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Blood Test Tracker

Phosphorus (Phosphate)

Phosphorus is a mineral that builds bones and powers cells. Its blood level mostly reflects your kidneys, parathyroid glands, vitamin D and diet.

AI-verified against cited sources. Awaiting expert reviewAlso called PO4, PhosLOINC 2777-1

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What it is

Phosphorus works with calcium to build strong bones and teeth, and it helps your cells make and store energy. Most of it is stored in your bones. The blood test measures phosphate, which is phosphorus bound to oxygen. Your kidneys, intestines, parathyroid hormone (PTH) and vitamin D keep the level within a narrow range. You get phosphorus from foods such as nuts and seeds, dairy, dried beans, meat, poultry and eggs.

Why it matters

Phosphorus is usually read together with calcium, PTH, vitamin D and kidney tests. A high level most often reflects reduced kidney function. In people with advanced kidney disease, a level that stays high over time is linked to bone and blood vessel problems. Low levels are less common and can follow illness, alcohol use, poor nutrition or certain medicines. Mild changes often cause no symptoms and can be short-lived, for example after a sugary meal.

If it’s low

A result below your lab's range is called low phosphorus (hypophosphatemia). Mild lows usually cause no symptoms and are often short-lived, for example after a meal high in sugar or during recovery from an illness. Longer or deeper lows can come from heavy alcohol use, poor nutrition, problems absorbing food, low vitamin D or overactive parathyroid glands. Very low levels can cause muscle weakness, bone pain and, rarely, confusion or breathing and heart problems. Your clinician can sort out which cause fits.

Common causes

  • Recent carbohydrate intake or insulin, which move phosphate into cells
  • Alcohol use disorder, especially with vomiting or poor eating
  • Starvation or poor nutrition, or restarting food after a long period of undernutrition
  • Recovery from diabetic ketoacidosis
  • Overactive parathyroid glands (hyperparathyroidism)
  • Low vitamin D
  • Problems absorbing nutrients, or long-term diarrhea
  • Some medicines, such as antacids that contain aluminum or calcium and certain water pills (diuretics)
  • Severe burns, or fast breathing that lowers carbon dioxide (respiratory alkalosis)

What can help

  • Ask your clinician whether your diet, alcohol use, digestion or a medicine could explain it
  • Ask whether calcium, vitamin D, PTH, magnesium and potassium should be checked too
  • Eat foods that contain phosphorus, such as dairy, nuts and seeds, dried beans, meat, poultry and eggs
  • Talk with your clinician before taking any phosphate supplement

If it’s high

A result above your lab's range is called high phosphorus (hyperphosphatemia). Most people with a mild increase have no symptoms. The most common true cause is reduced kidney function, because healthy kidneys remove extra phosphate. Other causes include low parathyroid hormone, too much acid in the blood and large amounts of phosphate from laxatives or enemas. A sample with damaged blood cells, or one that sat too long before it was spun down, can also read falsely high. Children and teenagers normally run higher than adults. Your clinician can put your result in context.

Common causes

  • Growing children and teenagers, whose ranges are higher than adult ranges
  • A sample problem, such as damaged red cells (hemolysis) or serum left too long on the clot
  • Reduced kidney function, especially advanced chronic kidney disease
  • Low parathyroid hormone (hypoparathyroidism) or a reduced response to it
  • Too much acid in the blood (acidosis), including in poorly controlled diabetes
  • Phosphate-containing laxatives or enemas
  • Too much vitamin D
  • Rapid release of phosphate from cells, for example from severe muscle injury or tumor breakdown
  • Very high protein, fat or bilirubin levels in the blood, which can make the test read falsely high

What can help

  • Ask your clinician whether to repeat the test and whether calcium, PTH, vitamin D and kidney function should be checked
  • Tell your clinician about every laxative, enema, antacid and supplement you use
  • If you have kidney disease, ask whether meeting with a dietitian who knows kidney diets would help

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

The usual range depends on sex and age. Choose women or men to see the one that applies.

Children and teens 7 ranges

See Phosphorus by age and sex, with the U.S. survey numbers where they exist.

If this applies to you

Phosphorus means a little more for some people. Open the one that fits you.

Kidney disease or at risk

Changes in phosphorus, calcium and PTH start early in CKD and grow as eGFR falls, a pattern called CKD-mineral and bone disorder. Phosphorus is read together with calcium and PTH rather than alone.

Blood tests for kidney health

How common is your level?

NHANES describes the US civilian, non-institutionalized population, not a 'healthy' reference group. People with disease, on medication, fasting or not (except fasting-subsample markers), smokers and so on are all included. A percentile says where a value sits among Americans of that sex and age, not whether it is normal.

How Phosphorus varies with age in the U.S.Shaded: middle 90% and middle 50% of people. Line: the median. mg/dL. Point at any age, or focus the chart and use the arrow keys, to read both sexes in words.

Use the left and right arrow keys to read each age. Shift moves five years.

Reference data derived from CDC/NCHS National Health and Nutrition Examination Survey (NHANES), cycles 2015-2016, 2017-March 2020 (pre-pandemic), August 2021-August 2023. Analysis by bloodtesttracker.com; not endorsed by CDC, NCHS or HHS.

Explore Phosphorus by age and sex: every percentile as a table, with a download.

What can affect your result

  • Age: children and teenagers run higher than adults because their bones are growing, and Mayo Clinic Laboratories lists separate ranges up to age 17
  • Time of day and meals: levels can vary by as much as about 2 mg/dL (0.65 mmol/L) across the day and fall after a meal high in carbohydrate
  • Kidney function, parathyroid hormone and vitamin D
  • Acid-base balance and insulin
  • Pregnancy: Labcorp notes a slight decrease in the last trimester
  • Potassium intake: Labcorp notes that more dietary potassium has been reported to raise serum phosphate
  • Sample handling: damaged red cells or delayed separation raise the result
  • Lab differences: adult intervals run from 2.8–4.1 mg/dL in men and 3.0–4.3 in women at Labcorp to 2.5–4.5 mg/dL at Mayo Clinic Laboratories
  • Damaged red blood cells (hemolysis) raise the result. Mayo Clinic Laboratories, ARUP and Labcorp reject hemolyzed samples.
  • Serum left on the clot or not separated from the cells quickly enough can read falsely high. Labcorp asks for separation within 45 minutes, and Mayo Clinic Laboratories and ARUP within 2 hours.
  • Very high protein (as in multiple myeloma), fat or bilirubin levels can make the result read falsely high (Merck Manual).
  • A recent meal high in carbohydrate, or IV glucose or insulin, can lower the result by moving phosphate into cells (Labcorp).
  • Phosphate-containing laxatives and enemas can raise the result. Antacids that bind phosphate and some water pills can lower it.
  • Tubes containing fluoride, oxalate or citrate are not acceptable (ARUP).

Phosphate rises and falls through the day, by as much as about 2 mg/dL (0.65 mmol/L) according to Labcorp, so testing at a similar time each visit makes results easier to compare. Hours for shifts caused by meals, insulin or illness. Longer-term changes follow kidney, parathyroid and vitamin D status.

When to talk to a clinician

Contact the clinician who ordered the test soon if your phosphorus is outside your lab's range, especially if you have kidney disease, muscle weakness, bone pain or confusion. ARUP treats results below 1.0 or above 9.0 mg/dL (about 0.32 and 2.9 mmol/L) as urgent, and Quest's priority list calls results of 1.0 mg/dL or lower. Call 911 or your local emergency number for a seizure, severe confusion, trouble breathing, chest pain or fainting.

Questions you could ask

  • Should I fast before this test?
  • How often should my phosphorus be rechecked?
  • Could my kidney function, a medicine, a laxative or a supplement explain a higher result?
  • Should we repeat the test and check calcium, PTH, vitamin D and kidney function?
  • Could my diet, alcohol use, digestion or a medicine explain a lower result?
  • Should we check calcium, magnesium, potassium and vitamin D too?

Questions people ask

Is phosphorus the same as phosphate?

On a lab report they mean the same test. Phosphate is the form found in the body, and the result is shown as the amount of phosphorus in it. Your report may say either word.

Does the time of day or what I ate matter?

Yes. Phosphate in the blood rises and falls through the day, and it can drop after a meal high in sugar or carbohydrate. Some providers ask you to fast for several hours (MedlinePlus), so follow the instructions you were given. Testing at a similar time each visit makes your results easier to compare.

