Cyanosis is a blue coloration of the skin and mucous membranes due to the presence of deoxygenated hemoglobin in blood vessels near the skin surface. It occurs when the oxygen saturation of arterial blood falls below 85-90% (1.5g/dl deoxyhemoglobin).

Although human blood is always a shade of red (except in rare cases of hemoglobin-related disease), the optical properties of skin distort the dark red color of deoxygenated blood to make it appear bluish[1].