This paper presents a new approach with frequency selection for fast medical tests. Many medical tests need high speed sensor because of its short reaction time. High frequency biosensor with high sensitivity plays a vital role in fast medical tests. Each medical test has its own reaction time. How to select proper frequency for specific medical test is an important issue. In this paper, one frequency-based capacitive sensor is proposed to prove this new approach with salt water. According to the experimental results, the limit of detection (LOD) can be improved if proper working frequency is selected. The 0.1%wt salt water in 20ul can be detected on proposed system and coefficient of variation (CV) is only 0.19% at 40MHz.