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Consider the sentence:

He offered to be an HSC.

vs:

He offered to be a HSC.

In the example above, HSC stands for Health Service Consultant. If one were saying the sentence aloud, one would say an HSC because an obviously proceeds the spoken letter H, but the H actually represents the word Health, at which point the sentence should perhaps read a HSC because one would obviously say a Health Service Consultant. If one considers what the letter H actually stands for, in a strange way, both versions could perhaps appear to be incorrect depending on whether the acronym is viewed by the reader solely as a contraction or as its real meaning.

I would surmise that an HSC would be the more correct grammar, but I'd like opinion.

Pronunciation of the following sound is the only thing that dictates the use of a vs. an: vowel sounds get "an," consonants get "a." In particular, in the case of initialisms, what matters is how the letter is pronounced, not whether the letter represents a vowel or consonant sound. The letter "h" (usually) represents a consonant sound (though it is often silent - whether you say "a herb" or "an herb" depends on whether you pronounce the "h" in your dialect), but when we say "h" we're really saying something along the lines of "aitch," which begins with a vowel sound (EDIT: see comments, this appears to not always be the case, but in either case you'd use whatever's appropriate for the sound).

Therefore, you'd say "an HSC."

As several people have already stated, in both speech and writing, the only thing that matters is how the writer/speaker would pronounce the sound that follows the indefinite article. If that sound is pronounced as a vowel, it should be 'an', and if it is pronounced as a consonant, it should be 'a'.

Now we apply that rule to your case. In American English as well as in standard UK English, the 'H' in HSC is pronounced 'aitch'. That begins with a vowel sound, and so should be preceded by 'an'. Thus, it should be written as

He offered to be an HSC.

However, as Ian MacDonald and tmgr have pointed out (and which this article confirms), in the UK, people increasingly pronounce 'H' as 'haitch'. For such speakers, it would be written as

He offered to be a HSC.

Here is how this is explained in the Chicago Manual of Style:

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7.33: “A” and “an” before abbreviations, symbols, and numerals

Before an abbreviation, a symbol, or a numeral, the use of a or an depends on (or, conversely, determines) how the term is pronounced. In the first example below, “MS” would be pronounced em ess; in the second, it would be pronounced manuscript. In the last two examples, “007” would be pronounced oh oh seven and double oh seven, respectively.

an MS treatment (a treatment for multiple sclerosis)

a MS in the National Library

an NBC anchor

a CBS anchor

a URL

an @ sign

an 800 number

an 007 field (in a library catalog)

a 007-style agent

10.9: “A,” “an,” or “the” preceding an abbreviation

When an abbreviation follows an indefinite article, the choice of a or an is determined by the way the abbreviation would be read aloud. Acronyms are read as words and are rarely preceded by a, an, or the (“member nations of NATO”), except when used adjectivally (“a NATO initiative”; “the NATO meeting”). See 10.2; see also 7.33.

an HMO

a UFO

a NATO member

a LOOM parade

an AA meeting

a AA battery (pronounced double A)

an NAACP convention

an NBA coach

an HIV test

an MS symptom (a symptom of multiple sclerosis)

but


a MS by... (would be read as a manuscript by...)

Initialisms, which are read as a series of letters, are often preceded by a definite article (“member nations of the EU”). Whether to include the article may depend on established usage. For example, one would refer to the NBA and the NAACP, on the one hand, but to W3C, PBS, and NATO, on the other—though all these organizations include the definite article in spelled-out form. If no established usage can be determined, use the definite article if it would be used with the spelled-out form. Some terms, such as DIY (do it yourself), do not ordinarily require a definite article in spelled-out form and therefore do not require one as an initialism.

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