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Does this test have other names?

Serum theophylline concentrations, blood theophylline level

What is this test?

This test measures the level of theophylline in your blood.

Theophylline is a chemical similar to caffeine. It's sometimes used as a medication to treat lung conditions such as asthma, chronic obstructive pulmonary disease, and bronchiolitis. Sometimes it's prescribed to babies, especially premature infants, to help their breathing and lung function.

Theophylline reduces inflammation or irritation in lungs and airways, relaxes smooth muscles in the airways and digestive system, and stimulates the central nervous system. But theophylline can be harmful in high doses or if the theophylline level in your blood is too high, so your health care provider may want to test your level of theophylline.

Why do I need this test?

You will need this test if your doctor gives you theophylline for a medical condition. The test will help your doctor figure out the proper dose and see if you have too much theophylline in your body. Too much theophylline can be life-threatening. Signs and symptoms of too much theophylline include:

  • Vomiting, heartburn, and abdominal pain

  • Rapid heart rate or other changes in heart rhythm

  • Muscle aches and tremors

  • Anxiety

  • Difficulty sleeping

  • Restlessness

  • Seizures

  • High blood sugar

  • Low potassium

  • Low blood pressure

What other tests might I have along with this test?

Your doctor may order other tests if you have taken too much theophylline. These include:

  • Tests to measure other substances in your blood, such as blood sugar and electrolytes

  • Blood gas test to check the oxygen and carbon dioxide levels in your blood

  • Liver function tests

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

Results are given in micrograms per milliliter (mcg/mL). Safe levels for people taking theophylline are usually 8 to 20 mcg/mL. Levels may be harmful at 20 to 40 mcg/mL if you've taken too many pills within a short time period. Such cases might include accidentally taking extra pills or having a prescription that was written for too high a dose. This may also happen if someone was using the drug to attempt suicide.

Levels of 20 to 30 mcg/mL may cause serious or even fatal problems if you've regularly taken too much of the medication over time.

A safe level for you depends on how much alcohol you drink, whether you smoke, whether you take herbal medications, and how nutritiously you eat.

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm.

Does this test pose any risks?

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.

What might affect my test results?

Certain medications cause theophylline to break down more slowly in the body, which can lead to higher test results. These include:

  • Cimetidine, used to treat problems related to stomach acid

  • Birth control pills

  • Certain antibiotics, such as erythromycin, clarithromycin, ciprofloxacin, and norfloxacin

  • Fluvoxamine, a drug for anxiety

Other drugs cause theophylline to leave your body more quickly, leading to lower test results. Smoking also has this affect on results.

How do I get ready for this test?

Tell your health care provider how much theophylline you have taken. In addition, be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.