Lab reference
Glycosylated ferritin
What the research found.
In one study, glycosylated ferritin averaged 15.9 percent in adult-onset Still disease patients, against 31.5 percent in the controls. The spread around each average was 11.9 and 18.7. Those spreads are wide enough that plenty of controls sat in the range typical of patients.
One rule pairs a glycosylated ferritin at or below 20 percent with a total ferritin above the top of normal. That pair caught 70.5 percent of cases and cleared 83.2 percent of the people who didn't have the disease. Neither test on its own gets close to that.
Fautrel and colleagues, Journal of Rheumatology, 2001
Across eight studies, the glycosylated ferritin cut-offs used ran from 16 to 33 percent. So no single cut-off has anything settled behind it, and the familiar 20 percent sits toward the cautious end of the range. That makes it a habit rather than a finding.
Pooled across those studies, glycosylated ferritin caught 74 percent of cases and cleared 80.5 percent of healthy people. Total ferritin caught 60.4 percent and cleared 85.7 percent. So the glycosylated test catches more and raises slightly more false alarms than the total does.
Lee and Song, Zeitschrift für Rheumatologie, 2025
What the number means.
| Band | Value | What it means | Source |
|---|---|---|---|
| Adult-onset Still disease, average | 15.9 percent | Against 31.5 percent in controls. The spread around each was 11.9 and 18.7. | Fautrel 2001 |
| The paired rule | 20 percent or below, with total ferritin above normal | Caught 70.5 percent of cases and cleared 83.2 percent, when both are required together. | Fautrel 2001 |
| Cut-offs used across studies | 16 to 33 percent | No single cut-off has anything settled behind it. | Lee 2025 |
| Pooled figures | Caught 74 percent. Cleared 80.5 percent. | Total ferritin caught 60.4 and cleared 85.7. So this one catches more and clears fewer. | Lee 2025 |
| A level to follow over time | None | This test helps make a diagnosis. It isn't something to track. | No source, because none exists |
A reference range is not a target. Where a range on this page differs from the one your laboratory prints, the difference is explained above and sourced below. Where no trial has tested a target, this page says so rather than offering one.
What the test measures
Most of the ferritin in your blood has sugar groups stuck to it, added as the protein passes out through the cell's machinery. The glycosylated ferritin test measures what share of your total ferritin has been through that step, so the result comes back as a percentage rather than an amount. That makes it easy to mix up with the total ferritin, which is an amount and moves the other way.
That share falls in adult-onset Still disease, and it falls while total ferritin is climbing steeply. In the one study this page rests on, it averaged 15.9 percent in the patients against 31.5 percent in the controls. The two tests moving apart is the whole reason this test exists, and it's why the two get ordered together rather than one at a time.
Why the share falls
The accepted explanation is about where the ferritin is coming from. Normally ferritin reaches your blood by being secreted, and on the way out it passes the machinery that attaches sugar groups. In illnesses with a lot of cell activation or cell damage, ferritin gets released straight out and never goes through that step.
So the total climbs from a source with no sugar groups on it, and the glycosylated share of that total falls. Note what that means, though. It's a proposed mechanism rather than a finding, because the diagnostic studies measured the two values and never tested why the two diverge. Whether the mechanism is right doesn't change how the test performs.
The numbers
The foundational study compared 49 patients who had adult-onset Still disease against 120 controls. Glycosylated ferritin averaged 15.9 percent in the patients and 31.5 percent in the controls, with the spread around each average at 11.9 and 18.7. So the difference between the two averages is large, and the scatter around each of them is large too.
Those spreads deserve a look. The control group's spread is wide enough that plenty of controls had values in the range typical of patients, which is why the test gets used in a pair rather than alone. It's also why one result close to any cut-off shouldn't decide anything.
Pairing the two tests is what makes the numbers usable. Take a glycosylated ferritin at or below 20 percent, together with a total ferritin above the top of normal, and that pair caught 70.5 percent of cases while clearing 83.2 percent of healthy people. Now ask for the total ferritin to be five times normal instead, and the figures move to 43.2 percent and 92.9 percent.
So that change spent 27 points of catching to buy about 10 points of clearing, which is a great deal to pay for a little. Which trade to make depends on your goal. Hunting for cases and avoiding false alarms are two different jobs, and no cut-off does both.
Why the 20 percent rule was never checked
The figure of 20 percent turns up all through the research and in routine practice, and the pooled evidence shows it's a habit rather than a finding. A 2025 review pooled eight studies covering 556 patients with adult-onset Still disease and 763 controls. The glycosylated ferritin cut-offs across those studies ran from 16 to 33 percent.
So no single number has been settled on, and the familiar one is not the only one in use. Twenty percent sits toward the low end of that range, which makes it a convention rather than an established threshold. That distinction is worth holding on to, because it changes how much weight a result close to the line deserves, and results do land close to the line.
Here is what follows for you. A result of 22 or 25 percent shouldn't be read as ruling out adult-onset Still disease, because several studies in that pooled set would have counted it as positive. A number near a cut-off is a number near a cut-off, and the cut-off itself is a convention.
