Hemoglobin (Hb, Hgb, HGB, Haemoglobin (British spelling), Hemoglobin level, Hgb blood test)

How to read your result

Reading a low result: first ask how low. A few tenths under the cutoff is a gray zone that hydration and lab variation can explain, and is usually rechecked; clearly low values mean anemia, and the real question becomes why — iron deficiency, blood loss, vitamin deficiency, kidney disease, or chronic illness. Below about 7 g/dL, labs flag the result as critical and same-day care is standard. In men, even mild anemia is always worked up because there is no menstrual loss to explain it; in menstruating women, iron deficiency is the leading cause but still deserves confirmation.

Reading a high result: mild elevations are most often concentration effects (dehydration) or adaptations (smoking, altitude, sleep apnea), so the first step is a well-hydrated repeat. Persistent values above roughly 16.0 g/dL in women or 16.5 g/dL in men — the current WHO (2016) polycythemia vera thresholds — earn a workup that may include erythropoietin levels and JAK2 testing. At the extreme top of the scale, around 20 g/dL, most US labs flag the result as a critical value needing same-day attention. In both directions, compare against the range on your own report and your own prior results — your personal trend beats any generic chart.

What can shift this value

• Hydration — dehydration concentrates the blood and can raise hemoglobin by up to about 1 g/dL; heavy hydration dilutes it
• Smoking — chronic carbon monoxide exposure drives the body to make extra red cells
• Altitude — people living at elevation (roughly above 5,000 feet) run naturally higher values
• Pregnancy — plasma volume expands faster than red cells, so hemoglobin normally falls and lower anemia cutoffs apply
• Menstruation and blood donation — recurring blood loss gradually depletes iron stores
• Medications — testosterone and anabolic steroids raise hemoglobin; chemotherapy and some other drugs lower it
• Chronic disease — kidney disease lowers hemoglobin by reducing erythropoietin; lung disease and untreated sleep apnea raise it
• Endurance training — expanded plasma volume can produce a dilutional "athlete's pseudoanemia" in healthy athletes
• Recent illness, surgery, or blood loss — even modest bleeding shows up in the number for weeks
• Lab variation — reference ranges, instruments, and even posture and tourniquet time at the draw shift results slightly

When it's tested

Hemoglobin comes standard on every complete blood count (CBC), so it is checked at routine physicals, in anemia and fatigue workups, before surgery, during pregnancy care, and while monitoring chronic kidney disease, bleeding disorders, or chemotherapy. It is not part of a metabolic panel (CMP) or lipid panel. You may also meet it as a quick fingerstick check before donating blood — donation centers screen it every time.

US reports use g/dL; multiply by 10 to get the g/L used in the UK and much of the world (13.5 g/dL = 135 g/L), and a few countries report mmol/L, where about 8.4 mmol/L corresponds to 13.5 g/dL.