Phosphorus (Phosphate) Blood Test

Serum phosphorus reference range

High phosphorus (hyperphosphataemia)

Low phosphorus (hypophosphataemia)

Questions to ask your doctor

• Is my high phosphorus related to kidney disease?
• Do I need a phosphate binder medication?
• Is my calcium level also abnormal?

Frequently Asked Questions

Why phosphate matters in kidney disease

Phosphate balance is maintained largely by the kidneys, so it becomes progressively harder to control as kidney function declines. In chronic kidney disease, phosphate accumulates, which in turn stimulates parathyroid hormone and disturbs calcium and vitamin D metabolism: a cluster of problems known as CKD mineral and bone disorder.

Over time this contributes to bone weakness and to calcium deposition in blood vessels, which is one reason phosphate is monitored regularly in kidney disease and managed with dietary advice and, where needed, phosphate binders taken with meals.

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

1. Hypophosphatemia. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK493172

References

Sources cited on this page. PubMed links open the original abstract.

1. Uribarri J. Phosphorus homeostasis in normal health and in chronic kidney disease patients with special emphasis on dietary phosphorus intake. Semin Dial. 2007;20(4):295–301. PMID 20689978 · doi:10.1111/j.1525-139X.2007.00309.x

What phosphorus blood levels actually reflect

The serum phosphate test measures inorganic phosphate (Pi) in the bloodstream. However, only about 1% of the body's total phosphorus is in blood – the remaining 99% is in bone (as hydroxyapatite) and inside cells (as ATP, DNA, and phospholipids). Blood levels are therefore an imperfect proxy for total body phosphorus and can be misleading in certain clinical contexts.

Normal serum phosphate: 0.8–1.5 mmol/L (adults). Levels are higher in children (up to 1.9 mmol/L) due to growth-related bone metabolism, which is clinically important – adult reference ranges should not be applied to paediatric samples.

Causes and consequences of low phosphate (hypophosphataemia)

Hypophosphataemia (below 0.8 mmol/L) is more clinically significant than commonly appreciated. Severe hypophosphataemia (below 0.3 mmol/L) can cause:

• Muscle weakness and respiratory failure (diaphragmatic muscle depends on adequate intracellular ATP)
• Haemolytic anaemia (red cells lyse without sufficient phosphate-dependent ATP)
• Rhabdomyolysis (skeletal muscle breakdown)
• Confusion and neurological dysfunction

Common causes: malnutrition and refeeding syndrome (a critical complication of too-rapid nutritional replenishment in malnourished patients – phosphate, potassium, and magnesium shift into cells causing life-threatening deficiencies), primary hyperparathyroidism (PTH promotes phosphate excretion by the kidney), vitamin D deficiency (impairs phosphate absorption), antacid overuse (calcium or aluminium-containing antacids bind phosphate in the gut), and X-linked hypophosphataemia (an inherited tubular disorder of phosphate reabsorption).

Causes and consequences of high phosphate (hyperphosphataemia)

Hyperphosphataemia (above 1.5 mmol/L) is most commonly a consequence of chronic kidney disease – the kidneys are the primary route of phosphate excretion, and as eGFR falls below 30, phosphate accumulates. The consequences in CKD are severe:

• Secondary hyperparathyroidism (PTH rises to try to excrete phosphate, but at the cost of calcium loss from bone)
• Renal osteodystrophy (disordered bone metabolism causing fractures and deformity)
• Vascular calcification (calcium-phosphate crystals deposit in arterial walls, significantly increasing cardiovascular mortality in CKD patients)

Management includes dietary phosphate restriction, phosphate binders (calcium carbonate, sevelamer, lanthanum carbonate), and calcitriol (activated vitamin D). The CKD-MBD (mineral and bone disorder) guidelines set targets: phosphate below 1.5 mmol/L, PTH within 2–9 times the upper limit of normal for the CKD stage.