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Phenobarbital/Creatinine Ratio, Urine

Published: August 9, 2026Updated: August 17, 2026

A urine test that compares phenobarbital with creatinine, helping assess drug exposure while accounting for differences in urine concentration.

Quick summary

["The result can help estimate recent phenobarbital exposure, but it should be interpreted with symptoms, medication history, and other clinical findings."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 17, 2026Editorial policy

Key facts at a glance

LOINC
64133-2
Short code
Phenobarb/Creat Ur
Synonyms
Phenobarb/Creat Ur
Category
kidney
Sample
urine
Container
Sterile urine container
Units
N/A
Ref low
4.2
Ref high
5.9
Critical range
2 N/A – 7 N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
CPT
85041
SNOMED
900000000000003001
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?

This test measures phenobarbital in urine relative to the amount of creatinine in the same sample. Creatinine is used as a reference because urine concentration can vary widely.

Why might I need this test?

A clinician may order it to assess recent phenobarbital exposure, monitor medication use, or investigate suspected overdose. It may also be part of a broader urine drug-testing panel.

How is the test performed?

A urine sample is collected and analyzed in the laboratory for phenobarbital and creatinine. The laboratory then reports the amount of phenobarbital relative to creatinine.

How do I prepare for the test?

Collect urine exactly as your clinician or laboratory instructs, including any timed or first-morning collection requirements. Tell the healthcare team about phenobarbital use and other medications, and do not change prescribed doses without medical advice.

Reference range

Adult males: 4.2-5.9; Females: 3.8-5.1 x 10^12/LN/A

Specimen: urine

No special preparation required. Stay well hydrated.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Urine phenobarbital/creatinine ratio

Frequently asked questions

Does this test show how much phenobarbital is in my blood?
No. It measures phenobarbital in urine and compares it with urine creatinine. A blood phenobarbital level may be needed for more detailed medication monitoring.
Why is creatinine included in the result?
Urine can range from very dilute to highly concentrated. Comparing phenobarbital with creatinine helps reduce the effect of these concentration differences.
Does a detectable result mean I am taking too much phenobarbital?
Not necessarily. Detectable or higher levels can reflect prescribed use or recent dosing. Your clinician should interpret the result using your symptoms, dose, medication timing, kidney function, and other relevant information.
  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 17, 2026. · Read our editorial policy