Abstract: A method is provided for detecting an analyte indicative of a cancer or a metastatic disease condition, which utilizes the ability of the analyte to induce permeability in a barrier. The method includes providing a biosensor having a barrier which is substantially impermeable to an ion, a permeable membrane which is selective for the ion, and a detector capable of detecting the ion. The biosensor is contacted with a sample including at least one of the ion and the analyte, wherein the analyte causes at least a portion of the ion to pass through the barrier and the membrane. Passage of the ion through the barrier and membrane allows detection of the ion, providing indirect detection of the analyte. In one embodiment, the barrier is a cell monolayer, the membrane is selective for potassium, and the analyte is vascular endothelial cell growth factor.