In depth
Adult-onset Still's and blood tests
Ferritin is the test everybody links with this disease, and on its own it caught about 60 percent of the cases. It cleared about 86 percent of the people who did not have it. The interesting test is the one measured alongside it, and this page is mostly about that one.
What the research found.
One review pooled the studies of both tests and reported them side by side. Glycosylated ferritin caught 74 percent of the cases and cleared 80.5 percent of the people who did not have the disease. Ferritin alone caught 60.4 percent of the cases and cleared 85.7 percent of the rest.
One rule pairs two results, needing a glycosylated ferritin of 20 percent or less and a ferritin above the upper limit of normal. That pair caught 70.5 percent of the cases and cleared 83.2 percent. Requiring five times normal ferritin changed both figures, with catching falling to 43.2 percent and clearing rising to 92.9 percent.
One study tested a ferritin cut-off of 3,500 micrograms per liter, and it was looking for macrophage activation syndrome. That cut-off caught 85 percent of the people who had it. When it read below the cut-off, it was right 97 percent of the time, which is where its value sits.
A set of rules from 2016 spots macrophage activation syndrome, and one team used them on adults with Still's disease. The rules caught 100 percent of the cases and cleared 70 percent of the people who did not have it. The ferritin rule on its own cleared only 15 percent.
What the number means.
| Test | What it is | How it performs | What it can't do |
|---|---|---|---|
| Ferritin | An iron storage protein that rises when there's inflammation | Caught 60.4 percent of the cases. Cleared 85.7 percent of the rest. Area under the curve 0.778 | Tell Still's disease from other causes of a high ferritin on its own |
| Glycosylated ferritin | The share of your ferritin with sugar groups attached, given as a percentage | Caught 74 percent. Cleared 80.5 percent. Published cut-offs range from 16 to 33 percent | Stand alone. It works alongside ferritin rather than instead of it |
| The two together | A low sugar-attached share with a raised ferritin | Caught 70.5 percent and cleared 83.2 percent, at above-normal ferritin | Catch well and clear well at the same time |
| Ferritin above 3,500 micrograms per liter | A cut-off used for macrophage activation syndrome | Caught 85 percent of it. Right 97 percent of the time when it read below | Confirm the complication. It's best at ruling it out |
| The ferritin rule inside the 2016 rule set | One item in the rules for that complication | Cleared only 15 percent when used on adults | Sort one group from the other, because ferritin is already high in this disease |
| Diagnostic odds ratio | How much a test moves the odds overall | 11.32 for ferritin and 10.14 for glycosylated ferritin | Stand in for the clinical picture, which does most of the work |
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.
Ferritin, and what it is worth
Ferritin is the test people link with adult-onset Still's disease, and the link is earned. Levels here often run far above what other inflammatory diseases produce, and an extreme result is memorable to anybody who has seen one. The question is what a given result is worth as evidence.
Pooled across the studies, ferritin caught 60.4 percent of the cases and cleared 85.7 percent of the people who did not have the disease. The area under the curve was 0.778 and the diagnostic odds ratio was 11.32. Those numbers describe a useful test rather than a decisive one. So roughly two in five people with the disease test normal on it, and about one in seven people without it test high.
Ferritin also rises in infection, in cancer, in liver disease, and when the body holds too much iron. So the diagnosis rests on the fever, the rash, the joints, and exclusion. The blood test supports that picture rather than establishing it, and the difference is the whole of this page.
The sugar-attached share, and why it adds something
Some of the ferritin in your blood has sugar groups attached, and that share falls in Still's disease. Reporting that share as a percentage adds something. It is a second reading from one sample, and it turns out to be the better catcher of the two tests. That is not what most people assume about the newer test.
Glycosylated ferritin caught 74 percent of the cases against 60.4 percent for ferritin. It cleared 80.5 percent of the rest against 85.7 percent, and the diagnostic odds ratios were close, at 10.14 and 11.32. So the sugar-attached share earns its place by catching cases plain ferritin misses, which is a real gain in a disease diagnosed by exclusion.
The published cut-offs vary more than most people realize, ranging from 16 to 33 percent across the studies. That is worth knowing before you compare your result against a figure you read somewhere else, or against somebody else's. It is also a reason to ask which cut-off your own laboratory uses.
