See below under "3 Special billing terms"

"If there is no explicit code for a test and/or analyte in the GOÄ, analog billing is applied. Under Section 6(2) GOÄ and the General Provisions No. 8 of Section M, independ-ent medical services not listed in the fee schedule may be settled using a service of the fee schedule that is equiva-lent to the service in question in terms of type, cost and time expended. For services under sections MII to MIV, the fee schedule item chosen for the purpose of analog billing for a service not listed in the schedule must be identified by prefixing it with the letter “A”. In case of doubt, the GOÄ commentary18 will be consulted.However, such analog billing is not binding for private health insurers; reimbursement problems may arise with private insurers even in the event of correct billing. Insur-ers frequently reimburse a lower amount, saying that a different analog code, for a lower-priced service, should have been used in their view." https://www.researchgate.net/publication/279217536_A_systema...