The utility of ADAMTS13 in differentiating TTP from other acute thrombotic microangiopathies: results from the UK TTP Registry

Thrombotic microangiopathies (TMAs) are frequently difficult to differentiate clinically, and measurement of ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) remains vital in thrombotic thrombocytopcnic purpura (TIP) diagnosis. We retrospectively reviewed cases referred for ADAMTS13 testing, using UK TIP Registry screening data. Of a total 810 cases, 350 were confirmed as TIP. The 460 non-TIP cases comprised secondary TMAs (24.57%) and haemolytic uraemic syndrome (HUS) (27.17% aHUS, 2.83% Shiga-like toxin-producing E. coli [STEC]-HUS); the remainder were TMAs with no clear association, not TMAs, or had no confirmed diagnosis. ADAMTS13 levels were significantly lower in TIP than STEC-HUS, aHUS and other TMAs. TIP patients had significantly lower platelet count (15 x 10(9)/l; range 0-96) than aHUS (57 x 10(9)/1; range 13-145, P < 0.0001) or STEC-HUS (35 x 10(9)/l: range 14-106, P < 0.0001); they also had lower creatinine levels (92 mu mol/l; range 43-374) than aHUS (255 mu mol/l; range 23-941, P < 0.0001) and STEC-HUS (324 mu mol/l; range 117-639, P < 0.0001). However, 12/34 (35.3%) aHUS patients had a platelet count <30 x 10(9)/l and 26/150 (17.3%) of TIP patients had a platelet count >30 x 10(9)/l; 23/150 (15.3%) of TIP patients had a creatinine level >150 mu mol/l. This study highlights the wide variety of TMA presentations, and confirms the utility of ADAMTS13 testing in TTP diagnosis.