Table 1.

Advantages and limitations of current enrichment methodologies.

	Method	Advantage	Limitation	Current improvement
Physical properties	Size/ deformability based filtration	Easy to use High throughput High CTC cluster capture efficacy	Loss of CTCs with size equal to and smaller than pore size of filter Low purity CTC damage due to hemodynamic stress	Introduce CTC size-amplification strategy to reduce the loss of CTC with small size Employ 3D filter to improve cell viability Use microfluidic devices to reduce shear forces and to exclusively enrich clusters
	Density-gradient centrifugate	Easy to use High throughput	Loss of CTCs with high density Low purity	Depletion of leukocytes by bi-specific antibodies to improve purity Integrate filter into centrifuge tube to reduce the loss of cluster and CTC with large size Use microfluidic devices designed with inertial focusing and vortex function
	Electric property based dielectro-phoresis	High CTC viability	Loss of CTCs with electrical property similar to leukocytes Low purity
Biological properties	Positive selection	High purity	Loss of CTC subpopulations including EMTed CTCs, clusters, and CTCs cloaked by blood cells Tumor-specific assays may be required since CTC phenotype varies by cancer type and over disease progression and treatment course	Search new markers, e.g. actin bundling protein plastin 3 Utilize combination of antibodies Use microfluidic devices integrated with micromixer to increase capture efficacy
	Negative selection	Potential to enrich all CTC subpopulations	Low purity	Introduce microfluidic devices to improve purity