▁or ▁M aint rac . ▁In ▁very ▁rare ▁cases , ▁C TC s ▁are ▁present ▁in ▁large ▁enough ▁quantities ▁to ▁be ▁visible ▁on ▁routine ▁blood ▁sm ear ▁exam ination . ▁This ▁is ▁referred ▁to ▁as ▁car cin oc yth emia ▁or ▁car cin oma ▁cell ▁le uk emia ▁and ▁is ▁associated ▁with ▁a ▁poor ▁pro gn osis . ▁ ▁D ete ction ▁methods ▁ ▁To ▁date , ▁a ▁variety ▁of ▁research ▁methods ▁have ▁been ▁developed ▁to ▁isol ate ▁and ▁enumerate ▁C TC s . ▁The ▁only ▁U . S . ▁Food ▁and ▁D rug ▁Administration ▁( F DA ) ▁cleared ▁method ology ▁for ▁enumer ation ▁of ▁C TC ▁in ▁whole ▁blood ▁is ▁the ▁Cell Search ▁system . ▁Ext ensive ▁clin ical ▁testing ▁done ▁using ▁this ▁method ▁shows ▁that ▁presence ▁of ▁C TC s ▁is ▁a ▁strong ▁pro gn ost ic ▁factor ▁for ▁overall ▁surv ival ▁in ▁patients ▁with ▁met ast atic ▁breast , ▁col ore ct al ▁or ▁pro state ▁cancer . ▁ ▁C TC s ▁are ▁p iv otal ▁to ▁understanding ▁the ▁bi ology ▁of ▁met ast asis ▁and ▁promise ▁potential ▁as ▁a ▁bi om ark er ▁to ▁non in vas ively ▁evaluate ▁tum or ▁pro gression ▁and ▁response ▁to ▁treatment . ▁However , ▁isol ation ▁and ▁character ization ▁of ▁C TC s ▁represent ▁a ▁major ▁techn ological ▁challenge , ▁since ▁C TC s ▁make ▁up ▁a ▁minute ▁number ▁of ▁the ▁total ▁cells ▁in ▁circul ating ▁blood , ▁ 1 – 1 0 ▁C TC s ▁per ▁m L ▁of ▁whole ▁blood