ADAMTS13 Level of 71% Does Not Indicate TTP and Plasmapheresis Should Be Stopped
An ADAMTS13 activity level of 71% effectively rules out thrombotic thrombocytopenic purpura (TTP) as the diagnosis, and plasmapheresis should be discontinued immediately if it was initiated based on clinical suspicion alone. 1, 2
Diagnostic Threshold for TTP
- ADAMTS13 activity <10% is the diagnostic threshold for TTP according to the International Society of Thrombosis and Haemostasis 1
- A level of 71% is well above this cutoff and indicates normal ADAMTS13 function 1
- When ADAMTS13 activity is >10%, alternative causes of thrombotic microangiopathy (TMA) must be investigated, including complement-mediated TMA, antiphospholipid syndrome, or drug-induced TMA 1
Clinical Decision Algorithm
If plasmapheresis was already initiated:
- Stop plasmapheresis immediately, as the patient does not have TTP 2, 3
- Continuing unnecessary plasmapheresis exposes the patient to procedural risks (line complications, citrate toxicity, allergic reactions) without therapeutic benefit 2
Next diagnostic steps:
- Evaluate for alternative causes of TMA if clinical features suggest microangiopathy 1
- Check complement levels (C3, C4, CH50) and complement inhibitory antibodies for atypical hemolytic uremic syndrome 4
- Screen for antiphospholipid antibodies if thrombosis is present 1
- Review medication history for drug-induced TMA (calcineurin inhibitors, quinine, chemotherapy agents) 4
- Consider Shiga toxin testing if diarrheal illness preceded symptoms 4
Understanding ADAMTS13 Levels in Context
- Severely reduced ADAMTS13 (<10%) with inhibitor antibodies indicates acquired (autoimmune) TTP 1, 5
- Severely reduced ADAMTS13 (<10%) without inhibitor antibodies suggests congenital TTP 1
- Moderately reduced ADAMTS13 (10-50%) can occur in sepsis, liver disease, or pregnancy but does not cause TTP 5
- Normal or near-normal ADAMTS13 (>50%) excludes TTP as the diagnosis 1, 6
Common Pitfall to Avoid
The critical error is continuing plasmapheresis "just to be safe" when ADAMTS13 results return normal. While guidelines appropriately recommend starting plasmapheresis empirically in patients with high clinical suspicion before ADAMTS13 results are available (because delays increase mortality), once results confirm ADAMTS13 >10%, the diagnosis of TTP is excluded and treatment must be stopped 1, 2, 3. The case report literature documents scenarios where clinicians struggled with this decision when platelet counts remained low despite normal ADAMTS13, but the correct approach is to stop TPE and investigate alternative diagnoses 3.
When to Continue Monitoring
- If the patient has ongoing hemolysis and thrombocytopenia despite normal ADAMTS13, focus shifts to identifying the correct alternative TMA diagnosis 1
- Repeat ADAMTS13 testing is not indicated unless clinical deterioration suggests evolving autoimmune TTP (extremely rare scenario) 3
- Serial platelet counts, LDH, haptoglobin, and schistocyte monitoring guide evaluation of the alternative TMA cause 4