Where the threshold is set changes everything
The classic rule pairs two results, which are a sugar-attached share of 20 percent or less and a ferritin above the upper limit of normal. That pair caught 70.5 percent of the cases and cleared 83.2 percent of the rest. Raising the ferritin bar to five times normal made it a different instrument, with catching falling to 43.2 percent and clearing rising to 92.9 percent.
Look at what that trade costs, because it is easy to miss when it is written in percentages. Twenty-seven points of catching were spent to buy about ten of clearing. For a disease that is still being searched for rather than confirmed, that is a bad exchange. The cases lost are people who have the disease and whose result now looks perfectly normal to a clinician.
This is the general form of every threshold decision in medicine, and it is unusually stark here. A high bar is right when you are confirming a suspicion you already hold. It is the wrong tool when you are still deciding whether to hold a suspicion at all, and that is the harder case. Knowing which of those two situations you are in is what makes a threshold useful rather than arbitrary.
Macrophage activation syndrome, and the test that watches for it
The most serious complication of this disease is macrophage activation syndrome, in which the immune response gets out of control. Ferritin is the test used to watch for it, and one study tested a cut-off of 3,500 micrograms per liter. It caught 85 percent of the people who had the complication, and when it read below the cut-off it was right 97 percent of the time. That is a monitoring test rather than a diagnostic one.
Being right when it reads low is where the practical value sits, because a result below the cut-off makes the complication unlikely. That is what a monitoring test is for. Extreme values at the start of the disease counted too, and a ferritin above 10,448 micrograms per liter at diagnosis marked a raised risk of getting the complication later.
There is a strange finding worth understanding alongside that one. A set of rules from 2016 spots this complication in children, and one team used those rules on adults with Still's disease. The rules caught 100 percent of the cases and cleared 70 percent of the people who did not have it. The ferritin item on its own cleared just 15 percent. The reason is built into the illness, because nearly everyone with the underlying disease already has a high ferritin. So the measurement cannot sort the people developing the complication from the people who are not. The researchers then dropped triglycerides and re-tuned the cut-offs, and that got the rules to catching 100 percent and clearing 93 percent of people.
What these numbers decide
Less than their prominence suggests, which is true of most laboratory tests in rheumatology and unusually true in this disease. The 2024 European guideline for this disease covers four things, and it is worth knowing which of them ferritin belongs to. It covers diagnosis, blocking interleukin-1 and interleukin-6, short courses of steroids, and watching for complications. Ferritin's day-to-day job is the last one of those.
The practical reading is short enough to state in three sentences. A ferritin rising in somebody already diagnosed prompts a look for macrophage activation syndrome, and a normal one makes that unlikely. Neither result is a treatment decision on its own, and neither replaces a look at how the person in front of you is doing.
For diagnosis, pair ferritin with its sugar-attached share, because that is the best blood evidence there is. It is a supporting argument rather than a verdict. Four other things do the rest, which are fever, rash, joint involvement, and the careful ruling out of infection and cancer. That is why the diagnosis takes time, and why a single blood result rarely settles it.
Common misconceptions.
Myth. A very high ferritin confirms Still's disease.
Reality. It raises the suspicion without confirming anything, and the numbers show why. Ferritin cleared 85.7 percent of the people who did not have the disease, so about one in seven of them still tested positive. Ferritin also rises in infection, in cancer, in liver disease, and in iron overload. The diagnosis is made on the fever, the rash, the joints, and ruling everything else out.
Myth. A normal ferritin rules Still's disease out.
Reality. Ferritin caught only 60.4 percent of the cases, so roughly two in five people with the disease test normal on it. Glycosylated ferritin is better at 74 percent, and it still misses a quarter of them. Neither of the two tests rules the disease out. People do get missed in this disease, and a reassuring ferritin is one real way for that to happen to somebody.
Myth. Ferritin five times normal is the meaningful threshold.
Reality. Raising the bar from above-normal to five times normal cost 27 points of catching and bought about 10 points of clearing. That is a poor trade while the diagnosis is still being sought. A high bar is useful for confirming a suspicion you already hold, and it is the wrong tool for finding the disease in the first place.
