Rising Rate of XLH Seen in UK Since 1995, Possibly Due to Better Testing, Study Says

Rising Rate of XLH Seen in UK Since 1995, Possibly Due to Better Testing, Study Says
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The prevalence of X-linked hypophosphataemia (XLH) rose in both children and adults across the United Kingdom between 1995 and 2016, a study found, likely due to improvements in clinical testing.

Mortality rates were higher in elderly people with XLH than in people of similar age without this disorder serving as controls, its researchers found.

The study, “Prevalence and mortality of individuals with X-linked hypophosphataemia: a United Kingdom real world data analysis,” was published in the Journal of Clinical Endocrinology and Metabolism.

Prior research into the prevalence of XLH in children have reported conflicting rates. In adults, the lack of standard disease management guidelines had led to a lack of accurate data.

Scientists in the United Kingdom addressed this gap using data from the National Health Service (NHS) healthcare system covering 1995 to 2016. Because the U.K. has near universal coverage, NHS data enable a precise assessment of the prevalence of rare diseases such as XLH and its associated mortality rate.

Each patient with XLH was matched by age, gender and general physician (GP) practice with up to four controls.

Due to the large variability in how patients are diagnosed, researchers developed an algorithm to identify an individual’s likelihood of having XLH. It used laboratory test results; prescriptions such as activated vitamin D (calcitriol), oral phosphate, and potassium or calcium supplements; and whether medical description included terms that would suggest other diagnoses that could mimic XLH.

The algorithm divided patients into five categories, depending on a probability of XLH: “highly likely,” “likely,” “possible,” “unlikely” or “unable to determine.”

From among 522 potential cases, 122 (23.4%) were scored as at least “possible” XLH, with 27 of these cases “highly likely” and 37 “likely.” The remaining 58 were possibilities for the disorder. Ages of people in these groups ranged from 1 to 61, and they had a follow-up duration of one to 14 years.

Results also showed that the prevalence of XLH increased from 3.1 per million in 1995–99 to 14 per million in 2012–16.

Among children over these two period, XLH prevalence rose from 9.4 to 17 cases per million, and from 1.8 to 13.3 per million among adults. Prevalence of people “likely” or “highly likely” to have XLH also rose from 3 per million in 1995–99 to 8.1 per million in 2012–16.

“The secular trend is most likely driven by changes in clinical practice for laboratory testing, documentation and quality of coding during this time,” the researchers wrote, noting a steady decline in the number of at least possible cases without basic blood tests.

Nine XLH patients (7.4%, median age of 64 ) and 14 controls (2.9%, median age of 72.5) died over a median follow-up of about 4.5 years.

“We observed an unexpected reduction in survival among XLH cases relative to controls (with average age at death being approximately 8 years younger in cases relative to controls),” the researchers wrote.

Compared to controls, mortality was almost three times higher (an increase of about 2.9-fold) in older people with at least “possible” for XLH, and more than six times higher (nearly 6.6-fold) in people “likely” or “highly likely” to have XLH.

“There was an unexpected increase in mortality in later life which may have implications for other FGF23-related disorders,” the team added.

FGF23 (fibroblast growth factor 23) is a hormone produced in excessive amounts in people with XLH. Tumor-induced osteomalacia, characterized by bone pain, fractures and muscle weakness, is an example of an FGF32-related disease.

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