Intact Fibroblast Growth Factor 23 (FGF23), Plasma — Post-Challenge Mass Concentration
Measures intact FGF23 in plasma after a challenge to assess regulation of phosphate, vitamin D, and calcium metabolism.
["This blood test measures active FGF23 and may help evaluate disorders of phosphate and mineral metabolism."]
Key facts at a glance
- LOINC
- 53420-6
- Short code
- FGF-23.intact sp2 p chal Plas IA-mCnc
- Synonyms
- FGF-23.intact sp2 p chal Plas IA-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
- Method
- ia
- 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 test measures intact fibroblast growth factor 23, or FGF23, in blood plasma. FGF23 is a hormone that helps control phosphate levels and vitamin D activity.
Why might I need this test?
The test may be used to investigate unusual phosphate or calcium levels, kidney-related mineral abnormalities, or suspected disorders that alter FGF23 production. When it is part of a challenge test, responses before and after the challenge are compared.
How is the test performed?
A blood sample is collected and analyzed to determine the mass concentration of intact FGF23. For a post-challenge test, the sample is taken at the time specified by the laboratory or ordering clinician.
How do I prepare for the test?
Follow the clinic's instructions about the challenge, fasting, and exact collection time. Do not change medicines or vitamin or mineral supplements unless your clinician tells you to do so.
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 abnormal FGF23 result may reflect altered phosphate and vitamin D regulation, chronic kidney disease, or another condition affecting mineral metabolism. High FGF23 generally increases phosphate loss through the kidneys, while low levels can occur in rare disorders that impair FGF23 activity. Results must be interpreted with the challenge findings and other laboratory tests.
Short name
Intact FGF23, Plasma
Frequently asked questions
What does FGF23 do?
Why is the test called a post-challenge test?
Does an abnormal 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