Continuous measurement of the hematocrit level in blood can potentially be performed using optical fibre sensors. The Fabry-Perot interferometric sensors are a promising candidate in this application. The most important step in the design of the sensor is design of the sensing interferometer. Adequate cavity length and high interference contrast are two most important requirements in this application. The design method of the sensing interferometer presented in this paper uses a Gaussian beam approximation. In order to verify the design, an optical fibre Fabry-Perot interferometer with adjustable cavity length was built. Its performance was tested, confirming validity of the design approach.