Haemoglobin is the iron-containing protein in red blood cells responsible for binding oxygen in the lungs and delivering it to tissues throughout the body. It also carries carbon dioxide from tissues back to the lungs for exhalation. Haemoglobin is the most direct measure of the blood's oxygen-carrying capacity.

Low haemoglobin defines anaemia — a condition that causes fatigue, breathlessness, pallor, poor concentration, and reduced exercise tolerance. The most common causes include iron deficiency, vitamin B12 deficiency, folate deficiency, chronic disease, and blood loss. The pattern of red blood cell indices (MCV, MCH, MCHC) alongside haemoglobin helps identify the type of anaemia and guide further investigation.