<p>Accurate and rapid detection of red blood cells (RBCs) is essential for anemia diagnosis, disease monitoring, and blood group determination. This study developed an ion-sensitive field-effect transistor (ISFET)-based sensor using zinc oxide (ZnO) nanorods as the sensitive layer and anti-antigen A as the sensing layer. X-ray diffraction and scanning microscopic images were used to verify the ZnO fabrication, and after determining the number of cells, dilution was performed, and the RBCs in plasma and urine as electrolytes was investigated using the source-drain current (I_DS) versus source-drain voltage measurement technique at constant gate voltages applied to the working electrode (Ag/AgCl). The results indicated the ability to detect 770 cells/mL with sensitivities of 0.13 and 0.09 μA/cell in plasma and urine, respectively. The proposed sensor was capable of counting the number of cells over time, and the kinetic mechanism of investigation was pseudo-first-order reaction and then the efficiency of the sensor depends on environmental conditions such as pH, temperature, and electrolyte concentration in addition to the number of RBCs. The proposed ISFET-based sensor with rapid, label-free, real-time, and highly sensitive RBC detection can be a suitable alternative to the methods commonly used in biomedical research and clinical applications.</p>