We all learned that you use an “a” before words that start with consonants and “an” before words that start with vowels.
But what about abbreviations? Should you use an “a” or an “an” before abbreviations?
The accepted rule is to use the choice that matches how the abbreviation is pronounced rather than how it is spelled. For example, HIV begins with a consonant but is pronounced āch-ˌī-ˈvē. In other words, HIV is pronounced as starting with a long “a” which is a vowel; therefore, it should be proceeded by “an.” The following sentence illustrates the correct usage:
An HIV positive patient was transferred to the nursing unit.
Here is an example of an abbreviation that begins with a consonant and is pronounced as a consonant; therefore, it is preceded by “a”:
A BID order for the drug was written last evening.