Characterization Panels: Identification, ER/PR/Her2, PD-L1

CellsWay CTC analysis uses three characterization panels: the CTC Identification Panel, the ER/PR/Her2 Panel and the PD-L1 Panel. This article summarizes the composition and reading of the panels.

CTC Identification Panel

• CD45⁻ (leukocyte negative selection): CD45-positive cells are excluded as leukocytes.
• Cytokeratin⁺ (CK8/18/19): epithelial tumor cell marker.
• DAPI⁺: intact nuclear staining — cell integrity confirmed.
• Epithelial CTCs are identified by the triple phenotype (CD45⁻/CK⁺/DAPI⁺); counts are reported.

ER/PR/Her2 Panel

• Designed especially for breast cancer follow-up.
• Surface markers (ER, PR, HER2) are evaluated simultaneously on the CTC; it provides research data on treatment selection and response.
• Guidance: HER2 expression on CTCs may differ from the tissue profile — dynamic phenotype change is a research topic.

PD-L1 Panel

• The immune checkpoint marker PD-L1 is evaluated on the CTC surface.
• Used in immunotherapy response prediction studies; CTC-based PD-L1 complements (does not replace) tissue PD-L1.

Reading Principles

• The panels are used together with the identification kit; characterization is not interpreted without identification.
• An FMO/control strategy applies to every panel; for continuous markers (PD-L1, ER), threshold interpretation is control-based.
• All panels are for research use only (RUO); they cannot serve as a rationale for clinical decisions.