The dielectric properties of blood are extremely important for electromagnetic medical applications such as imaging, hyperthermia and ablation. The aim of this paper is to investigate the dielectric properties of human blood and examine the effects of anti-coagulant agents over a broad frequency range. The measurements were performed using freshly extracted human blood samples with and without the added agents. The measurements were performed within 20 mins after extracting the blood sample. The obtained results demonstrate a significant variation between the dielectric properties of blood with and without anti-coagulant agents. The difference is larger in conductivity than in relative permittivity. The results also demonstrate clear variation in the dielectric properties of female and male blood samples. The findings in this paper suggest that measurements conducted on blood samples with the added agents may not represent the natural in-vivo and inter-patient variations in blood properties. In order to have an accurate representation of in-vivo blood dielectric properties, all factors should be considered carefully.