Behcet's disease and blood tests

What the research found.

• One international study collected data on 2556 people with clinically diagnosed Behcet's disease and 1163 controls from 27 countries. In the validation set, the newer International Criteria caught 94.8 percent of cases, on an interval from 93.4 to 95.9. The older International Study Group criteria caught 85.0 percent of them.

  International Team for the Revision of the International Criteria, Journal of the European Academy of Dermatology and Venereology, 2014

• That study reported the other side of the trade as well, which is the part that counts. The newer criteria cleared 90.5 percent of people without the disease, from 87.9 to 92.8, and the older criteria cleared 96.0 percent. So the newer set catches more cases and clears fewer controls.

  International Team for the Revision of the International Criteria, Journal of the European Academy of Dermatology and Venereology, 2014

• In the group where a pathergy test was used, adding a point for it raised the catch rate from 95.5 percent to 98.5 percent. It lowered the clearing rate from 92.1 percent to 91.6 percent. So it buys a real gain in catching cases at a very small cost on the other side.

  International Team for the Revision of the International Criteria, Journal of the European Academy of Dermatology and Venereology, 2014

• One review pooled 78 independent studies covering 4800 people with Behcet's disease and 16289 controls. Having HLA-B51 or B5 gave an odds ratio of 5.78 for having the disease, on an interval from 5.00 to 6.67. Estimates by region ran consistently from 5.31 to 7.20.

  De Menthon and colleagues, Arthritis and Rheumatism, 2009

• One study looked at family history and neurological involvement together. Neurological involvement was present in 20 percent of those with an affected sibling, against 7.5 percent of those with no family history, at P under 0.001. The odds ratio was 3.01, from 1.54 to 5.88.

  Ata and colleagues, Irish Journal of Medical Science, 2026

What the number means.

No blood test, but real criteria

No blood test makes this diagnosis, and this site holds no source on how any individual test performs here. So no figure appears for any of them anywhere on the page. What exists instead is a good deal better than what's available for several of the rarer diseases on this site.

There are two published sets of criteria, and one large study measured how well each of them works. That study collected data on 2556 people with clinically diagnosed Behcet's disease and on 1163 controls. All of it came from 27 countries, which means the figures below rest on a group that is both large and varied.

What the two sets of criteria do

The newer set is called the International Criteria for Behcet's Disease, and the older one is called the International Study Group criteria. Both of them score the whole clinical picture rather than any single test result. That's the first thing worth knowing about either.

In the validation set the newer criteria caught 94.8 percent of cases, where the true value ranges from 93.4 to 95.9. The older criteria caught 85.0 percent of them. That looks like a clear win until you read the other side of it.

The newer criteria cleared 90.5 percent of people without the disease, on an interval ranging from 87.9 to 92.8, and the older criteria cleared 96.0 percent. So the newer set finds nearly ten percent more cases and labels more people who turn out not to have the disease. Both of those things follow from the one change.

Why neither one is simply right

This trade recurs through every test and every rule set in medicine rather than being peculiar to this disease. Making it easier to catch a case makes it easier to catch somebody who doesn't have the condition, because the two move together. So no set of criteria does both jobs perfectly.

The question is which mistake costs more, and that's what the two sets answer differently. Missing a case means somebody goes untreated in an illness that can affect the eyes, the blood vessels, and the nerves. A wrong label means somebody lives with a diagnosis they don't have, along with the treatment and the worry that come with it.

Which way a team leans on that is a clinical judgment rather than a fact anybody can look up. It's also a fair thing to ask about at an appointment. Which set your team uses changes what a borderline picture gets called, and that's worth knowing.

The pathergy test

One test gets its own figure in that study, and it's the pathergy test. It's a skin reaction to a needle prick, read a day or two afterward. In the group where it was used, adding a point for it raised the catch rate from 95.5 percent to 98.5 percent.

It lowered the clearing rate as well, from 92.1 percent to 91.6 percent, which is a very small loss against a real gain. So this is an unusual test, because the trade goes almost entirely one way. It finds cases and gives up half a percentage point on the other side.

HLA-B51

One gene marker gets discussed in this disease a great deal more than any blood test does, and it's called HLA-B51. Its close relative is called B5, and the studies here report the two together. One review pooled 78 independent studies covering 4800 people with Behcet's disease and 16289 controls between them.

Having HLA-B51 or B5 gave an odds ratio of 5.78 for having the disease, where the true value ranges from 5.00 to 6.67. The studies disagreed moderately, at an I-squared of 61 percent. The result held across the world, with estimates by region ranging from 5.31 to 7.20.

What that number means

That's a tight spread for a marker studied on several continents, and it's a reason to trust the pooled figure. An odds ratio of 5.78 is a strong association rather than a test performance figure. That difference is easy to lose and worth holding onto.

The review reports how much more likely people with the marker are to have the disease. It doesn't report how often the marker catches cases, and it doesn't report how often it clears people who don't have the disease. Those are different questions, and this review answers only the first of them.

So a positive HLA-B51 moves the probability upward a long way without making a diagnosis, because plenty of people have it and never develop anything. A negative one doesn't clear the question either. This page has no figure for how often people with the disease lack the marker, because the review doesn't report one.

Family history

One study asked whether having an affected relative changes anything about the course of the illness. It looked at siblings, and it looked at neurological involvement in particular. Neurological involvement was present in 20 percent of people with an affected sibling, against 7.5 percent of those with no family history.

That difference came in at P under 0.001, and in the full model the odds ratio was 3.01, from 1.54 to 5.88. The two groups didn't differ in age, in sex, in body mass index, or in smoking status. So the family history is doing the work in that finding rather than something else about the two groups.

Hold the size of it loosely all the same. That study was retrospective and drawn from referral centers, where families with more than one affected member are over-represented, and family history was reported by patients rather than checked against records. What it supports is telling your team if a sibling has this, and it doesn't predict your own course.

There is a guideline here

This deserves saying, because several rare diseases here have nothing of the kind. A European task force published an updated set of recommendations in 2018, and it covers six areas of the body. Those are mouth and skin involvement, the joints, the eyes, the blood vessels, the nerves, and the gut.

It added five guiding principles and a new recommendation on surgery for blood vessel involvement. This page names no treatment from it, because what's worth knowing here is that the document exists and what it covers. That lets you ask whether the care you're getting follows what the guideline says.

Its own authors add a caution worth taking away. They say the guidance sets out to highlight the shortcomings of the research behind it, which is an unusually honest thing for a guideline to say about itself. That sentence is worth remembering whenever you meet confident advice about this illness from anywhere.

Common misconceptions.

Cautions specific to this condition.

• Ask which set of criteria your team is using. The two differ by nearly ten percentage points in how many cases they catch.
• Don't take a positive HLA-B51 as a diagnosis. It raised the odds by about six times across 4800 patients, and plenty of carriers never develop the disease.
• Don't take a negative one as an all-clear either. The review here reports odds of having the disease rather than how well the marker works as a test.
• Tell your team if a sibling has this. Neurological involvement was nearly three times commoner in that group in one study.
• This page gives no reference range and no threshold. The criteria are scoring sets rather than laboratory values.
• Expect the diagnosis to rest on the whole picture. Mouth ulcers, skin, eyes, joints, blood vessels, nerves, and gut all feed into both sets of criteria.

Questions patients ask.