Intact Fibroblast Growth Factor 23 (FGF23), Prechallenge, Plasma — Mass Concentration
Measures intact FGF23 in plasma before a specified challenge. This test helps evaluate how the body regulates phosphate, calcium, vitamin D, and bone mineralization.
["An elevated intact FGF23 level may indicate increased signaling to remove phosphate from the body, often related to kidney disease or disorders of phosphate and bone metabolism."]
Key facts at a glance
- LOINC
- 53418-0
- Short code
- FGF-23.intact pre chal Plas-mCnc
- Synonyms
- FGF-23.intact pre chal Plas-mCnc
- Category
- liver
- Sample
- plasma
- Container
- Lavender-top (EDTA) or Green-top (Heparin)
- Units
- N/A
- Ref low
- 44
- Ref high
- 147
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- CPT
- 84075
- SNOMED
- 27550007
- Specimen volume
- 5 mL
- Stability
- Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
- Hemolysis max
- 500
- Lipemia max
- 1000
- Icterus max
- 500
- TAT
- 4–24 hours
- Order
- routine
What is this test?
This blood test measures intact fibroblast growth factor 23, or FGF23, in plasma. FGF23 is a hormone made mainly by bone that helps control phosphate levels, vitamin D activation, and calcium balance.
Why might I need this test?
A clinician may order this test when there is concern about excessive phosphate loss in the urine, low vitamin D activity, unusual bone findings, or a bone disorder such as osteomalacia. A prechallenge sample may be obtained as part of a planned phosphate or metabolic challenge.
How is the test performed?
Blood is drawn into a tube, and the plasma is separated and analyzed to determine the intact FGF23 concentration. When this is a prechallenge test, the sample is collected before the planned challenge is given, according to the testing protocol.
How do I prepare for the test?
No special preparation is usually required for the blood draw itself. Because timing can be important, follow your clinician's instructions about medications, dietary restrictions, and when the challenge will be given.
Reference range
Adult: 44-147 U/LN/A
No special preparation required. Avoid fatty meals before the test if possible.
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
An elevated intact FGF23 level can occur with reduced kidney function and can also be seen in disorders that cause the body to lose too much phosphate. This may contribute to low vitamin D activity, bone softening, or other disturbances of mineral metabolism. Results should be interpreted with kidney function, phosphate, calcium, vitamin D, and other clinical findings.
Short name
Intact FGF23, Prechallenge, Plasma
Frequently asked questions
What does an intact FGF23 test measure?
Why is the sample described as prechallenge?
Does an elevated FGF23 result diagnose a specific condition?
Sources
- 1
- 2Lab Tests OnlineSource
- 3
Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.
This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.
This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.
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Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy