Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (664 page)

   ACTH is unstable in blood, and proper handling of specimen is important.

   Most commercial RIAs are insensitive and nonspecific, measuring intact ACTH as well as precursors and fragments. Highly sensitive IRMAs measure intact ACTH only.

   RIAs are recommended for investigating ectopic ACTH–producing tumors, because some of the tumors secrete ACTH precursors and fragments. IRMAs are more sensitive than RIAs and are useful for investigating disorders of the hypothalamic–pituitary–adrenal system.

   Patients taking glucocorticoids may have suppressed levels of ACTH with an apparent high level of cortisol.

   Pregnancy, menstruation, and stress increase secretion.

ALLERGEN TESTS, SPECIFIC IMMUNOGLOBULIN E (IgE)

   Definition

   Allergic diseases are manifested as hyperresponsiveness in the target organ, whether the skin, nose, lung, or GI tract. Most tests for “allergy” are actually tests for allergic sensitization, or the presence of allergen-specific IgE.

   Most patients who experience symptoms upon exposure to an allergen have demonstrable IgE that specifically recognizes that allergen, making these tests essential tools in the diagnosis of allergic disorders.

   
In vitro
testing for allergy has certain advantages:

   It poses no risk to the patient of an allergic reaction.

   It is not affected by medications (antihistamines, etc.) the patient may be taking.

   It is not reliant upon skin integrity or affected by skin disease.

   It can be more convenient for the patient. In vitro testing requires submitting a blood sample and does not necessitate a separate visit for skin testing.