Early diagnosis is particularly important for XLH patients. The current treatment regimen (phosphorus and Calcitriol/Rocaltrol) is generally not begun at birth, but should, ideally, begin before the child begins to crawl and walk, placing full weight on soft bones.
You would think that diagnosing the child of someone who has XLH would be relatively simple, given that, statistically, the children of a woman with XLH have a 50/50 chance of inheriting the condition, and ALL of the daughters of a man with XLH will inherit the condition. Nevertheless, both the fluctuations of mineral levels in infant blood and the rarity of the condition leads to delays in diagnosis, even when the parent knows he/she has XLH.
One problem is that the "normal" values for the phosphorus in a child's bloodstream are higher than in an adult, and doctors (and labs) unfamiliar with XLH may not be aware of this. As a result, the doctor/lab uses the lower "normal" values for adults for comparison, instead of the higher values for a child, which makes the blood tests look normal. One thing you can do for an XLH child is to make sure you get copies of all lab reports. That way, you can be sure that they're comparing the results to the child's normals (we have links elsewhere here) and get a second opinion if it doesn't look right to you. (NOTE: reading and making recommendations based on lab tests requires professional medical expertise, so you should not make any treatment decisions. It can't hurt, however, to familiarize yourself with the tests, so you'll understand what you're being told by the doctor and you'll notice over time when something changes, so you can discuss it with the doctor.)
Diagnosis is more complicated for "spontaneous" cases (where neither parent has XLH, and the condition is a new mutation in the child). Based on the combined experiences of the Network's members, it appears that diagnosis generally happens around age three, when the effects of weight-bearing on too-soft bones become too severe to dismiss as merely an optical illusion caused by diapers (the "all babies look bow-legged" response too many parents are familiar with), generally after a year or more of the parent seeking a diagnosis before the doctor acknowledges that there's a problem, goes through more common conditions and finally thinks to test for phosphorus wasting.
Either way, a preliminary diagnosis can be arrived at through simple blood tests, once XLH is suspected. When the phosphorus level is low and the calcium level is normal, that's highly suggestive of XLH in a child who has either bowed legs or an XLH parent. More refined testing can be pursued after that, including more blood tests (creatinine and parathyroid hormone levels) and x-rays. In some cases genetic testing can be pursued to identify the exact location of the defect in the DNA. It should be noted, however, that the specific gene where the defect occurs does not appear to affect either the symptomatology or the treatment.
