In the writing sub-test for OET candidates quite often start a referral letter this way; "I'm writing to refer my patient John Smith who requires..........." There is nothing wrong with this in terms of language, but when you have a limit of approximately 200 words conciseness is very important. It is obvious you are writing, (it's a letter!) so why say it? The same can be said of using the word "referral". The reader will know it is a referral because of what you are going to ask them to do with your patient. Therefore you can simply write; "My patient John Smith requires......... " Think about what your reader will want to know, and what they need to know so that your communication is clear.