Fructosamine Average Glucose Converter

Introduction to fructosamine and its two-to-three week glucose window

Fructosamine and A1C are the same chemistry read on different clocks. Both measure glycation — glucose bonding non-enzymatically to a protein, at a rate set by how much glucose the protein was bathed in and for how long. A1C reads that signal off haemoglobin, which lives inside a red cell for about 120 days, so it reports a weighted average of roughly the last two to three months. Fructosamine reads it off serum proteins, mostly albumin, which turn over in about two to three weeks. Same reaction, different exposure time, different answer.

That difference is the entire clinical point. When a clinician changes insulin, adds a drug or starts someone on a very different diet, an A1C ordered six weeks later is still half-full of the old regimen. Fructosamine has already forgotten it. The other reason to reach for fructosamine is that it does not care about red cells at all: in anaemia, after recent blood loss or transfusion, in haemoglobin variants such as sickle cell trait or thalassaemia, and in chronic kidney disease with shortened red cell survival, A1C can read low for reasons that have nothing to do with glucose. Pregnancy combines both problems — fast-moving targets and shifting red cell mass — which is why fructosamine appears in gestational diabetes management.

The catch is that fructosamine is not a familiar number. Nobody has an intuition for 312 µmol/L. This page converts it into the two units people do think in, estimated average glucose and A1C, and it does the conversion in both directions so you can also ask the reverse question: what fructosamine would correspond to the A1C target I have been given?

How to use the fructosamine converter

1. Pick which number you have. The mode selector decides which single field is live; the other two are filled in for you when you convert, rather than being left for you to work out. Switching mode never discards what you typed.
2. Choose your glucose unit. mg/dL is standard in the United States; mmol/L is standard almost everywhere else. The two ADAG equations are separate published fits, not a unit conversion applied afterwards, so the converter uses whichever one matches your selection.
3. Read all three outputs, not just one. A converted A1C and a converted average glucose carry exactly the same information as the fructosamine you started from — they are one number in three costumes. The value of seeing all three is that your care team, your meter and your lab each speak a different one.
4. Check the scale. The result includes a scale bar showing where the fructosamine value sits relative to the interval most laboratories quote for people without diabetes, so you can see at a glance whether a result is near the top of normal or well beyond it.
5. Share the link. The copy-link button produces a URL that reopens the page with the same values, which is easier than retyping a lab number into a message to your clinic.

The conversion formulas and where each one comes from

Two published linear regressions do all the work, and the converter chains them through A1C so that every mode is an inversion of the same pair. The first maps fructosamine to A1C, from Cohen and colleagues' 2003 study of discordance between the two markers:

with in µmol/L and A1C in DCCT percent. The second maps A1C to estimated average glucose, from the A1c-Derived Average Glucose (ADAG) study, which fitted continuous glucose monitoring against A1C in 507 participants:

Composing them gives the direct fructosamine-to-glucose relationship, which is where the frequently quoted coefficients come from:

Reversing is algebra, not a second fit. Given an A1C target, the fructosamine that would produce it is:

Because A1C is the hub, a round trip is exact: convert a fructosamine to A1C, convert that A1C back, and the original fructosamine returns. That is a property of the arithmetic, not evidence that the relationship is right for you — see the limitations below.

Worked example: a fructosamine of 300 µmol/L

A lab reports 300 µmol/L. Applying the Cohen equation:

and then ADAG, in both unit systems:

So 300 µmol/L corresponds to about 6.7% A1C, an average glucose near 146 mg/dL or 8.1 mmol/L. Now run it backwards. If the target discussed at the last appointment was an A1C under 6.5%, the fructosamine that would sit exactly on that line is (6.5 − 1.61) ÷ 0.017 = 287.6 µmol/L, an average glucose of 139.9 mg/dL. The 300 µmol/L result is therefore roughly 12 µmol/L above target — a gap that a fructosamine test can confirm or refute in three weeks, where an A1C would take three months.

The two rows at 205 and 285 are worth noting because they bracket the interval most laboratories quote for adults without diabetes. Converted, that interval spans an A1C of roughly 5.1% to 6.5% — which is wide, and which already tells you something important about the precision available here.

Limitations that change what a fructosamine number means

Albumin is the hidden variable. Fructosamine measures glycated serum protein, and albumin is most of it. Low albumin means less substrate, so nephrotic syndrome, cirrhosis, protein-losing enteropathy and severe malnutrition all push fructosamine down without any improvement in glucose. Hyperthyroidism speeds protein turnover and does the same. In any of these settings the number understates glycaemia and so does every conversion on this page. Some laboratories report an albumin-corrected fructosamine or a glycated albumin percentage instead; if yours does, use that.

The assay is not standardised. A1C has been harmonised internationally against a reference method. Fructosamine has not. Two laboratories can return meaningfully different µmol/L values on the same sample, which is why repeat testing at the same laboratory matters more here than it does for A1C, and why a general reference interval is a weaker guide than the one printed on your own report.

These are population fits with individual scatter. The Cohen relationship was derived in a specific cohort on a specific assay, and the ADAG relationship, although large and well conducted, still showed individual average glucoses spanning a wide band at any given A1C. Two people with the same fructosamine can genuinely have different average glucose. Treat the output as a translation with error bars, not a measurement.

No conversion between these markers is validated for diagnosis. No regulator endorses a fructosamine-to-A1C equation, and a converted A1C is not an A1C. It cannot be used to diagnose diabetes, to satisfy a treatment threshold, or to replace the test your clinician actually ordered.

Assay interference is real. High-dose vitamin C and vitamin E can interfere with the colorimetric fructosamine methods some laboratories use, biasing results low. Severe hyperbilirubinaemia, haemolysis and lipaemia can also interfere depending on method. Tell whoever interprets the result what supplements you take.

The converter refuses inputs outside the fitted range. Extrapolating the Cohen line below about 100 µmol/L or an A1C below the intercept of 1.61% produces negative average glucose or negative fructosamine — arithmetically fine, physiologically meaningless. The input bounds exist to stop the page printing numbers that cannot exist.

Questions about fructosamine conversion

Sources. The fructosamine-to-A1C relationship A1C = 0.017 x fructosamine + 1.61 is the linear fit reported by Cohen RM, Holmes YR, Chenier TC and Joiner CH, "Discordance between HbA1c and fructosamine: evidence for a glycosylation gap and its relation to diabetic nephropathy", Diabetes Care 2003;26:163-167. The A1C-to-average-glucose relationships eAG (mg/dL) = 28.7 x A1C - 46.7 and eAG (mmol/L) = 1.5944 x A1C - 2.5944 are from the A1c-Derived Average Glucose (ADAG) study, Nathan DM et al., "Translating the A1C assay into estimated average glucose values", Diabetes Care 2008;31:1473-1478. The reference interval of roughly 205 to 285 micromol per litre for adults without diabetes and with normal serum albumin is the range commonly published by clinical laboratories; the fructosamine assay is not internationally standardised, so your own laboratory's printed interval takes precedence. This page performs a units translation only. It is not a diagnostic device, it does not store or transmit anything you type, and no result here should change a medication or a monitoring plan without the clinician who ordered the test.