How gradient enrichment works:

The SurePath collection device "head" is dislodged into the transport fluid, and this 100% sample is transported to the lab. About 80% of the ethanol-fixed SurePath (does not distort the cells) thoroughly-homogenized sample volume (homogenized cellularity from thetotally submitted collection-device end...cellularity within sample fluidand mechanically dislodged from the components of the collection device) is molecularly "strained". Straining happens as differential centrifugation (x2) drives proper density cells (the gradient fluid is said to be formulated to especially favor malignant & benign epithelial cells, regardless of size) through a density-gradient-barrier watery fluid to form a "cleansed" proper-density cellular-concentrate "centrifugation pellet". Too-little-density material (mucous & mucous threads, tumor diathesis protein & fibrin, RBC membranes & platelet clumps, & at least 50% of polys) fails to move through the gradient fluid. Material that is dense enough but small (polys & Trich.) to very small (bacteria) gets stratified within the 3cm. of gradient fluid layer and partly diluted away. Excess sample after the "soft spin" straining is pipetted & disposed of and the remaining sample "hard spin" centrifuged to compact the strained/sifted sample pellet. All gradient-fluid excess is then rapidly poured off. The pellet is then resuspended & additionally homogenized; an aliquot of this homogenate is placed into a  disposable "settling chamber" temporarily affixed to the surface of the diagnostic positively "charged" slide. The charge draws a representative population of (all cells...epithelia, inflammatory, & organisms...are negatively charged) cells tightly (but without distortion) to the slide surface to form a thin and uniform layer of enough negatively charged cells to balance the positively charged slide. Cells do not compete with mucous, protein, platelet clumps, RBC membrane clumps, or excessive polys for space on the slide. Slide is then stained for screening & diagnostic interpretation.  [We use "charged" slides in surgical pathology all of the time to assure better "sticking" of the specimen section to the slide through the sometimes violent staining process (especially in IHC antigen retrieval maneuvers).] 

How membrane filtration works:

A portion of the homogenized methanol-fixed ThinPrep (methanol distorts cells by "rounding" them, & methanol "clots" sample mucous) sample is swished into the collection container from the collection device and gotten to the lab. Part of this partial sample volume is "strained" by a suction vacuum through a cloggable fixed-pore-size thin mechanical membrane until a negative-pressure vacuum sensor detects that membrane "clogging" has occurred...implying that a proper & possibly-concentrated cell-layer has collected on the filter membrane surface (including clotted mucous, protein, blood, and excess polys) mixed with everything larger than the pores in the barrier filter. The more densely mucoid and particulate the sample is, the quicker membrane clogging occurs (quick clogging means less suspensate fluid strained for abnormal epithelial cells). With that thin cell-material layer in place on the filter, the filter is slapped against a diagnostic slide (it can be a charged slide) to leave a cellular monolayer "touch prep" on the slide. The problematic thing here is that the system has no way of knowing whether the vacuum sensor is triggered to stop suction because of a properly cellular surface clogging or due to a heavy component of debris and/or clotted mucous. Slide is then stained for screening & diagnosis.