Electrochemical immunosensors are simple, portable diagnostic tools that are used for the detection and quantification of protein biomarkers. While electrochemical immunosensors are capable of rapid and high sensitivity measurements, they require complicated and/or long sample processing procedures or suffer from poor sensitivity using whole blood samples. Therefore, the goal of this research is to investigate new strategies to achieve rapid, high sensitivity protein measurements in complex biological matrices, such as blood serum and whole blood. This thesis introduces two novel electrochemical immunosensing platforms. The first is a microfluidic magneto-immunosensor based on a unique sensing scheme utilizing dual-labeled magnetic nanobeads for immunomagnetic enrichment and signal amplification. The functionality of this immunosensor was validated by using it to detect SARS-CoV-2 nucleocapsid protein, which could be detected at concentrations as low as 50 pg/ml in undiluted human serum in 55 minutes. This magneto-immunosensor was further developed into an electricity-free, mobile phone-based point-of-care (mPOC) diagnostic platform for the quantification of Plasmodium falciparum histidine-rich protein 2 (PfHRP2), a malaria parasite biomarker, which could be detected in whole blood at concentrations as low as 180 pg/mL in 15 minutes. The functionality of this mPOC platform was validated using 31 clinical blood samples obtained from malaria patients, which revealed its ability to quantify PfHRP2 with similar accuracy as a commercial ultrasensitive PfHRP2 ELISA kit. The second platform that was developed was an alternating current electrothermal flow (ACEF)- enhanced immunosensor, which utilizes electrothermal flow-induced mixing for enhanced immunocomplex formation and electrochemical detection. Proof of concept was carried out by performing measurements of buffer samples spiked with PfHRP2, which could be detected at concentrations as low as 1 ng/mL in 10 min. This ACEF-enhanced immunosensing technique was combined with our magneto-electrochemical immunosensor, which resulted in a significant improvement in the analytical performance, resulting in a lower limit of detection of 5.7 pg/mL in whole blood. Furthermore, each measurement could be completed in 7 minutes. In conclusion, we have presented several unique microfluidic magneto-electrochemical immunosensors capable of rapid, high sensitivity measurements in whole blood and blood-derived samples, which are promising diagnostic tools for rapid diagnostic testing and point-of-care testing.