The present invention relates to the diagnosing of colorectal cancer. The inventors have thus designed an efficient, rapid and cost-effective method to purify and analyse mRNA from small volumes of blood plasma. They found that the RPS28, B2M, TIMP-1 and CLU levels were significantly higher in metastatic colorectal cancer patients. Thus, the present invention relates to a method for diagnosing a colorectal cancer in a subject in need thereof comprising i) determining in a sample obtained from the subject the expression levels of at least one biomarker selected from the group consisting of RPS28, B2M, TIMP-1 and CLU mRNAs.