A method for detecting a breast cancer biomarker using a biosensor based on a fiber-optic ball resonator, comprising an ultra-sensitive fiber-optic sensor with a low detection limit, wherein detection is performed in vitro by simulating venous blood circulation for measuring CD44 protein levels, the system comprising a syringe pump configured to provide adjustable flow rates approximating venous blood flow, a tubing system through which CD44 protein in concentrations ranging from attomolar to nanomolar is passed, wherein the tubing is pre-treated with a blocking agent, specifically bovine serum albumin (BSA), to reduce non-specific binding, the biosensor being enclosed in a catheter-like device formed of a flexible polymeric cannula with an inner diameter and length suitable for sensor insertion, and connected to an optical backscatter reflectometer for real-time signal detection, the system being further configured to assess specificity using control proteins and to remain functional under variable pressure conditions.