Hemoglobin (Hb) is an oxygen-carrying protein found inside your red blood cells. A hemoglobin test measures the amount (concentration) of hemoglobin in your blood level, usually reported in grams per deciliter (g/dL). Because hemoglobin binds oxygen in the lungs and delivers it to the rest of the body, your Hb level is a key indicator of how well your blood can carry oxygen to your organs and muscles.

In plain language, this test helps answer: Do you have enough healthy, oxygen-carrying red blood cells? A low hemoglobin result often suggests anemia—meaning your blood may not be carrying enough oxygen. This can lead to symptoms like fatigue, weakness, shortness of breath, pale skin, headaches, and dizziness. One very common cause is iron deficiency anemia, when your body doesn’t have enough iron to make adequate hemoglobin.

A high hemoglobin result can happen when your body makes extra red blood cells (for example, in response to low oxygen from smoking, chronic lung disease, or high altitude) or when you’re dehydrated and your blood becomes more concentrated. Your clinician interprets Hb results along with other blood tests—especially the complete blood count (CBC), including hematocrit, red blood cell indices (MCV/MCH), and sometimes iron studies—to understand the cause and next steps.

The hemoglobin test is commonly ordered as part of a complete blood count (CBC) during routine checkups, pre-surgery evaluations, pregnancy care, or when symptoms suggest anemia or another blood disorder. It may be recommended if you have:

Symptoms that may indicate anemia or low oxygen delivery such as ongoing tiredness, weakness, reduced exercise tolerance, dizziness or lightheadedness, headaches, rapid heartbeat, chest discomfort, shortness of breath, cold hands/feet, or pale skin.

Risk factors for low hemoglobin, including heavy menstrual bleeding, pregnancy, recent childbirth, a diet low in iron, B12, or folate, gastrointestinal blood loss (such as ulcers, hemorrhoids, or colon polyps), frequent blood donation, chronic kidney disease (reduced erythropoietin production), inflammatory conditions, or a history of anemia.

Reasons to evaluate or monitor known conditions, such as previously diagnosed anemia (including iron deficiency anemia), treatment response to iron supplements, B12/folate therapy, or erythropoietin; monitoring after bleeding, surgery, or hospitalization; and follow-up for inherited blood conditions like sickle cell anemia or thalassemia.

Reasons to check for high hemoglobin, such as symptoms or history that suggest increased red blood cell production (polycythemia), living at high altitude, smoking, sleep apnea, or chronic lung/heart disease that can lower oxygen levels and stimulate the body to make more hemoglobin.

• Anemia (general)
• Iron deficiency anemia
• Anemia of chronic disease/inflammation
• Vitamin B12 deficiency anemia
• Folate deficiency anemia
• Sickle cell disease (sickle cell anemia)
• Thalassemia
• Chronic kidney disease–related anemia
• Gastrointestinal bleeding (e.g., ulcers, colon polyps, colorectal cancer)
• Polycythemia vera or secondary polycythemia (high hemoglobin)
• Chronic lung disease (e.g., COPD) and sleep apnea–related high hemoglobin

• Screening for anemia and maintaining healthy energy levels
• Evaluating causes of dizziness, fatigue, weakness, or shortness of breath
• Monitoring iron status and response to iron supplementation (especially in iron deficiency anemia)
• Supporting pregnancy wellness by checking for pregnancy-related anemia
• Assessing overall wellness as part of a routine CBC and preventive care
• Pre-operative assessment to reduce surgical risk from low blood counts
• Monitoring chronic conditions (e.g., kidney disease, inflammatory disease) that can affect hemoglobin
• Tracking inherited blood disorders (e.g., sickle cell anemia, thalassemia) over time