ADAMTS13 Activity Test
🧬 ADAMTS13 Activity Test — Complete Guide
Key diagnostic test for thrombotic thrombocytopenic purpura (TTP)
Principle · ELISA method · Reference range · Clinical significance · Interpretation points
Professional guide to ADAMTS13 Activity by a medical specialist
1. What Is ADAMTS13?
ADAMTS13 (A Disintegrin And Metalloprotease with ThromboSpondin type 1 motif, 13) is a plasma metalloprotease that cleaves ultra-large von Willebrand factor (UL-vWF) multimers.
When ADAMTS13 activity is severely reduced:
- UL-vWF is not cleaved
- Platelets become hyper-aggregated
- Microvascular thrombi form
This mechanism explains the pathogenesis of TTP (Thrombotic Thrombocytopenic Purpura).
⮕ ADAMTS13 is essential for preventing microvascular thrombosis.
2. Indications for Testing
✔ 1) Diagnosis of TTP
- ADAMTS13 activity <10% → highly suggestive of TTP
- Detection of ADAMTS13 inhibitors confirms immune-mediated TTP (iTTP)
✔ 2) Differentiating TTP from Other Forms of MAHA
Useful for distinguishing:
- DIC
- HUS (especially Shiga toxin–associated)
- HELLP syndrome
- Sepsis-associated TMA
✔ 3) Monitoring Treatment and Relapse Risk
- Used during plasma exchange (PEX) and immunosuppressive therapy
- Activity reduction may precede relapse
✔ 4) Diagnosis of Congenital TTP (Upshaw–Schulman Syndrome)
3. Test Method: ELISA-Based ADAMTS13 Activity Assay
ADAMTS13 activity can be measured using various platforms, but ELISA or Fluoro-ELISA–based assays are most widely used in clinical laboratories.
🔬 Principle of the ELISA Method
- A synthetic peptide mimicking the vWF A2 domain is coated onto the plate.
- Patient plasma is added → ADAMTS13 cleaves this substrate.
- The cleavage exposes a specific neo-epitope.
- A detection antibody (anti-cleaved vWF) labeled with peroxidase binds the epitope.
- Color development is proportional to the amount of substrate cleaved.
- Optical density is converted into percent activity (%).
⮕ Low activity reflects impaired cleavage ability → supportive of TTP.
4. Reference Range
Reference intervals vary among laboratories, but generally:
▶ Normal: >40%
▶ Partial deficiency: 10–30%
▶ Severe deficiency: <10% (highly specific for TTP)
| ADAMTS13 Activity | Interpretation |
|---|---|
| <10% | Strongly suggests TTP |
| 10–30% | Partial deficiency; consider TTP or secondary TMA |
| >40% | Normal |
5. Clinical Significance
🩸 1) Diagnosis of TTP
- <10% + inhibitor present → immune-mediated TTP (iTTP)
- <10% + no inhibitor → congenital TTP (cTTP)
(Upshaw–Schulman syndrome)
🩸 2) Distinguishing TTP From Other TMAs
| Condition | ADAMTS13 | Key Clues |
|---|---|---|
| TTP | <10% | MAHA + thrombocytopenia |
| DIC | Mild–moderate decrease | PT/aPTT prolonged, fibrinogen low |
| HUS | Normal–mild decrease | Shiga toxin, renal failure heavy |
| HELLP | Normal range | Pregnancy, ↑AST/ALT |
| Sepsis-TMA | Moderately reduced | Severe infection |
🩸 3) Therapeutic Use
- Plasma exchange must begin before results return if TTP is clinically suspected.
- Recovery of ADAMTS13 activity during therapy predicts remission.
6. Conditions Associated With Increased or Decreased Levels
📉 Decreased ADAMTS13 Activity
| Condition | Mechanism |
|---|---|
| Immune-mediated TTP (iTTP) | Autoantibody inhibition |
| Congenital TTP (cTTP) | Genetic mutations |
| Severe liver disease | Decreased synthesis |
| DIC | Factor consumption |
| Cancer-related or sepsis-related TMA | Consumption and endothelial injury |
📈 Increased/Normal-High Levels
- Usually clinically insignificant
- Often due to biological variability
7. Key Interpretation Points
✔ 1) Activity <10% = TTP in most cases
Inhibitor assay is essential to classify iTTP vs cTTP.
✔ 2) Normal activity does NOT exclude TMA
Consider:
- DIC
- HUS
- HELLP
- Malignant hypertension
- Drug-induced TMA
✔ 3) Start PEX immediately when TTP is suspected
Do NOT wait for ADAMTS13 results (turnaround may be slow).
✔ 4) Anticoagulants (heparin, DOACs) do not affect the assay
✔ 5) Specimen quality matters
- Hemolysis and lipemia interfere with ELISA quantification.
8. Important Precautions
⚠ 1) Post-PEX Samples Can Give False-Normal Results
Plasma exchange artificially increases ADAMTS13 levels.
⮕ Draw blood before starting PEX.
⚠ 2) Always Perform an Inhibitor Assay
(Bethesda or ELISA-based inhibitor test)
Critical for diagnosing immune TTP and predicting relapse.
⚠ 3) Newborns Have Physiologically Low Levels
Age-specific reference ranges are required.
✔ Summary
- ADAMTS13 is the enzyme that cleaves UL-vWF and prevents platelet-rich microthrombi.
- ELISA-based assays quantify activity as a percentage.
- Severe deficiency (<10%) strongly indicates TTP, especially with inhibitors.
- Essential for distinguishing TTP from other TMAs such as DIC, HUS, and HELLP.
- Blood sampling must be done before plasma exchange, and inhibitor assays should be performed in parallel.
Rapid Malaria Test: Guide – MedLab Insight
https://pubmed.ncbi.nlm.nih.gov/16672704
📚 References
- George JN. Thrombotic Thrombocytopenic Purpura. NEJM.
- Sadler JE. Von Willebrand Factor, ADAMTS13, and TTP. Blood.
- ISTH Guidelines on Thrombotic Thrombocytopenic Purpura.
- Tsai HM. ADAMTS13 and Microangiopathic Hemolytic Anemia. Curr Opin Hematol.
- CLSI. Laboratory Guidelines for the ADAMTS13 Assay.