An IgE allergy (for example, a peanut allergy) will usually show up on skin testing and/or RAST.  That's a fairly easy thing to diagnose.  "Atopy" is a very general term which means that you're prone to allergic reactions.  A lot of conditions fall under this heading... food allergies, grass/pollen allergies, oral allergy syndrome, asthma, eczema, etc...  

MCAD has only recently been defined by the WHO.  It's an umbrella term that includes three different types...

- Primary (SM, TMEP, UP, clonal MCAS)
- Secondary (activation due to an IgE allergy and/or underlying autoimmune disorder or malignancy)
- Idiopathic (raises tryptase during a crisis, but otherwise all of the tests are normal)

It's possible to have a combination of these.  For example, I'm allergic to shrimp, ragweed, and latex, plus I have two autoimmune disorders.  At the same time, not all of my reactions fall under these categories... I've also been diagnosed with Idiopathic Anaphylaxis.  My baseline tryptase is normal and I don't have the c-Kit mutation.

Here's another thread I started about testing for mast cell disorders:
http://mastcelldisorders.wallack.us/yabb/YaBB.pl?num=1296696091

Heather

Here's the abstract:

http://www.ncbi.nlm.nih.gov/pubmed/22041891

Definitions, Criteria and Global Classification of Mast Cell Disorders with Special Reference to Mast Cell Activation Syndromes: A Consensus Proposal.

Valent P, Akin C, Arock M, Brockow K, Butterfield JH, Carter MC, Castells M, Escribano L, Hartmann K, Lieberman P, Nedoszytko B, Orfao A, Schwartz LB, Sotlar K, Sperr WR, Triggiani M, Valenta R, Horny HP, Metcalfe DD.

Source
Division of Hematology and Hemostaseology, Department of Internal Medicine I, Medical University of Vienna, Vienna, Austria.

Abstract
Activation of tissue mast cells (MCs) and their abnormal growth and accumulation in various organs are typically found in primary MC disorders also referred to as mastocytosis. However, increasing numbers of patients are now being informed that their clinical findings are due to MC activation (MCA) that is neither associated with mastocytosis nor with a defined allergic or inflammatory reaction. In other patients with MCA, MCs appear to be clonal cells, but criteria for diagnosing mastocytosis are not met. A working conference was organized in 2010 with the aim to define criteria for diagnosing MCA and related disorders, and to propose a global unifying classification of all MC disorders and pathologic MC reactions. This classification includes three types of 'MCA syndromes' (MCASs), namely primary MCAS, secondary MCAS and idiopathic MCAS. MCA is now defined by robust and generally applicable criteria, including (1) typical clinical symptoms, (2) a substantial transient increase in serum total tryptase level or an increase in other MC-derived mediators, such as histamine or prostaglandin D(2), or their urinary metabolites, and (3) a response of clinical symptoms to agents that attenuate the production or activities of MC mediators. These criteria should assist in the identification and diagnosis of patients with MCAS, and in avoiding misdiagnoses or overinterpretation of clinical symptoms in daily practice. Moreover, the MCAS concept should stimulate research in order to identify and exploit new molecular mechanisms and therapeutic targets.

Copyright © 2011 S. Karger AG, Basel.