Abstract: The present invention provides a quantitative liquid biopsy diagnostic system and methods for performing diagnostic assays. The system offers a liquid biopsy method using circulating tumor cells (CTCs) or White Blood Cells (WBC) subpopulations for precision cancer diagnosis, early detection of disease evolution, and cancer patient management. The invention utilizes selective plane illumination microscopy (SPIM) to deliver high sensitivity and specificity for the detection and isolation of individual CTCs, superseding the efficacy of existing methodologies for early cancer detection. Isolated CTCs can be analyzed for their molecular fingerprint, which can lead to matching genetic abnormalities with specific drug treatments. The system allows ex vivo observation of live CTC or WBC response to treatment.