Methods and compositions are provided for specific aptamers and aptamer pools that bind biomarkers of interest such as microvesicle surface antigens or functional fragments of microvesicle surface antigens. In various embodiments, aptamers of the invention are used in diagnostic, prognostic, or theranostic processes to screen a biological sample for the presence or levels of biomarkers such as microvesicles that are determined to provide a diagnostic, prognostic, or theranostic readout. The diagnosis, prognosis, or theranosis may be related to cancer or other diseases and disorders. The invention also provides methods and composition to facilitate aptamer library screening and aptamer detection methods.