The present invention provides a novel method of in vitro sample preparation for the analysis of proteins. In a first aspect, the present invention is a method comprising the separation and individual treatment of blood platelets and platelet-poor plasma obtained from a platelet-rich plasma sample. This allows for optimal treatment of both fractions and overcomes many of the problems associated with current methods. In a second aspect of the present invention the platelet sample preparation method can be implemented before a biochip based immunoassay for the detection of platelet proteins in combination with plasma proteins.