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Daily Urinary Free Creatinine-to-Serum Creatinine Ratio

Published: August 9, 2026Updated: August 16, 2026

This test compares creatinine in a timed urine collection with the creatinine level in blood. It can help evaluate muscle mass and how the kidneys handle creatinine.

Quick summary

["The result is influenced by muscle mass, age, hydration, kidney function, and the completeness of urine collection."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
48987-2
Short code
PYD.free/Creat Ur-sRto
Synonyms
PYD.free/Creat Ur-sRto
Category
kidney
Sample
urine
Container
Sterile urine container
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Specimen volume
10 mL
Stability
Room temperature: 2h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
0
Lipemia max
0
Icterus max
0
TAT
4–24 hours
Order
routine

What is this test?

Creatinine is a waste product made by muscles and removed through the kidneys. This test measures creatinine in a urine collection and compares it with the blood creatinine level.

Why might I need this test?

A clinician may use this ratio to assess muscle mass, monitor certain muscle or kidney conditions, or interpret another urine measurement. It may also help determine whether a timed urine collection was complete.

How is the test performed?

You provide urine for the collection period specified by your clinician, and a blood sample is usually collected at the same time. The laboratory measures creatinine in both samples and calculates the urine-to-serum ratio.

How do I prepare for the test?

Follow the urine-collection instructions exactly and record the start and end times. If a blood sample is needed, avoid strenuous exercise beforehand when possible and tell the clinician if you have recently eaten a high-meat meal or taken creatine.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Urine/Serum Creatinine Ratio

Frequently asked questions

Do I need to fast for this test?
Fasting is usually not required, but meat intake and creatine supplements can affect creatinine levels. Ask your clinician whether dietary restrictions are needed for your specific test.
What if I miss part of the urine collection?
Tell the laboratory or clinician because an incomplete collection may make the calculated ratio inaccurate. Depending on the collection period, you may need to repeat the test.
Does exercise affect the result?
Yes. Strenuous exercise and high muscle mass can increase creatinine, while reduced intake or muscle mass can lower it. The result should therefore be interpreted alongside your medical history and other laboratory findings.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

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.

Medical Disclaimer

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.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy