Challenge Test: What to Expect and How to Prepare for Your Appointment

A challenge test is a procedure in which your care team gives you a small, carefully measured amount of a substance, such as a food, medicine, or airborne allergen, under close medical supervision to see how your body reacts. It helps confirm or rule out an allergy or sensitivity when other tests have not given a clear answer.

The 17-Hydroxyprogesterone (17-OHP) test measures the amount of this hormone in your blood. 17-Hydroxyprogesterone is a substance produced by the adrenal glands, which are small glands located on top of your kidneys. This hormone is an important intermediate in the production of cortisol, the body's main stress hormone.

The test is done after the challenge because your provider needs to observe your body's actual response to the exposure. Comparing measurements taken before the challenge with those taken afterward shows whether the substance triggered symptoms, such as breathing changes or a skin reaction, and how strong that response is.

This test is often performed as part of an ACTH stimulation test (also called a Cosyntropin stimulation test). During this test, you will receive an injection of ACTH (adrenocorticotropic hormone), which signals your adrenal glands to produce hormones. The blood sample taken 45 minutes after this injection shows how your adrenal glands respond to stimulation. This helps doctors determine if your adrenal glands are functioning properly.

During the test, your care team first records baseline information, such as your vital signs, breathing measurements, or skin appearance. You are then given the challenge substance in gradually increasing doses while staff watch you closely for any symptoms. Afterward, you are monitored for a set period of time so the team can treat any reaction right away.

First, a healthcare provider will draw a baseline blood sample. Then, you will receive an injection of synthetic ACTH (cosyntropin) into your arm. After exactly 45 minutes, another blood sample will be drawn. The entire procedure usually takes about 1-2 hours. You may be asked to fast overnight before the test.

To prepare, follow your provider's instructions about which medicines to stop before the test, since some, such as antihistamines, can interfere with the results. Avoid the challenge substance beforehand if you are told to do so, eat and drink as directed, wear comfortable clothing, and plan to stay at the office for several hours. Bring a list of your current medications and arrange a ride home if advised.

Follow these instructions: Fast for 8-12 hours before the test (usually overnight). Inform your doctor about all medications you are taking, as some medications can affect test results. The test is often scheduled in the morning. Wear comfortable clothing with sleeves that can be easily rolled up. Tell your doctor if you are pregnant or have any bleeding disorders.