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Theophylline Trough Level (Serum/Plasma)

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

This test measures the amount of theophylline, a medication used to treat breathing problems like asthma or chronic obstructive pulmonary disease (COPD). It helps ensure your dose is safe and effective.

Quick summary

["A theophylline trough level helps doctors monitor your medication levels to avoid side effects or reduce the need for adjustments."]

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

Key facts at a glance

LOINC
72673-7
Short code
Theophylline Trough SerPl-mCnc
Synonyms
Theophylline Trough SerPl-mCnc
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
10
Ref high
20
Critical range
— – 20 N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
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

Why Is This Test Done?

Your doctor may order a theophylline trough level if you’re taking a long-term theophylline medication. This test checks your blood level just before the next dose to ensure it stays within a safe range. It helps prevent both too much and too little of the drug in your body.

What Does the Test Measure?

Theophylline is a bronchodilator that relaxes muscles in your airways, making it easier to breathe. The test measures its concentration in your blood (either serum or plasma) to guide your treatment.

How Is the Test Performed?

A blood sample is taken from a vein, usually in your arm. The sample is sent to a lab where it’s analyzed to measure theophylline levels.

What Are Common Results?

Results are typically reported as a mass concentration (e.g., micrograms per milliliter). Your doctor will interpret the result based on your specific needs and medication regimen.

Reference range

Therapeutic: 10-20 mcg/mLN/A

Specimen: serum

No special preparation usually required. Stay well hydrated. Inform your healthcare provider of all medications and supplements you are taking.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Theophylline Level

Frequently asked questions

How often should I expect this test?
Your doctor will determine how often you need this test based on your treatment plan, medication dose, and how your body responds. It may be done weekly or monthly, depending on your condition.
What should I do if my test results are outside the expected range?
If your theophylline level is too high or too low, your doctor may adjust your dose or medication timing. Always follow their instructions carefully and never change your medication without approval.
Can I eat or drink before this test?
You can usually eat and drink normally before this test unless your doctor specifies otherwise. However, avoid caffeine-rich foods or drinks (like coffee or tea) for 12 hours before testing, as they can affect theophylline levels.
  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