BACKGROUND: Severe deficiency of von Willebrand factor-cleaving protease 
(ADAMTS-13) activity (<5% of normal) is specific for classical thrombotic 
thrombocytopenic purpura (TTP), a disorder presenting with thrombocytopenia, 
microangiopathic haemolytic anaemia and often with organ dysfunction such as 
neurological symptoms, renal failure, and fever. A certain, though according to 
several case series, variable percentage of patients with clinically diagnosed 
TTP and most patients with other forms of thrombotic icroangiopathies (TMA) do 
not show severe ADAMTS-13 deficiency.
METHODS: We determined ADAMTS-13 activity in 508 plasma samples of 309 patients 
referred to our laboratory in 2001 and 2002. Plasma samples with ADAMTS-13 
activity <5% were additionally tested for the presence of inhibitory antibodies. 
Patients were assigned to ten predefined clinical categories according to 
information provided in the referral letter (TMA not specified; TMA associated 
with neoplasia or chemotherapy; TMA following haematopoietic stem cell 
transplantation; TMA with additional disorder; idiopathic TTP; 
haemolytic-uraemic syndrome (HUS) not specified; HUS with diarrhoea prodrome; 
atypical HUS; other haematological disorder; no clinical information available).
RESULTS: We detected 50 (16%) patients with severe ADAMTS-13 deficiency. 
Forty-four (88%) of these patients had been classified as idiopathic TTP, 2 as 
neoplasia- or chemotherapy-associated, and 4 as non-specified TMA. Among the 
patients labelled as acute idiopathic TTP, the prevalence of severe ADAMTS-13 
deficiency was 63% (44/70). Inhibitory antibodies were found in 31 (62%) 
patients with ADAMTS-13 activity <5%. Of the 44 patients with acute idiopathic 
TTP, at initial presentation or at relapse, with ADAMTS-13 activity <5%, 11 were 
identified to have (probable) constitutional severe ADAMTS-13 deficiency.
CONCLUSION: Severe ADAMTS-13 deficiency is found in about 60% of patients 
diagnosed with idiopathic TTP but in none of 111 diagnosed with HUS. Plasma 
ADAMTS-13 activity <5%, however, does not identify all patients clinically 
diagnosed with TTP. Detection of inhibitory antibodies against ADAMTS-13 helps 
to differentiate between acquired and constitutional forms of TTP, which may be 
important for treatment strategies.