Ristocetin-induced platelet aggregation explained

Ristocetin-induced platelet aggregation
Purpose:	It measures platelet aggregation with the help of von Willebrand factor

The ristocetin-induced platelet aggregation (RIPA) is an ex vivo assay for live platelet function. It measures platelet aggregation with the help of von Willebrand factor (vWF) and exogenous antibiotic ristocetin added in a graded fashion.^[1] It is similar to the ristocetin cofactor assay but has the added benefit in that it helps in the diagnosis of type 2B/pseudo von Willebrand disease (vWD) and Bernard–Soulier syndrome because it uses patient's live endogenous platelets, whereas ristocetin cofactor assay tests the function of only the vWF and not the platelets. Ristocetin cofactor assay uses the patient's platelet poor plasma (with vWF but no platelets) and adds ristocetin and exogenous formalin-fixed platelets which can passively agglutinate (but not actively aggregate, as they are dead). Formalin does not allow the extrinsic platelets to secrete the vWF of their α-granules, and thus only the activity of the intrinsic vWF is tested.

In an unknown fashion, the antibiotic ristocetin causes von Willebrand factor to bind the platelet receptor glycoprotein Ib (GpIb), so when ristocetin is added to normal blood, it causes agglutination of fixed platelets or initiates the initial agglutination phase of aggregation of live platelets.

The results of the ristocetin-induced platelet aggregation in some characteristic diseases are the following:

• Type 1 vWD: hypoactive agglutination occurs (consistent with ristocetin cofactor assay results)
• Type 2A vWD: hypoactive agglutination occurs (consistent with ristocetin cofactor assay results)
• Type 2B vWD: hyperactive agglutination occurs (when the ristocetin cofactor assay is used, the agglutination is hypoactive)
• Type 2M vWD: hypoactive agglutination occurs ^[2] (consistent with ristocetin cofactor assay results)
• Type 2N vWD: normal agglutination occurs (consistent with ristocetin cofactor assay results)
• Type 3 vWD: no agglutination occurs (consistent with ristocetin cofactor assay results)
• Pseudo-vWD: hyperactive agglutination occurs (when the ristocetin cofactor assay is used, the agglutination is hypoactive)
• Bernard-Soulier syndrome: hypoactive agglutination occurs (when the ristocetin cofactor assay is used, the agglutination is normal)^[3]

INTERPRETATION OF TESTRAT+ Ve= normal (gp1b & vWF present)RAT-ve=abnormal(gp1b & vWF absent)

Notes and References

1. Coller BS, Gralnick HR . Studies on the mechanism of ristocetin-induced platelet adhesion. Effects of structural modification of ristocetin and vancomycin . The Journal of Clinical Investigation . 60 . 2 . 302–12 . August 1977 . 17620 . 372370 . 10.1172/JCI108778.
2. Riddell AF, Jenkins PV, Nitu-Whalley IC, McCraw AH, Lee CA, Brown, SA . Use of the collagen-binding assay for von Willebrand factor in the analysis of type 2M von Willebrand disease: a comparison with the ristocetin cofactor assay . British Journal of Haematology . 116 . 1 . 187–92 . January 2002 . 11841416 . 10.1046/j.1365-2141.2002.03199.x. 40676013 . free .
3. Pham A, Wang J. Bernard-Soulier syndrome: an inherited platelet disorder. Arch Pathol Lab Med (2007);131(12):1834-6.