Hereditary hypophosphataemic rickets represents a group of rare genetic conditions that affect how the body handles phosphate, a mineral essential for healthy bones and teeth. Early recognition and proper testing can make a significant difference in managing this lifelong disorder.

Introduction: When to Seek Diagnostic Testing

Children and adults who show certain warning signs should undergo testing for hereditary hypophosphataemic rickets. In children, the most important time to seek evaluation is during the first two years of life, especially when leg bowing or other bone deformities become noticeable as the child begins to walk and bear weight. Parents may observe that their child is growing more slowly than other children of the same age, or notice that the legs curve outward in an unusual way.^[1]

If a child experiences unexplained bone pain, muscle weakness, or dental problems such as abscesses despite good oral hygiene, these symptoms warrant medical attention. Children who have delayed motor development or seem weaker than their peers should also be evaluated. In families where hereditary hypophosphataemic rickets has already been diagnosed in one member, siblings and other close relatives should undergo screening even if they appear healthy, as the condition can vary greatly in severity even within the same family.^[1]

Adults should seek diagnostic testing if they experience persistent bone pain, unexplained fractures, or a history of childhood bone problems that were never properly diagnosed. Sometimes adults discover they have the condition only after their child is diagnosed, as mildly affected individuals may have gone through childhood with minimal symptoms. Adults with muscle weakness, joint stiffness, or difficulty with mobility should also discuss testing with their healthcare provider.^[3]

Diagnostic Methods for Identifying the Disease

Diagnosing hereditary hypophosphataemic rickets requires a combination of clinical observations, laboratory tests, imaging studies, family history assessment, and genetic testing. The process begins with a thorough physical examination where the doctor looks for characteristic signs such as bowed legs, knock knees, short stature compared to family members, or widening of the wrists and ankles. In children, the doctor will measure height and compare it to growth charts to see if the child is shorter than expected.^[5]

Blood and Urine Tests

The cornerstone of diagnosis is laboratory testing that reveals abnormal levels of certain substances in the blood and urine. A blood test will typically show low levels of phosphate, which is called hypophosphataemia. This is the defining feature of the condition. The test also usually reveals elevated levels of alkaline phosphatase, an enzyme that indicates active bone disease.^[3]

Importantly, calcium levels in the blood are usually normal in people with hereditary hypophosphataemic rickets. This helps doctors distinguish it from other types of rickets that are caused by calcium or vitamin D deficiency. Vitamin D levels, specifically the form called 25-hydroxyvitamin D, are also typically normal. However, levels of the active form of vitamin D, called 1,25-dihydroxyvitamin D3, may be normal or slightly low, which is unusual because low phosphate levels would normally cause this hormone to increase.^[3]

Urine tests are essential to demonstrate that the kidneys are wasting phosphate. The kidneys normally reabsorb phosphate back into the bloodstream, but in people with hereditary hypophosphataemic rickets, too much phosphate is lost in the urine. Doctors may measure something called tubular maximum for phosphate reabsorption per glomerular filtration rate, which sounds complicated but simply means they calculate how well the kidneys are holding onto phosphate. In this condition, that ability is reduced.^[3]

Imaging Studies

X-rays play a critical role in diagnosing hereditary hypophosphataemic rickets by showing the effects of the disease on bones. In children, X-rays of the legs, wrists, and sometimes the ribs can reveal changes in the growth plates, which are areas of developing tissue near the ends of long bones. These growth plates may appear widened, irregular, or frayed. X-rays may also show bone deformities such as bowing of the legs or curvature of other bones.^[5]

Occasionally, X-rays reveal fractures that occurred without significant trauma, which are called stress fractures or pseudofractures. In adults, X-rays may show signs of osteomalacia, which is softening of the bones. Kidney ultrasound may be performed to check for kidney stones, which can develop as a complication of the condition or its treatment.^[5]

Family History and Genetic Testing

A detailed family history is an important part of diagnosis because hereditary hypophosphataemic rickets runs in families. The doctor will ask about relatives who have had similar symptoms, short stature, bone problems, or dental issues. However, it is important to know that about 20 to 30 percent of people with the condition have no family history at all because their condition resulted from a new genetic change that was not inherited from either parent.^[6]

