What does a low ADAMTS13 activity laboratory result indicate?

Low ADAMTS13 Activity: Clinical Significance

A low ADAMTS13 activity level, particularly when <10% of normal (<10 IU/dL), strongly indicates thrombotic thrombocytopenic purpura (TTP) in the appropriate clinical context and requires urgent evaluation and treatment to prevent life-threatening microvascular thrombosis and organ damage. 1

What ADAMTS13 Does

ADAMTS13 is a plasma protease that cleaves ultra-large von Willebrand factor (UL-VWF) multimers. This cleavage is essential for preventing excessive platelet adhesion and aggregation 1, 2. When ADAMTS13 activity is severely deficient, UL-VWF accumulates on endothelial surfaces and in circulation, triggering widespread microvascular thrombus formation throughout the body, leading to ischemic organ damage—the hallmark of TTP 1, 2.

Diagnostic Threshold and Clinical Context

The critical diagnostic threshold is ADAMTS13 activity <10 IU/dL (or <10% of normal). When this level is found in a patient presenting with:

• Severe thrombocytopenia (usually <30 × 10⁹/L)
• Microangiopathic hemolytic anemia (low hemoglobin, elevated LDH, schistocytes)
• Variable degrees of end-organ damage (neurological, renal, cardiac)

The diagnosis of TTP should be strongly considered 1.

Two Main Causes of Low ADAMTS13

1. Immune TTP (iTTP) - Most Common

• Caused by autoantibodies against ADAMTS13 1, 2
• Can occur at any age, though more common in adults
• Testing for anti-ADAMTS13 IgG antibodies helps confirm this diagnosis

Critical caveat: Absence of detectable antibodies during the initial acute episode does NOT rule out iTTP 1. Free antibodies may be undetectable by ELISA due to:

• Lack of free antibodies (all bound to ADAMTS13)
• Accelerated clearance of antibody-ADAMTS13 complexes

2. Congenital TTP (cTTP) - Rare

• Caused by inherited mutations in the ADAMTS13 gene 1
• Approximately 250 causative mutations identified
• Most patients have compound heterozygous mutations
• Diagnosis requires: repeated negative antibody testing PLUS positive genetic testing for ADAMTS13 mutations

Beyond TTP: Other Conditions with Low ADAMTS13

While severe deficiency (<10%) is specific for TTP, mild to moderate reductions in ADAMTS13 activity occur in several other conditions 3, 4:

• Secondary thrombotic microangiopathies
• Severe infections (including COVID-19)
• Sepsis
• Severe trauma (associated with coagulopathy and mortality) 5
• Ischemic stroke risk (lowest quartile associated with 1.65-fold increased risk) 6
• Inflammatory processes
• Liver disease
• Malignancies

These conditions represent relative ADAMTS13 deficiencies that contribute to a prothrombotic state but do not typically reach the severe deficiency seen in TTP.

Clinical Urgency

TTP is potentially fatal without treatment—historically nearly 100% mortality, now >95% survival with prompt therapy 2. The key challenge is that ADAMTS13 assay results may take several days, necessitating empirical treatment based on clinical suspicion before laboratory confirmation.

Use clinical scoring systems while awaiting ADAMTS13 results:

• French score
• PLASMIC score

These help assess the likelihood of severe ADAMTS13 deficiency and guide the decision to initiate empirical treatment 1.

Rapid Testing Advances

Recent rapid ADAMTS13 assays (turnaround time <1 hour) now allow earlier diagnostic certainty 7:

• HemosIL AcuStar CLIA: Sensitivity 0.98, specificity 0.99—can reliably rule out TTP and avoid unnecessary empirical treatment
• Technofluor FRET and Technoscreen assays also available with slightly lower performance

Treatment Implications

The finding of low ADAMTS13 activity directly determines treatment strategy:

For iTTP:

• Therapeutic plasma exchange (TPE)
• Corticosteroids
• Rituximab
• Caplacizumab (anti-VWF nanobody)

For cTTP:

• Strong 2025 recommendation: Recombinant ADAMTS13 over fresh frozen plasma for patients in remission 1
• If recombinant ADAMTS13 unavailable: fresh frozen plasma
• Prophylactic treatment significantly improves relapse-free survival 8

The distinction between iTTP and cTTP is critical because it fundamentally changes long-term management—iTTP requires immunosuppression while cTTP requires lifelong ADAMTS13 replacement.