Myth. Ferritin tells my team whether I'm developing macrophage activation syndrome.
Reality. It is better at ruling that out than at finding it. One cut-off of 3,500 micrograms per liter caught 85 percent of the people who had it, and when it read below it was right 97 percent of the time. The ferritin rule inside the 2016 rule set cleared only 15 percent in adults. The reason is that ferritin is already high in the disease itself.
Questions patients ask.
Why is ferritin the test associated with this disease?
Because it is often extremely high in it, running far above what most inflammatory conditions produce. That is odd enough to be memorable, and the numbers still show a middling test rather than a sure one. Ferritin caught 60.4 percent of the cases and cleared 85.7 percent of the rest, at an area under the curve of 0.778. That describes a useful test rather than a decisive one.
What is glycosylated ferritin?
Some of the ferritin in your blood has sugar groups attached, and this test measures what share of it does. So the result is a percentage rather than an amount. In Still's disease that share falls while the total climbs, and a low share alongside a high total tells you more than either result alone. The test caught 74 percent of the cases and cleared 80.5 percent of the rest, so it is the better catcher of the two and slightly the worse clearer.
What threshold is used for glycosylated ferritin?
The published cut-offs range from 16 to 33 percent, which is a wide spread and worth knowing before you compare results between laboratories. The classic rule uses 20 percent or less, paired with a ferritin above the upper limit of normal. That pair caught 70.5 percent of the cases and cleared 83.2 percent of the rest. There is not a single agreed cut-off in this literature.
Should the ferritin threshold be set higher?
It depends what you are using it for, and for finding the disease the trade is bad. Pairing a low sugar-attached share with a ferritin above normal caught 70.5 percent and cleared 83.2 percent. Requiring five times normal caught 43.2 percent and cleared 92.9 percent, so 27 points of catching were spent on about 10 of clearing. That is reasonable for confirming and poor for searching.
What is macrophage activation syndrome and how is it watched for?
It is a severe complication in which the immune response gets out of control, and it is the reason ferritin gets rechecked in this disease. One study tested a cut-off of 3,500 micrograms per liter, which caught 85 percent of the people who had it. When it read below the cut-off, it was right 97 percent of the time. Extreme values at diagnosis counted too, and a ferritin above 10,448 micrograms per liter marked a raised risk of getting it later.
Why did the ferritin rule clear only 15 percent in one study?
Because of what it was being asked to do. A set of rules from 2016 spots this complication in children, and one team used those rules on adults with Still's disease. The rules caught 100 percent of the cases and cleared 70 percent of the people who did not have it. The ferritin item on its own cleared just 15 percent. Almost everyone with the underlying disease already has a high ferritin, so it cannot sort one group from the other.
What do these tests decide about my treatment?
Less than the attention they get suggests, which is worth saying plainly. The 2024 European guideline covers four things, which are diagnosis, blocking interleukin-1 and interleukin-6, short courses of steroids, and watching for complications. Ferritin's main job is that last one, and it is not used to set your drug doses. The clinical picture does most of the work in every decision that gets made.
References.
- 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
- 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
- Javaux C; El-Jammal T; Neau P et al. Detection and Prediction of Macrophage Activation Syndrome in Still’s Disease. Journal of Clinical Medicine. 2021;11:206. 10.3390/jcm11010206Retrospective multicentre observational cohort of 206 patients with Still's disease
- Tada Y; Inokuchi S; Maruyama A et al. Are the 2016 EULAR/ACR/PRINTO classification criteria for macrophage activation syndrome applicable to patients with adult-onset Still’s disease?. Rheumatology International. 2018;39:97-104. 10.1007/s00296-018-4114-1Retrospective diagnostic accuracy study applying the 2016 sJIA MAS criteria to 76 adult-onset Still's disease patients
- Fautrel B; Mitrovic S; De Matteis A et al. EULAR/PReS recommendations for the diagnosis and management of Still's disease, comprising systemic juvenile idiopathic arthritis and adult-onset Still's disease. Annals of the Rheumatic Diseases. 2024;83:1614-1627. 10.1136/ard-2024-225851International guideline
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.
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