Abbreviations and Articles

When you are dealing with an abbreviation, the choice of "a" or "an" will depend on whether you pronounce the expression letter by letter or as a word. Abbreviations pronounced letter by letter are called "initialisms"; abbreviations pronounced as words are called "acronyms."

An FBI agent

A MADD fund-raising drive

An NBC news report

A SWAT team

from: http://www.basic-learning.com/