Methods and systems for selectively capturing analytes, such as cells, e.g., circulating tumor cells (CTCs), from fluid samples are disclosed. The methods include contacting the sample with an analyte binding moiety that selectively binds to the analytes; optionally separating first components of the sample including a majority of the analytes bound to the binding moieties from second components of the sample using size-based separation, e.g., in a microfluidic channel; adding to the first components of the sample a plurality of binding agents under conditions that enable a plurality of the binding agents to be linked to the analyte binding moieties to form multivalent tagging agents bound to the analyte; passing the first components of the sample past a surface to which is attached a plurality of capture agents that selectively bind to the binding agents; and capturing the analytes by providing conditions that enable the multivalent tagging agents bound to the analytes to bind to one or more of the capture agents.