Cyanosis is blue or purple coloration of skin or mucous membranes, due to tissue near skin surface having low oxygen saturation. Signs usually first appear on the lips or fingers.
It occurs when >5g/dL of deoxyhemoglobin is present. As such, if there is a low hemoglobin count, such as in anemia, the bluish color will be less apparent. It is also more difficult to detect on deeply pigmented skin