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Serum theophylline level

What is this test?

This test measures the level of a bronchodilator drug called theophylline in blood. Bronchodilators open up the airways to assist breathing. This test is used to monitor blood levels of this drug during treatment or when an overdose is suspected[1][2][3][4][5].

Why do I need this test?

Laboratory tests may be done for many reasons. Tests are performed for routine health screenings or if a disease or toxicity is suspected. Lab tests may be used to determine if a medical condition is improving or worsening. Lab tests may also be used to measure the success or failure of a medication or treatment plan. Lab tests may be ordered for professional or legal reasons. You may need this test if you have:

  • Theophylline monitoring
  • Theophylline overdose

When and how often should I have this test?

When and how often laboratory tests are done may depend on many factors. The timing of laboratory tests may rely on the results or completion of other tests, procedures, or treatments. Lab tests may be performed immediately in an emergency, or tests may be delayed as a condition is treated or monitored. A test may be suggested or become necessary when certain signs or symptoms appear.

Due to changes in the way your body naturally functions through the course of a day, lab tests may need to be performed at a certain time of day. If you have prepared for a test by changing your food or fluid intake, lab tests may be timed in accordance with those changes. Timing of tests may be based on increased and decreased levels of medications, drugs or other substances in the body.

The age or gender of the person being tested may affect when and how often a lab test is required. Chronic or progressive conditions may need ongoing monitoring through the use of lab tests. Conditions that worsen and improve may also need frequent monitoring. Certain tests may be repeated to obtain a series of results, or tests may need to be repeated to confirm or disprove results. Timing and frequency of lab tests may vary if they are performed for professional or legal reasons.

This test may need to be done more than once[1][2][4].

How should I get ready for the test?

Venous blood or capillary blood:

Before having blood collected, tell the person drawing your blood if you are allergic to latex. Tell the healthcare worker if you have a medical condition or are using a medication or supplement that causes excessive bleeding. Also tell the healthcare worker if you have felt nauseated, lightheaded, or have fainted while having blood drawn in the past.

Some medications may affect the results of this test. Before the test, tell the healthcare worker all the medications you are currently taking[6].

Avoid physical activity, such as walking or jogging, before the test[6].

Tell the person doing the test if you smoke[6].

How is the test done?

A sample of venous blood or capillary blood may be collected for this test.

Venous blood:

When a blood sample from a vein is needed, a vein in your arm is usually selected. A tourniquet (large rubber strap) may be secured above the vein. The skin over the vein will be cleaned, and a needle will be inserted. You will be asked to hold very still while your blood is collected. Blood will be collected into one or more tubes, and the tourniquet will be removed. When enough blood has been collected, the healthcare worker will take the needle out.

Capillary blood:

Common sites to collect a capillary blood sample are the fingertip and earlobe. Infants often have a capillary blood sample taken from the heel of the foot. Once the site is selected, the healthcare worker may heat the area with a warm compress to ensure adequate blood flow. The area will be cleaned with antiseptic. A small needle is used to make a cut in the skin surface, and the area may be squeezed gently to produce blood. The blood is collected in small collection device.

How will the test feel?

The amount of discomfort you feel will depend on many factors, including your sensitivity to pain. Communicate how you are feeling with the person doing the test. Inform the person doing the test if you feel that you cannot continue with the test.

Venous blood or capillary blood:

During a blood draw, you may feel mild discomfort at the location where the blood sample is being collected.

What should I do after the test?

Venous blood:

After a blood sample is collected from your vein, a bandage, cotton ball, or gauze may be placed on the area where the needle was inserted. You may be asked to apply pressure to the area. Avoid strenuous exercise immediately after your blood draw. Contact your healthcare worker if you feel pain or see redness, swelling, or discharge from the puncture site.

Capillary blood:

After capillary blood collection is complete, cotton will be placed over the site and held firmly until the bleeding has stopped. A bandage or cotton may be secured over the site.

What are the risks?

Blood: During a blood draw, a hematoma (blood-filled bump under the skin) or slight bleeding from the puncture site may occur. After a blood draw, a bruise or infection may occur at the puncture site. The person doing this test may need to perform it more than once. Talk to your healthcare worker if you have any concerns about the risks of this test.

What are normal results for this test?

Laboratory test results may vary depending on your age, gender, health history, the method used for the test, and many other factors. If your results are different from the results suggested below, this may not mean that you have a disease. Contact your healthcare worker if you have any questions. The following are considered to be normal results for this test:

  • Adults: 8-20 micrograms/mL (44-111 micromols/L) [7]
  • Neonatal apnea: 6-11 mg/L [1]

What might affect my test results?

  • Serum concentration increased in [4]:
    • Obesity
    • High carbohydrate, low protein diet
    • Premature babies and neonates
    • Elderly patients
    • Prolonged fever has been shown to slow the elimination of theophylline [2]
  • Serum concentration reduced in:
    • Low carbohydrate, high protein diet [4]
      • Physical activity [6]
    • Cigarette or marijuana smoking increases the elimination of theophylline 1.5 to 2 times [2][5]

What follow up should I do after this test?

Ask your healthcare worker how you will be informed of the test results. You may be asked to call for results, schedule an appointment to discuss results, or notified of results by mail. Follow up care varies depending on many factors related to your test. Sometimes there is no follow up after you have been notified of test results. At other times follow up may be suggested or necessary. Some examples of follow up care include changes to medication or treatment plans, referral to a specialist, more or less frequent monitoring, and additional tests or procedures. Talk with your healthcare worker about any concerns or questions you have regarding follow up care or instructions.

Where can I get more information?

Related Drugs

  • Theophylline
  • Theophylline


[1] Pesce AJ, Rashkin M, & Kotagal U: Standards of laboratory practice: theophylline and caffeine monitoring. Clin Chem 1998; 44(5):1124-1128.

[2] Weinberger M & Hendeles L: Theophylline in asthma. N Engl J Med 1996; 334:1380-1388.

[3] Shannon M: Predictors of major toxicity after theophylline overdose. Ann Intern Med 1993; 119:1161-1167.

[4] Aronson JK, Hardman M, & Reynolds DJ: ABC of monitoring drug therapy. Theophylline. BMJ 1992; 305(6865):1355-1358.

[5] Emerman CL, Devlin C, & Connors AF: Risk of toxicity in patients with elevated theophylline levels. Ann Emerg Med 1990; 19:643-648.

[6] Tietz NW (Ed): Clinical Guide to Laboratory Tests, 3rd ed. W. B. Saunders, Philadelphia, PA, 1995.

[7] Kratz A, Ferraro M, Sluss PM, et al: Case records of the Massachusetts General Hospital: laboratory values. N Engl J Med 2004; 351(15):1549-1563.

Last Updated: 11/4/2014

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