The health board had refused to supply medical details to a person who sought them through the OIA. After a complaint to the Ombudsman, it posted the personal records to prove they should be kept secret.
Instead of addressing the letter to Richard Fisher, of the Ombudsman's office, the letter was addressed to this reporter at the Herald.
Mr Bell asked that the letter be returned without being read - which the Herald did.
It was opened in front of a witness, viewed long enough to identify details described by Mr Bell, then sealed and returned by post.
Board spokesman Mark Fenwick said the contents of the envelope referred to the person who had sought the information and the person whose information was sought.
"ADHB clinical privacy staff are reviewing this information and intend on disclosing and apologising to the persons referred to in the letter intended for the Ombudsman but sent in error to the Herald."
He said the board would learn from the mistake and make changes as needed.