Sensors based on functionalized optical resonators provide high specificity, high sensitivity, very high detection limit and fast response. The sensing principle of such sensors is based on the detection of a change of environment at the vicinity of the optical resonator surface. This change induces an effective index variation of the guided mode circulating in the resonator, resulting in a resonance spectral shift in the optical resonator response. The detection limit (i.e. the smallest amount of analyte that can be detected by sensors) depends on the resolution of the sensor and of its background noise. Improving the detection limits of sensors requires then to minimize their background noise by exploring various real-time configurations. In this report, we present direct measurements methods at different points of the resonance transmittance response (at minimum point and at inflexion points where the slope of transmittance is maximum) and indirect methods (resonance transmittance fit with Lorentzian and microresonator transmission models) to determine the resonance wavelength. Using an optofluidic label-free sensor based on a polymeric vertically coupled optical microracetrack, we demonstrated that measuring a spectral shift at the minimum of the sensor spectral transmittance at resonance is not the best solution to reduce the measurement background noise of sensors. For different analyte concentrations, the background noise obtained with the inflexion point method is reduced 3 times as compared to the minimum point method. On another hand, the sensor response time is 1000 times less than that obtained with the method using transmission model fitting or Lorentzian fitting. This method of measurement can be extended for any sensors based on optical resonators.