methods for quantitating ADAMTS13 activity are manual and only available at specialized laboratories, which often results in initiation of specific treatments long before a diagnosis of TTP is established. METHODS: We compared the performance of the HemosIL, a novel and rapid automated method, and the current standard FRET (fluorescence resonance energy transfer) method in quantitating ADAMTS13 activity using 706 consecutive plasma samples collected over a period of 14 years. The clinical accuracy of both methods was further examined using 212 diagnostic samples. RESULTS: The correlation between the FRET and HemosIL methods in all 706 samples and in the 212 diagnostic samples was excellent (Pearson's r of 0.919 and 0.912, respectively). Both methods displayed a high degree of clinical accuracy using the current cutoff of ADAMTS13 activity <0.10 kIU/L (<10%) as diagnostic for TTP: the area under the curve (AUC) was 97.7% for the FRET method and 99.5% for the HemosIL method. When applying a lower cutoff (ADAMTS13 activity <0.05 kIU/L or <5%), the diagnostic accuracy of the HemosIL method increased further (AUC = 99.7%). CONCLUSIONS: A novel, rapid method for ADAMTS13 quantification is comparable to the more laborious FRET method in patients with possible TTP. A rapid analysis available in the acute setting assessing patients with possible TTP allows for improved care and optimized treatment of a life-threatening condition.