Adama S. claimed he was 15 years old when he arrived in France from Guinea in 2016 seeking asylum as an unaccompanied minor. But French authorities had doubts about the authenticity of his birth certificate. So, like many asylum seekers in France, Adama was given a bone test to determine his “true” age.

When the results came back, he was told he was between 20 and 30 years old, and denied the status of minor that would have allowed him access to state benefits under French law (link in French).

Adama challenged the test results in a case that reached France’s highest constitutional authority, with his lawyers arguing that bone tests were not reliable measures of age. Last week, the Constitutional Council (link in French) ruled against Adama’s case, upholding an article of the French civil code which allows judicial authorities to order bone exams, with a person’s consent, “in the absence of valid identity documents and where the alleged age is not plausible.”

Adama’s case is not unique. Bone tests are regularly used by immigration authorities around the world to establish the ages of refugees. Yet many immigration advocates and even medical professionals argue that the test is not precise enough to be the decisive factor in legal cases that can determine a person’s future.

The most common test (and the one used by French authorities) consists of scanning a child’s wrist and hands and comparing the degree of ossification of the bones to a 1959 atlas of X-rays compiled by anatomists William Walter Greulich and Sarah Idell Pyle.

Many things can influence a child’s growth—including nutrition, genetics, and exposure to environmental toxins—and not all children grow the same way. The Greulich & Pyle Atlas was built from a dataset of mostly white and well-off American children. Studies have found that while the test is effective in assessing the bone age of children from Europe, Australia, and the Middle East, it’s not so good at it with kids from Asia or Africa, where many refugees come from.

“It’s not possible to make a definitive, single age determination from X-rays or examination of bones or teeth,” says Elizabeth DiGangi, a forensic anthropologist at Binghamton University. That’s why “forensic anthropologists report age estimations as a range,” DiGangi says. “For example, rather than saying someone is 17 years and 8 months old, our estimation may be that she is between 17 and 20 years old.” But for someone whose entire case rests on the premise that she is below 18, the margin of error of bone tests can make all the difference.

In its decision, France’s Constitutional Council acknowledged the uncertainty tied to the test. The council said that the person performing it should always disclose the margin of error to the judge, and that it should never be the sole element in determining an applicant’s age. And in cases where the bone test results contradict other documents like birth certificates, the council ruled that the applicant should be considered a minor.

There is evidence that some refugees lie about their age in order to access the special protections and rights extended to minors in countries like France, Sweden, and Germany. But assessments of how common this is are limited, and have been contested. Activists argue that the actual proportion of refugees who lie about their age is very low and should not detract attention from those who need help. That hasn’t stopped conservative politicians across Europe from weaponizing the idea, and exaggerating the prevalence of migrants lying about their age to push anti-immigration policies. Still, some adult refugees do pose as teenagers; and in many cases, governments have not found a better way than bone tests to deal with it.

They are also overwhelmed by the demand: In France, the number of people claiming to be unaccompanied minors tripled between 2014 and 2017, while in the EU, that number quadrupled between 2014 and 2015 (pdf) (links in French). That’s partly why this controversial test persists.

For immigration activists, any amount of uncertainty is unacceptable considering what’s at stake: As DiGangi argues, “making age determinations based on X-rays alone…can have serious punitive consequences for young migrants.”