Haemoglobin (Hb)

What it is

Haemoglobin is the protein in red blood cells that carries oxygen from your lungs to your tissues. It’s measured as part of a full blood count (FBC), one of the most common blood tests, which also counts red cells, white cells and platelets.

Why it matters for training decisions

Endurance performance depends on delivering oxygen to working muscle, so haemoglobin matters. A low result can mean anaemia, which can cause tiredness, weakness and breathlessness. Common causes include low iron, vitamin B12 or folate, and blood loss, so a low result needs a doctor’s assessment rather than a training tweak.

Endurance athletes sometimes have a slightly lower haemoglobin concentration without being anaemic. Training expands plasma (the liquid part of blood), which dilutes the red cells even though the total amount of red cells and haemoglobin in circulation is actually higher than in untrained people. This is sometimes called “sports anaemia”, but it isn’t anaemia in the clinical sense.

Limits of the test

Hydration affects the result (dehydration can push it up), and each lab uses its own reference range. Haemoglobin also says little about iron stores on its own. Low ferritin can come first, while haemoglobin is still normal. Always compare results with the reference range on your report, and talk to your doctor about anything flagged.

How P247 uses it

If you turn on AI features, you can scan a blood test. P247 reads haemoglobin and the other markers with their reference ranges, tracks them over time and adds a plain-English summary for training. It isn’t a diagnosis.

This glossary is general information to help you understand your training data. It isn't medical advice, diagnosis or treatment. For injury, illness, medication, blood test results or any health concern, speak to a qualified health professional.

← Back to the glossary