It is courteous and professionally appropriate that we inform our referring colleague what we do with a client.  It is also important to development of trust and thereby referral source.  When we receive referral from a colleague who refers his client to us on basis of trust, there is a duty that we reply.  Finally, if we empower a referring doctor with knowledge about initial presentation and subsequent management, we establish additional support even when the client does not understand our plan, and worse, with adverse outcome.
 
A proper reply letter is addressed directly to the referring doctor, e.g., Dear Dr X, or Dear Joseph.  The presentation, examination findings, diagnosis, investigation and plan in care are then described.  It should always end with a message to encourage professional communication.  Please sign the letter after ‘Yours sincerely’ and similar phrases, but usually not ‘Yours faithfully’.  It is appropriate to attach a few (not usually more than 5) name cards of ours for subsequent referral.  I myself usually make sure that my emergency contact numbers are given to the referring FRIEND of mine.
 
I am taught by my tutor in HKU that doctors are brothers and sisters who call each other by first name.  We are a team to provide good care to our patients.  Looking at matters at utilitian approaches, the client listens to the referring doctor to see us: the trust rests with the referring doctor. It is therefore unwise not to engage the referring colleague. 
 
When I was very busy and even towards end of my direct service term, I still made a case to reply EACH referral at the time I finished my consultation (lest I forgot).  I usually cut-and-paste part of my progress into the reply letter.  Later I developed another way to do it.  I wrote a courteous and brief reply letter like this, and attach a copy of my medical notes.
 
“Dear Jane,
 
Thank you for allowing me the privilege to participate in the care of Madam So-and-so, whom I saw today.  My present diagnosis is XYZ.  I have attached a copy of my clinical notes today (which usually contains also indications about follow-up plans).  Upon further investigations to confirm my diagnosis, I would discuss with her about intervention with ABC.  Please kindly call me in case you want to discuss about her (give your pager once in case you are not a long standing friend of the referring doctor).
Yours sincerely,
 
Ares”
 
Now that we are communicating about letters, it may be appropriate to describe referral letters from our own desks.  Each time I felt very bad when I received a letter addressed ‘To whom it may concern’. 
 
A referral letter reflects our own intelligence, and an appropriate one brings about respect.  It should be addressed to ‘Dr AAA’, and the bare minimum to ‘The COS, Department of BBB’ or ‘Consultant in-charge’.  In the computer era, it is best to be issued from our computer so that everything is clearly read.  It should also end with ‘Yours sincerely,’ and be signed.  The contents should be professional instead of vague or over-general symptoms.