A haemoglobin test measures the amount of haemoglobin in your blood. Haemoglobin is a protein in your red blood cells that carries oxygen to your body's organs and tissues and transports carbon dioxide from your organs and tissues back to your lungs. If a haemoglobin test reveals that your haemoglobin level is lower than normal, it means you have a low red blood cell count (anaemia). Anaemia can have many different causes, including vitamin deficiencies, bleeding and chronic diseases. If a haemoglobin test shows a higher than normal level, there are several potential causes — the blood disorder polycythemia vera, living at a high altitude, smoking and dehydration.