How do I convert phosphorus between mg/dL and mmol/L?

Multiply mg/dL by 0.3229 to get mmol/L, so 3.5 mg/dL is about 1.13 mmol/L. To go the other way, multiply mmol/L by 3.097.

Show the science and sources

Units

  • 1 mg/dL = 1 mg/dL
  • 1 mg/dL = 0.3229 mmol/L

Open the Phosphorus unit converter

Is a change real?

Phosphorus naturally varies about 7.67% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 23% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: The European Biological Variation Study (EuBIVAS): a summary report.

For clinicians and researchers

About 85% of body phosphate is in bone as hydroxyapatite, and soft-tissue phosphate is mostly intracellular, so serum phosphate reflects intake, renal handling and transcellular shifts more than total body stores. Intestinal absorption is enhanced by vitamin D, and PTH reduces renal tubular phosphate reabsorption (as does FGF23). Results are reported as mg/dL of elemental phosphorus (multiply by 0.3229 for mmol/L); Labcorp uses a colorimetric method and ARUP spectrophotometry. Adult limits: Mayo Clinic Laboratories and the Merck Manual 2.5–4.5 mg/dL (0.81–1.45 mmol/L), Labcorp 2.8–4.1 mg/dL in men and 3.0–4.3 in women over 19. Phosphate is about 50% higher in infants and 30% higher in children (Merck). Serum levels show diurnal variation of up to about 2 mg/dL (Labcorp); KDIGO notes this too but did not include variable sampling times in its guideline. Hemolysis, delayed separation, hyperproteinemia, dyslipidemia and hyperbilirubinemia can falsely raise results. Hyperphosphatemia is usually renal (GFR below about 30 mL/min), but also occurs with hypoparathyroidism or pseudohypoparathyroidism, acidosis, rhabdomyolysis, tumor lysis, and phosphate loads; the Merck Manual links a calcium × phosphate product chronically above 55 mg²/dL² in CKD to soft-tissue calcification. Hypophosphatemia is common in alcohol use disorder, refeeding, diabetic ketoacidosis recovery, respiratory alkalosis, hyperparathyroidism, vitamin D deficiency and with aluminum-containing antacids. The Merck Manual grades hypophosphatemia as mild (2–2.5 mg/dL), moderate (1–2 mg/dL) and severe (below 1 mg/dL). KDIGO 2017 suggests lowering elevated phosphate toward the normal range in CKD G3a–G5D (grade 2C) and monitoring calcium and phosphate every 6–12 months at G3a–G3b, 3–6 months at G4 and 1–3 months at G5. Critical limits: ARUP below 1.0 or above 9.0 mg/dL, Houston Methodist 0.9 or lower and 9.0 or higher, Quest priority 1.0 or lower.

LOINC 2777-1 (primary), 14879-1. Specimen: serum-or-plasma.

Sources

  1. Phosphorus (Inorganic), Serum (test ID PHOS), Mayo Clinic Laboratories (catalog copy hosted by Schneck Medical Center)
  2. Phosphorus (test 001024), Labcorp
  3. Phosphorus (test 001024), sample patient report, Labcorp
  4. Phosphorus, Inorganic, Plasma or Serum (test 0020028), ARUP Laboratories
  5. Overview of Disorders of Phosphate Concentration, Merck Manual Professional Version (2025)
  6. Hypophosphatemia, Merck Manual Professional Version (2025)
  7. Hyperphosphatemia, Merck Manual Professional Version (2025)
  8. Phosphate in Blood, MedlinePlus (US National Library of Medicine)
  9. KDIGO 2017 Clinical Practice Guideline Update for the Diagnosis, Evaluation, Prevention, and Treatment of Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD), Kidney International Supplements (2017)
  10. Critical Values List (CORP-APPEND-0104A, Rev. 46), ARUP Laboratories (2026)
  11. Diagnostic Laboratories: Critical Values, Houston Methodist
  12. Quest Diagnostics Priority Result Reporting Policy (Client Synopsis), Quest Diagnostics (2025)
  13. Phosphorus (PubChem compound CID 5462309), National Library of Medicine, PubChem

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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