Examinations
The most common test procedure for diagnosing a ChU is the exertion test (Ill. 8). It is ideally conducted on a stationary bicycle, although stepping machines or treadmills are also possibilities. What's important is that the exertion test be conducted in "thick" clothing, since an elevation of the core body temperature by at least 1 °C is necessary for the triggering of a ChU. When a patient wears light sports clothing, the onset of sweating can lead to a quick cooling off of the body, whereby the exertion test then produces a false negative result, i.e. even though ChU is actually present, the test indicates that no ChU exists. The exertion test should first be initiated when the body is already well warmed up. Ideally the patient rides on the bicycle until he or she begins to sweat. The test itself then lasts 20 minutes while the patient rides the bicycle or climbs steps at a level that maintains the sweat. If the test is conducted properly, within several minutes a patient with ChU will develop strongly itchy wheals the size of a pencil rubber on his or her body.
- Ill. 8: Typical wheals and reddening following exertion tests for patients with cholinergic urticaria.
Another option for testing for a ChU is a hot bath. The hot bath should lead to an elevation of body temperature by 1–1.5 °C, which in turn will lead to wheal formation if ChU is present. This method is only rarely used anymore because unlike testing with a stationary bicycle, it provides only rough indications about the seriousness of a case of ChU.
For the sake of completeness, there are a few other less common intradermal tests associated with ChU. For patients with ChU the injection of what are known as cholinergic substances (e.g. methacholine, acetylcholine or pilocarpine) into the epidermis can lead to wheal formation at the injection point. Unfortunately the test also frequently produces false negatives, i.e. no wheals form although the patient does in fact suffer from ChU. For this reason the skin test is only used to confirm a diagnosis of ChU.
What to do if all tests turn up negative even though you are sure you suffer from ChU? In this case it may well be that the tests are being conducted during what is known as the skin's "refractory period." This is the period following a strong episode of ChU during which the skin's mast cells are no longer in a condition to release histamine. They first need to "recover" before they are again ready to react to stimulation. It's not precisely known whether histamine or some other unknown trigger of mast cell activity is absent during the refractory phase (e.g. a neuropeptide or an allergen). For this reason it is recommended that the exertion test be conducted at a time when the last episode of ChU was several days earlier.
