Anemia is commonly defined by low levels of hemoglobin or hematocrit in the blood, which are part of a complete blood count (CBC). Hemoglobin is the protein in red blood cells that carries oxygen, while hematocrit measures the proportion of blood that consists of red blood cells. These levels can vary based on age, sex, altitude, smoking habits, and other factors. For example, the World Health Organization defines anemia as hemoglobin levels below 13 g/dL for men and below 12 g/dL for women.

The causes of anemia can vary widely. It can result from chronic diseases such as kidney disease or inflammatory disorders, nutritional deficiencies like iron, vitamin B12, or folate, or inherited conditions like thalassemia. Lifestyle factors, such as a vegan diet lacking B12 or intense physical activity leading to dilutional anemia, can also contribute. Anemia can also be due to acute or chronic blood loss.

Symptoms of anemia include fatigue, weakness, pale skin, and shortness of breath. Diagnosis often starts with a CBC to check hemoglobin and hematocrit levels. Additional tests might include iron studies, vitamin levels, and a reticulocyte count to see how well your bone marrow is making red blood cells. A blood smear can reveal the shape and size of red blood cells, giving more clues to the cause of anemia.

Certain groups are more at risk for anemia, including older adults, young women due to menstruation, and people with chronic illnesses. Athletes and pregnant women might also face unique challenges due to their increased blood volume needs. It's important to monitor symptoms and get regular checkups if you're in one of these groups.

Anemia can have many causes and symptoms, so it's important to have regular checkups and maintain a balanced diet.