Monitoring the concentration of a therapeutic drug antibody, infliximab (IFX), is recommended for enhancing its efficacy in patients with inflammatory bowel disease (IBD). However, IFX concentrations are currently determined in patients' serum/plasma, which requires sample preparation from blood, hence hampering the turnaround time. In this paper, we present a short immunoassay (10 min) using a fiber-optic surface plasmon resonance (FO-SPR) biosensor for detection of IFX spiked in 100-fold diluted serum, plasma, and whole blood. The calculated limits of detection (LOD) based on calibration curves were 1.42, 1.00, and 1.34 ng/mL, respectively, which coincides with expected IFX concentrations in diluted samples from IBD patients. A linear correlation was established among different matrixes, indicating that the matrix effect was insignificant. The established point-of-care (POC) FO-SPR bioassay was also used to measure IFX in 100-fold diluted extracts of dried blood spots (DBS), and LOD achieved was below 2 ng/mL. Although DBS might be ideal for POC, this is the first report of using an SPR biosensor for measuring DBS samples. Finally, the POC FO-SPR immunoassay was validated by using matching serum and plasma samples from five IBD patients. A Pearson correlation of 0.968 was obtained between serum and plasma samples. IFX concentrations determined with FO-SPR were compared to a clinically validated enzyme-linked immunosorbent assay (ELISA), resulting in excellent Pearson correlation and intraclass correlation coefficient, both being 0.99 for serum and plasma samples. In conclusion, this paper demonstrates that our FO-SPR biosensor can be used as a true POC diagnostic tool for determining IFX concentrations in a variety of matrixes.