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A Practical Guide to Haemostasis


Ristocetin-Induced Platelet Agglutination [RIPA]

Introduction

Ristocetin is an antibiotic but was found to cause thrombocytopaenia and so removed from the market. Ristocetin induces binding of VWF to the GpIb complex by an alteration of the electrostatic forces between GpIb and VWF. In the micro-circulation.
Physiologically, it is the high shear rates in the capillary circulation that leads to the unfolding and then binding of VWF to platelets rather than a Ristocetin-like molecule.

Low-dose [<0.5-0.7 mg/mL] Ristocetin-Induced Platelet Agglutination [RIPA] can be of value in identifying patients with Type 2B VWD.  Low-dose RIPA will also be abnormal in patients with Platelet-Type VWD [PT-VWD] in whom there is a gain-of-function mutation in the Gp1b gene that leads to removal of VWF from the plasma, thrombocytopaenia and laboratory findings that are similar to Type 2B VWD.

Most cases of VWD show little response to low concentrations of Ristocetin apart from Type 2B VWD or Platelet-type VWD [PT-WD].


Principles & Methodology

The RIPA assay is based on Light Transmission Aggregometry [LTA] - see LTA - using varying concentrations of Ristocetin - Low dose Ristocetin 0.5mg/mL and High dose Ristocetin 1.5, 5mg/mL. 
As this is a LTA-based test, it employs Platelet Rich Plasma [PRP] from the patient. The agonist [Ristocetin] is added and the degree of agglutination is recorded

Results are expressed as the concentration of Ristocetin [mg/mL] able to induce 30% agglutination of platelets.


Interpretation

In cases in which there is enhanced agglutination with Ristocetin at low concentrations, it is important to establish whether this is a case of Type 2B VWD or Platelet-type von Willebrands disease - see below.


Reference Ranges

In cases of in which there is a low sensitivity to Ristocetin in LTA, this can suggest Type 1, 3, 2A or 2M
VWD. Enhanced agglutination of Ristocetin ie high sensitivity suggests either 2B VWD or PT-VWD.


What Test Next?

In cases in which there is enhanced agglutination to low dose Ristocetin, it is important to distinguish between Type 2B VWD and PT-VWD.  This can be undertaken by:
i. Screening for mutations in the VWF and GP1BA genes
ii. Mixing studies which involve:
   MIX A) Patient plasma and health control platelets
   MIX B) Donor plasma and patient platelets
LTA is performed using low doses of Ristocetin and the pattern of results observed. If the abnormality lies within the VWF protein ie Type 2B VWD then Mix A will demonstrate agglutination whereas Mix B will not whereas if the mutation is in the GpIB receptor then Mix A will show no agglutination but Mix B will.


  Mix A
Patient Plasma + Control Platelets
Mix B
Patient Platelets + Control Plasma
Type 2B VWD Agglutination No Agglutination
PT-VWD No Agglutination Agglutination