Blood tests are an important part of the diagnosis of X-linked hypophosphatemia (XLH), a genetic disorder characterized by low phosphate levels.

The condition is caused by mutations in a gene called PHEX that is situated on the X-chromosome. These mutations lead to high levels of a protein called fibroblast growth factor 23, or FGF23, which acts on the kidney and reduces the reabsorption of phosphate.

XLH patients exhibit symptoms such as short stature, bowed legs, and dental problems, including tooth cavities and abnormal enamel.

Phosphate levels

The normal phosphate concentration in the blood is 3.4 to 4.5 mg/dL. The phosphate concentration in the blood is generally higher in children compared to adults. A phosphate concentration below 2.5 mg/dl is considered as hypophosphatemia, with levels below 1 mg/dl referring to severe hypophosphatemia. Phosphate levels in the blood of XLH patients are significantly lower than normal.

The levels of phosphate in the blood are determined using a colorimetric assay called the Fiske-Subbarow method. In this method, the inorganic phosphate in the blood reacts with the  Fiske-SubbaRow reagent and produces a blue color.

FGF23 levels 

XLH patients have increased levels of FGF23 in their circulation. The excess FGF23 stimulates excessive phosphate excretion by the kidneys, thereby lowering phosphate levels in the body.

The levels of FGF23 in the blood are usually determined using ELISA assays, wherein a specific antibody against FGF23 is used to quantify the concentration of the protein in the blood.

Other blood tests

Patients suspected of having XLH also may be tested for levels of calcium, vitamin D, parathyroid hormone, and alkaline phosphatase in their blood.

The levels of calcium and vitamin D are within the normal range in the blood of XLH patients. In fact, the observed bone abnormalities are not caused by vitamin D or calcium deficiency but by low phosphate levels. Low levels of calcium and vitamin D in the blood may be indicative of other conditions such as rickets.

Parathyroid hormone levels may be normal or slightly elevated in the blood of people with XLH.

Alkaline phosphatase is characteristically elevated in children but returns to normal in adulthood with or without treatment.


Last updated: Jan. 2, 2020


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