I would not consider the "dear Dr" letter as the chart notes. There should be additional documentation that is backed up by the letter, specifically in the MD chart notes. Below is from the Medicare manual regarding documentation.
For any DMEPOS item to be covered by Medicare, the patientís medical record must contain sufficient documentation of the patientís medical condition to substantiate the necessity for the type and quantity of items ordered and for the frequency of use or replacement (if applicable). The information should include the patientís diagnosis and other pertinent information including, but not limited to, duration of the patientís condition, clinical course (worsening or improvement), prognosis, nature and extent of functional limitations, other therapeutic interventions and results, past experience with related items, etc. If an item requires a CMN or DIF, it is recommended that a copy of the completed CMN or DIF be kept in the patientís record. However, neither a physicianís order nor a CMN nor a DIF nor a supplier prepared statement nor a physician attestation by itself provides sufficient documentation of medical necessity, even though it is signed by the treating physician or supplier. There must be information in the patientís medical record that supports the medical necessity for the item and substantiates the answers on the CMN (if applicable) or DIF (if applicable) or information on a supplier prepared statement or physician attestation (if applicable).
See PIM, chapter 3, section 22.214.171.124, for additional instructions regarding review of documentation during pre- and post-payment review.