In this paper a GST-based perfect absorber is investigated for sensing applications. The simulation results show the high sensitivity and figure of merit of 900 nm per refractive index unit (RIU) and 15 RIU-1, respectively. The sensing parameters are also studied for different biomedical applications such as detection of glucose in water, malaria infection, bacillus bacteria, and cancer cells. The phase change material GST makes the structure tunable and switchable by changing the phase through different annealing temperatures. By changing temperature, the optical characteristics of GST are altered and so the perfect absorption can be tuned at different near-infrared wavelengths. Moreover, the sweep between the amorphous and crystalline states of GST results in switching capability with the high extinction ratio of 13.97 dB at λ = 1724 nm. Therefore, the perfect tunable absorber supports our new idea of multi-application plasmonic devices with two capabilities of sensing and switching, simultaneously. Also, the E-field distributions and analytical method of equivalent circuit model are utilized to verify the simulation results and demonstrate a better insight of the proposed structure performance. The polarization independency of the proposed structure, tunability, good sensing and switching performances are some other benefits of our proposed structure, which can pave the way for development of new investigations about the phase change material applications in plasmonic studies.