Written by Dr. Roupen Odabashian MD, FRCPC
Hematologist-Oncologist | Founder, MeDucation AI | Updated November 2026
The verdict: when you're paged about a hemoglobin drop, a severely low platelet count, and schistocytes on the smear, you do not wait for ADAMTS13 to come back before you act. Draw the ADAMTS13 activity and inhibitor level first, before any plasma or transfusion, then start therapeutic plasma exchange and corticosteroids without waiting for results[1]. Use the PLASMIC score at the bedside to estimate how likely this is immune TTP versus something else, because that number changes whether you also start caplacizumab before the ADAMTS13 result is back[1][2]. Everything else in this article is about sorting out which thrombotic microangiopathy (TMA) you're actually looking at once the first steps are handled.
Conflict of interest, up front: I'm a practicing hematologist-oncologist and I founded MeDucation AI, which sells a physician-written heme/onc question bank and AI study tools. I have no financial relationship with any TMA diagnostic, drug, or trial mentioned here. Where a competitor's resource or a guideline body did something well, I'll say so.
Entity | ADAMTS13 activity | First-line treatment | Where it goes wrong if you misdiagnose it |
|---|---|---|---|
Immune TTP (iTTP) | <10 IU/dL[1] | TPE + corticosteroids ± caplacizumab ± rituximab[1] | Delay in TPE is fatal; mortality was near 90% in the pre-plasma-exchange era[3] |
Hereditary (congenital) TTP | <10 IU/dL[1] | Plasma infusion or TPE; no rituximab target | Missed in children/pregnancy where it accounts for a much larger share of cases[1] |
Typical HUS (Shiga toxin-associated) | >10 IU/dL[1] | Supportive care; avoid antibiotics and antimotility agents[4] | Antibiotics can increase HUS risk in STEC infection[4] |
Atypical HUS (complement-mediated) | >10 IU/dL[1] | Eculizumab or ravulizumab[5] | TPE alone does not address the complement pathway driving the disease |
What should you actually do in the first hour when TMA is on the differential?
Draw the ADAMTS13 sample before you give any plasma product or start plasma exchange, then start treatment without waiting for the result[1]. The ISTH guideline panel, a multidisciplinary group of hematologists, an intensivist, a nephrologist, a clinical pathologist, a biostatistician, and patient representatives working with a McMaster methodology team, built its recommendations around exactly this sequencing problem[1]. Their first recommendation is explicit: acquire the ADAMTS13 sample, then start TPE and corticosteroids without waiting for results, and consider early caplacizumab before the ADAMTS13 activity comes back if your pretest probability is high[1].
The reason for this is turnaround time. The panel considered an ADAMTS13 result back in under 72 hours "ideal" and 72 hours to 7 days "acceptable"[1]. In most hospitals you are deciding on TPE and caplacizumab in real time, long before that number exists. That's what the PLASMIC score is for.
