Abstract: The present invention provides methods and compositions for isolating and detecting rare cells from a biological sample containing other types of cells. In particular, the present invention includes a debulking step that uses a microfabricated filters for filtering fluid samples and the enriched rare cells can be used in a downstream process such as identifies, characterizes or even grown in culture or used in other ways. The invention also include a method of determining the aggressiveness of the tumor or of the number or proportion of cancer cells in the enriched sample by detecting the presence or amount of telomerase activity or telomerase nucleic acid or telomerase expression after enrichment of rare cells.