Genetic testing can confirm the diagnosis by identifying mutations in the genes responsible for the condition. The most common form, called X-linked hypophosphataemic rickets, is caused by mutations in the PHEX gene. Other rarer forms are caused by mutations in different genes such as FGF23, DMP1, ENPP1, FAM20C, or SLC34A3. Genetic testing is particularly helpful when the clinical picture is unclear or when family planning decisions are being considered.^[1]

Specialized Measurements

In some specialized centres, doctors can measure levels of a hormone called fibroblast growth factor 23, or FGF23. This hormone is produced by bone cells and tells the kidneys to excrete phosphate. In most forms of hereditary hypophosphataemic rickets, FGF23 levels are elevated, which explains why the kidneys waste phosphate. Measuring FGF23 can help confirm the diagnosis and distinguish hereditary hypophosphataemic rickets from other conditions that cause low phosphate levels.^[2]

Distinguishing from Other Conditions

An important part of diagnosis is making sure the patient does not have a different condition that can cause similar symptoms. The most common condition to rule out is nutritional rickets, which is caused by deficiency of vitamin D or calcium. In nutritional rickets, vitamin D levels are low, calcium may be low, and the parathyroid hormone level is usually elevated. These findings are different from hereditary hypophosphataemic rickets, where vitamin D and calcium levels are typically normal.^[2]

Doctors also need to consider other rare genetic conditions that cause phosphate wasting, as well as acquired conditions such as tumours that produce FGF23. These tumours, which are usually benign, can cause a condition called tumour-induced osteomalacia that mimics hereditary hypophosphataemic rickets but develops later in life and can be cured by removing the tumour.^[3]

Diagnostic Testing for Clinical Trial Participation

When patients with hereditary hypophosphataemic rickets are being considered for participation in clinical trials, additional diagnostic tests and eligibility criteria are typically required. Clinical trials are research studies that test new treatments or approaches to managing the condition. These studies have specific requirements to ensure that participants are appropriate for the research being conducted and that results can be accurately interpreted.

To qualify for most clinical trials involving hereditary hypophosphataemic rickets, patients must first have a confirmed diagnosis. This usually requires documentation of low blood phosphate levels on multiple occasions, evidence of renal phosphate wasting demonstrated through urine tests, and either genetic testing confirming a mutation in one of the relevant genes or elevated FGF23 levels. Some trials require that genetic testing be completed before enrollment, while others may accept patients based on clinical and biochemical features alone.^[12]

Baseline X-rays are typically required for clinical trial participation to document the extent of bone disease before treatment begins. In children, X-rays of the knees and wrists are commonly obtained because these areas clearly show the changes of rickets. In adults, X-rays of areas affected by pain or previous fractures may be required. Some trials use specialized imaging techniques to measure bone mineral density or assess bone quality in more detail.

Blood tests performed for clinical trial screening usually include a comprehensive metabolic panel to assess kidney function, liver function, and calcium metabolism. Phosphate levels, alkaline phosphatase, parathyroid hormone, and vitamin D metabolites are measured to establish baseline values. Some trials require measurement of FGF23 levels at the start of the study to track how the experimental treatment affects this hormone. Urine tests to quantify phosphate wasting are also standard.^[9]

Clinical trials may have specific eligibility criteria related to disease severity or complications. For example, some trials may only accept patients with moderate to severe bone deformities, while others may exclude patients who have already had multiple surgeries. Trials testing new medications often require that patients stop their current treatments for a period of time before enrolling, which is called a washout period. This ensures that the effects of the new treatment can be properly evaluated without interference from previous therapies.

Additional specialized tests may be required depending on the nature of the clinical trial. Some studies include bone biopsies, which involve taking a small sample of bone tissue to examine under a microscope. Others may use advanced imaging such as dual-energy X-ray absorptiometry (DXA scans) to measure bone density, or high-resolution imaging to assess bone microarchitecture. Trials investigating treatments for dental complications may require dental X-rays and examinations by dental specialists.

For pediatric trials, age restrictions apply, and both parent or guardian consent and child assent are typically required. Some trials focus specifically on children, while others enroll both children and adults. Trials may also have restrictions based on previous treatments received, current medications, or the presence of certain complications such as kidney stones or secondary hyperparathyroidism. Patients interested in clinical trial participation should discuss options with their treating physician, who can help identify appropriate studies and facilitate referrals to research centres.^[12]