How well it performs overall
That review gives the pooled figures, and they put the two tests close together. Glycosylated ferritin comes out slightly ahead of total ferritin at catching cases and slightly behind at clearing healthy people, catching 74 percent and clearing 80.5 percent against total ferritin's 60.4 and 85.7 percent. So each test does slightly better than the other at a different job.
The odds figures put that in perspective. For glycosylated ferritin, a positive result multiplied the odds by 3.604 and a negative result multiplied them by 0.357, while the whole test scored 10.14 on one combined measure. For total ferritin the positive figure was 4.179 and the combined score was 11.32.
Read together, those describe two moderately useful tests. Both move the odds of a diagnosis without settling it, and that is how they should be used. Neither one is the sort of test that ends a conversation.
Where it fits in practice
This is a specialist test and many labs don't offer it, so it isn't something to expect on a routine panel. It earns its place when adult-onset Still disease is genuinely on the table, meaning somebody with the classic set of daily fevers, a rash that comes and goes, sore joints, and a sore throat, usually alongside a very high total ferritin. Outside that setting the test has no settled use.
Adult-onset Still disease is a diagnosis of exclusion, and that has to be said plainly, because no lab result changes it. An infection can produce a similar picture and similar ferritin findings, and so can a cancer, and both have to be ruled out before the diagnosis gets made. A low glycosylated ferritin backs the case in the right setting without shortening that work at all.
Suppose your result is back and you're trying to read it. Three other things bear on what it means, which are your total ferritin, your clinical picture, and what else has already been ruled out. Those are the pieces a rheumatologist puts together, and a percentage on its own tells you very little, so take the result to the rheumatologist who ordered it rather than reading it against a cut-off from a website.
Questions patients ask.
What is glycosylated ferritin and why is it measured?
Some of the ferritin in your blood has sugar groups stuck to it, and this test measures what share of it does, so the result comes back as a percentage rather than an amount. It gets measured because it behaves oddly in adult-onset Still disease, falling while total ferritin climbs steeply. The two moving apart is more telling than either value alone, which is why the two tests get ordered together.
Why does it fall when total ferritin rises?
Here is the usual explanation. Normally ferritin picks up its sugar groups on the way out of the cell, and in Still disease ferritin gets released a different way that skips that step. So as the total climbs from a source with no sugar groups, the glycosylated share of it drops. That's the accepted account, and it's a proposed mechanism, because none of the diagnostic studies tested it.
Is 20 percent the cut-off?
It's the figure people use most, and it isn't a checked one. Across eight studies the cut-offs ran from 16 to 33 percent, so the 20 percent rule is a habit rather than a finding. It does work reasonably well in practice, above all when paired with a raised total ferritin. Don't read a result of 22 percent as ruling anything out, because several studies in that pooled set would have called it positive.
How well does it perform?
Pooled across eight studies it caught 74 percent of cases and cleared 80.5 percent of the people who didn't have the disease, while total ferritin caught 60.4 percent and cleared 85.7 percent. So this test catches more cases and raises slightly more false alarms. A positive result multiplied the odds by 3.604, and the whole test scored 10.14 on one combined measure. That describes a test that moves things usefully without deciding anything.
Should I ask for this test?
It's a specialist test that many labs don't run, so ordering it makes sense only when adult-onset Still disease is genuinely on the table. Outside that setting it has no settled use. So if you've been told you might have this illness, the test belongs with the rheumatologist assessing you, sitting beside the total ferritin and beside the work of ruling out infection and cancer.
Does a low result confirm Still disease?
No, because adult-onset Still disease is a diagnosis of exclusion and no lab value changes that. A low glycosylated ferritin alongside a high total ferritin does back the diagnosis in somebody with the right picture, meaning daily fevers, a rash that comes and goes, sore joints, and a sore throat. That pair means something different in somebody with an untreated infection or a hidden cancer, so both have to be ruled out first, whatever this test shows.
References.
- Fautrel B; Le Moël G; Saint-Marcoux B et al. Diagnostic value of ferritin and glycosylated ferritin in adult onset Still's disease. The Journal of rheumatology. 2001;28:322-9. PMID 11246670Diagnostic accuracy study in adult-onset Still's disease versus a control group of patients with other conditions
- Lee Y; Gyu Song G. Diagnostic accuracy of ferritin and glycosylated ferritin in adult-onset Still’s disease: a meta-analysis. Zeitschrift für Rheumatologie. 2025;84:234-240. 10.1007/s00393-025-01672-6Diagnostic accuracy meta-analysis of 8 studies
This page gathers the published research on this subject into one place. The studies behind it were published between 1989 and 2026, and every figure links to the paper it came from. Those studies were peer reviewed. This summary of them was not. Dr. Sarah Luebker is reviewing these pages one at a time and has not reached this one yet, so it carries no medical review date and nothing here is her opinion or her advice to you. Each page gets updated as she reaches it. It is here in the meantime because the science is worth having in one organized place that is easy to find and easy to read. Talk to your own clinician before acting on any of it.