Hemoglobin

Also known as Hgb, Hb, haemoglobin

protein · no single molecular formula

LOINC 718-7 code verified whole_blood g/dL

unverified These explanations are written to be easy to read and have not yet been checked against a source. The ranges and measurement notes on this page show theirs.

definition unverified

The iron-rich protein inside red blood cells that carries oxygen around your body. It is what makes blood red.

Iron-containing tetrameric protein within erythrocytes; binds oxygen reversibly at the pulmonary capillary and releases it in tissue.

why measured unverified

It is the number that defines anaemia, and anaemia is one of the most common findings in any blood test.

No clinical version written yet.

if high unverified

Most often dehydration concentrating the blood. Less commonly the body is making extra red cells, which can happen at altitude, with smoking, or with untreated sleep apnoea.

No clinical version written yet.

if low unverified

This is anaemia. It says that oxygen carrying is reduced, but not why. The cell size result sitting next to it is what narrows down the cause.

No clinical version written yet.

caveats unverified

Men and women have different normal ranges. Living at high altitude raises it too, and that is not a problem.

No clinical version written yet.

What this substance is

A tetramer of two alpha and two beta chains, about 64.5 kDa, each carrying a haem group. The haem is a small molecule with a real structure; the analyte is the assembled protein.

UniProt P69905 · PDB 1HHO, 2HHB

Each molecule is drawn from ChEBI's curated 2D drawing, accepted only after its InChIKey, formula and charge matched the ChEBI release. Atom diagrams are checked against the NIST Atomic Spectra Database.

3D model

Deoxyhaemoglobin: two alpha and two beta chains (PDB 2HHB). Each chain holds one haem group. Experimental structure from the Protein Data Bank, entry 2HHB, shown as its biological assembly (4 chains). Every chain was checked against its UniProt sequence (at least 100.0% identical). Drag to rotate, scroll to zoom.

Reference ranges

Checking your own result against this range needs the running application; this is a static copy. Nothing you type anywhere in Empeiria is ever sent to a server.

This open edition shows a range only where a public-domain or openly licensed source states one, and none does for this marker yet. Compare your result with the range printed on your own lab report.

How it is measured

In practice

Hemoglobin is usually measured as part of a complete blood count (CBC). A health worker takes a small blood sample from a vein in your arm. You don't need to get ready in any special way unless other tests are done at the same time.

Venous whole blood in a K2 EDTA tube, usually as part of the CBC. Hemoglobin is measured directly (photometrically) by the analyser. No special preparation; fasting only if other tests are ordered.

Before the test

No special preparation; fast only if other blood tests ordered with it require it.

Where: laboratory

Sources: Hemoglobin Test, MedlinePlus (U.S. National Library of Medicine); Laboratory Procedure Manual: Complete Blood Count, NHANES 2015-2016, CDC National Center for Health Statistics cited · not yet checked by a person

In the lab

spectrophotometry

Sodium lauryl sulfate (SLS) haemoglobin

Blood contains several slightly different forms of haemoglobin. They are all converted into one form first, so a single measurement covers the lot.

All haemoglobin species are converted to a single stable SLS-haemoglobin derivative and measured by absorbance.

Read by: Absorbance at 555 nm
Standardised to: ICSH cyanmethaemoglobin reference

Why labs disagree: Automated analyser methods such as SLS are not the cyanmethaemoglobin (HiCN) reference method, and small but systematic differences between analyser results and HiCN have been shown. More broadly, different measurement procedures give different results, haemoglobin differs between men and women, and labs are advised to set intervals on their own equipment and routines.

Sources: Arbiol-Roca A et al. Reference intervals for a complete blood count on an automated haematology analyser Sysmex XN in healthy adults from the southern metropolitan area of Barcelona. EJIFCC 2018 (PMC5949618, open access); Stawschenko E et al. Current Status of Measurement Accuracy for Total Hemoglobin Concentration in the Clinical Context. Biosensors 2022 (PMC9775510, open access) cited · not yet checked by a person

What it reports on

erythrocyte structural component of
Hemoglobin is the iron-rich protein inside red blood cells that carries oxygen.
bone marrow synthesized by
Red blood cells, which contain hemoglobin, are made in the bone marrow.

Associated symptoms

When low

low Feeling tired all the time · non specific · common
low Getting out of breath easily · suggestive · common
Reduced oxygen-carrying capacity becomes limiting first under exertion.
low Looking unusually pale · suggestive · common
low Noticing your heartbeat · suggestive · occasional
Cardiac output rises to compensate for reduced oxygen delivery.
low Feeling lightheaded · non specific · occasional
low Headaches · non specific · occasional

Conditions this marker features in

When high

high Too many red blood cells · defining
Diagnostic threshold for polycythaemia vera (WHO 2016): above 16.5 g/dL in men and 16 g/dL in women. The same levels trigger referral for erythrocytosis. It should be confirmed on at least two blood counts.

When low

low Anaemia caused by long-term illness · defining
It is an anaemia: haemoglobin below 120 g/L in women or 130 g/L in men, together with inflammation and its typical iron pattern.
low Anaemia from red cells breaking down too fast · defining
Part of the definition (review article, not a formal diagnostic criterion): haemolysis becomes haemolytic anaemia once haemoglobin falls below the normal range. Haemoglobin also shows how severe it is and is used to follow treatment.
low Low iron causing anaemia · defining
Anaemia is defined as haemoglobin below the lower limit of normal. Iron stores can run low while haemoglobin is still normal.
low Carrier of beta-thalassaemia · supportive
Haemoglobin may be mildly low, or normal. Iron treatment does not raise it.
low Long-term reduced kidney function · monitoring
Anaemia is a common complication, and becomes more common as kidney function falls; blood counts are checked regularly.
low Low vitamin B12 · supportive
Anaemia (low haemoglobin) with large red cells is the most common blood finding, though nerve problems can occur without anaemia.

Ordered as part of

cbc