Protein C (PC) deficiency can lead to abnormal thrombus formation in blood vessels, obstructing oxygen and nutrient transport to various organs or tissues. Quantifying PC amount in blood plasma usually takes long time, is difficult and expensive due to its low concentration and other homologous proteins in it. A fiber-optic immunosensor has been under development for several years for the PC real-time assay. In this study, blocking buffers have been examined to reduce the noise caused by non-specific adsorption. Ethanolamine was found to reduce the noise significantly. The effect of various PC homologues (factors II, VII, IX, and X) on the biosensor performance was investigated. The sensor showed little cross reactivity with these homologues. Human serum albumin (HSA) in the sample decreased the signal intensity. However, PC could be still quantified in samples with HSA at the physiological level.