The symbol is written HbA1c: Hb is haemoglobin with a capital H, A is its adult form, and 1c is the fraction number, with a lower-case c in the subscript. The forms A1C and A1c are also found and mean the same thing; «НвА1с» in Cyrillic and «HBA1C» will not do, because every letter carries a meaning.
HbA1c per cent is a fraction of haemoglobin, not blood glucose: 7 % does not mean 7 mmol/L, and the average glucose at such a result is about 8.6 mmol/L. Nor is a millimole per mole a millimole per litre: for HbA1c the denominator is a mole of haemoglobin, not a litre of plasma.
A red blood cell with the symbol HbA1c inside slowly turns and glows, while glucose hexagons drift towards it from three sides and stick to its edge, where their predecessors already sit. On the right a marker runs along a chromatogram and climbs the tall peak of the A1c fraction, and below, the slider of a double ruler — per cent on top, millimoles per mole underneath — reaches the 6.5 % threshold, which reads as 48 on the lower scale.
Quantity data sheet
haemoglobin fraction · lab test sheet 1/1 · rev. 2026-09Glycated haemoglobin HbA1c · % · mmol/mol
Dimensionless quantity · fraction of glycated HbA; NGSP (%) and IFCC (mmol/mol)
A test that remembers three months
A glucose meter answers the question of how much sugar is in the blood right now, so its answer changes after every breakfast, whereas a doctor needs a different answer — what the sugar has been like over the whole recent period. Haemoglobin provides it: a red blood cell lives for about a hundred and twenty days, and throughout that time plasma glucose slowly and irreversibly attaches to its protein, so the share of “sugar-coated” haemoglobin becomes a record of average glucose over three to four months, one that cannot be touched up by dieting the day before the test.
The trouble is that this record can be read in two ways. American and most Asian laboratories report the result in per cent on the NGSP scale, tied to the DCCT clinical trial, while the United Kingdom, almost all of Europe and Australia have since 2009—2011 reported it in millimoles per mole according to the IFCC reference method. Both scales measure the same thing, but they are linked not by a factor but by a shift, which is why the diabetes threshold looks like 6.5 on one and 48 on the other.
This sheet shows how red blood cells of different ages add up to a single result, why the last month accounts for almost half of it, and why “lowering sugar by ten per cent” means different things on the two scales.
01 · Definition
two scales · average glucose · weight of the monthsHbA1c is the fraction of adult haemoglobin molecules in which a glucose molecule has irreversibly attached to the terminal amino acid of the β-chain, valine. The IFCC scale expresses this fraction literally, in millimoles of glycated haemoglobin per mole of total HbA, while the NGSP scale gives it in per cent, recalculated to match the method used to measure the participants of the DCCT trial in the 1980s.
The old methods captured a little of other haemoglobin forms along with the true fraction, so their percentages are shifted upwards by about 2.15 points relative to the exact fraction. Hence the key feature of the conversion: it is not proportional but affine, with a slope and a shift, and the line linking the scales does not pass through zero, so ratios and percentage changes differ between the two scales.
Average glucose, known as estimated average glucose, eAG, is derived from HbA1c with the formula of the ADAG study of 2008, and it too does not pass through zero. Moreover, HbA1c is a weighted average: young red blood cells carry traces of the last few weeks only, old ones of their whole life, and in the end the last month gives almost half of the result and the fourth only a sixteenth.
Why the test lags behind the glucose meter
The axis from left to right is the age of a red blood cell and, at the same time, the number of days ago: a cell at the 90 mark has lived through everything to its left. The upper band shows average glucose, crimson for the last thirty days and steel blue for the ninety before them, and the pale wedge beneath it is the weight of each day in the final result. The violet hexagons on the red cells are attached glucose: older cells carry more of them. The bar on the right is HbA1c itself, per cent on the left, millimoles per mole on the right.
20 5.7
39 6.5
48 10
86 14 %
130
As many young cells as old ones
Red blood cells of all ages live in the blood at the same time, from a day old to a hundred and twenty days old, and each yesterday has left its imprint on every cell older than a day, whereas a day four months ago is remembered only by the very oldest. So the weight of a day falls linearly with how long ago it was, and when the weights are added up, the last thirty days account for 44 % of the result, the next thirty for 31 %, and the fourth month for only 6 %. Hence the clinical rule: repeat the test no sooner than three months later, otherwise it will mostly show what was there before the change.
02 · Conversion
scales · thresholds · percentage trapFrom per cent to millimoles per mole and back
Enter a result in any of the accepted notations and the sheet will express it on both scales, convert it to average glucose, set it against the international thresholds and show how the same change looks in per cent and in millimoles per mole.
