Hematocrit (HCT): measures the percentage of your blood that is made up of red blood cells. Red blood cells carry oxygen to your body’s tissues, so this value helps show how well your blood can transport oxygen. Normal levels indicate a healthy balance of red blood cells in your blood. Low hematocrit may suggest anemia, blood loss, or nutritional deficiencies, which can cause fatigue, weakness, or shortness of breath. High hematocrit may indicate dehydration, living at high altitude, or certain medical conditions that increase red blood cell production.