🧬 ADAMTS13 Activity Test — Complete Guide

Key diagnostic test for thrombotic thrombocytopenic purpura (TTP)
Principle · ELISA method · Reference range · Clinical significance · Interpretation points

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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.

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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)

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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.

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🔬 Principle of the ELISA Method

1. A synthetic peptide mimicking the vWF A2 domain is coated onto the plate.
2. Patient plasma is added → ADAMTS13 cleaves this substrate.
3. The cleavage exposes a specific neo-epitope.
4. A detection antibody (anti-cleaved vWF) labeled with peroxidase binds the epitope.
5. Color development is proportional to the amount of substrate cleaved.
6. Optical density is converted into percent activity (%).

⮕ Low activity reflects impaired cleavage ability → supportive of TTP.

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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

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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.

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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

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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.

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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.

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✔ 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.

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📚 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.