A rule to remember: one NGSP percentage point is almost eleven millimoles per mole, and six and a half per cent is exactly forty-eight.
HbA1c misleads wherever red blood cells do not live a hundred and twenty days: in anaemia, after blood loss or transfusion, in pregnancy and kidney failure, and in carriers of haemoglobin variants. In these cases the result can differ from average glucose by a whole percentage point, and the doctor relies on other tests.
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04 · Measuring instruments
column · antibodies · cartridge · mass spectrometerFour ways to separate the sugar-coated protein
A column that separates relatives
The haemolysate is passed through an ion-exchange column, and haemoglobin molecules that differ in charge by a fraction of a unit leave it with different delays, so the detector draws a series of peaks, and the share of the A1c peak in the total area is the result. This is how the DCCT participants were measured, which is why chromatography became the basis of the percentage scale; the instrument also reveals haemoglobin variants that throw other methods off.
An antibody that recognises sugar
Immunoturbidimetry uses antibodies that recognise specifically the glycated end of the β-chain and do not bind to ordinary haemoglobin, and the turbidity of the solution after their reaction grows in proportion to the number of molecules caught. Most large laboratory analysers run on such reagents, and their calibrators are traceable to the IFCC reference, from which the result is converted to per cent with the equation linking the two scales.
A result in six minutes
Benchtop analysers in the doctor's office take a drop of fingertip blood into a disposable cartridge and show HbA1c within a few minutes, so treatment can be discussed at the same visit. They are convenient, but international guidelines allow diagnosis with them only for models that are NGSP-certified and run with quality control; otherwise an error of half a point turns a healthy person into a patient.
A reference that cuts the protein into pieces
The IFCC reference method cleaves haemoglobin with an enzyme and selects a single terminal fragment of the β-chain six amino acids long — with and without glucose — and then a mass spectrometer or capillary electrophoresis counts both fragments separately. In this way exactly the molecule named in the definition is measured, and a network of reference laboratories passes this value on to reagent manufacturers all over the world.
05 · Writing rules
two scales · points · glucoseAlways name the scale
The result is written with a unit that makes the scale clear, and a change in HbA1c in per cent is called percentage points, so that it is not mistaken for a fraction of the previous value.
The first two lines on the right attribute the number to the wrong scale, the third calls a difference a per cent when it actually equals 12.5 % of the previous value, and the fourth confuses a fraction of haemoglobin with a concentration of glucose per litre. The fifth forgets that even on the IFCC scale comparing results as a ratio calls for caution: doubling the fraction of glycated haemoglobin does not mean doubling the sugar, since average glucose is linked to HbA1c by a line with a shift. The last one breaks the letter case and substitutes Cyrillic for Latin letters.
06 · Neighbouring units
glucose · fructosamine · time in rangeHbA1c has three neighbours in purpose: instantaneous glucose, from which it is built up; fructosamine, which remembers only two to three weeks; and time in the target range from a continuous glucose monitoring sensor, which is increasingly placed next to it in discharge summaries.
The instantaneous value whose three-month average makes up HbA1c.
Glycated albumin and plasma proteins, which live shorter than a red blood cell.
The share of the day spent in the target range according to a sensor, which sees the swings that HbA1c averages out.
07 · Historical section
archive · from Tehran to two scalesHow a strange peak became the key number in diabetes
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Which month affects the test most
The height of a bar is the share of the result contributed by each of the four months, with a red blood cell lifespan of 120 days and an even age distribution of cells; the scale runs from zero to 44 %. The crimson bar is the last month; the frame visits the months in turn.
The three months between tests were not chosen by the calendar: by then almost all of the blood that remembered the old sugar levels has been replaced.
Why the exact scale was not made the only one
When the IFCC reference method appeared in 2002, it turned out that the old percentages were too high: the chromatograph peak captured neighbouring forms along with true HbA1c, and the exact fraction proved to be one and a half to two points lower than the familiar one. The simplest solution would have been to switch to the honest number, but everything known about which level is dangerous for the eyes, kidneys and heart had been obtained in the DCCT and UKPDS studies, precisely in the old percentages.
So in 2007 the IFCC, the diabetes associations and the manufacturers agreed not to abandon per cent but to tie it to the reference by an equation: the laboratory measures along the IFCC chain and, depending on the country's choice, prints either millimoles per mole or per cent recalculated as if the instrument stood in the DCCT laboratory. The new numbers were deliberately chosen to look unlike the old ones — 48 instead of 6.5 — so that they could not be confused.
Thus arose a pair rare in metrology: one measurand, one traceability chain and two legitimate notations of the result, linked by a line with a shift that the doctor has to keep in mind.
