Acdf

The patient had a prior anterior C5-6 fusion. Now needs fusion of C4-5 and C6-7. When coding out this anticipated procedure is the hardware code 22845 sufficient to cover all levels?  This is a projected procedure there is no operative note yet.

re: ACDF

Use 22845 when anterior instrumentation is applied to 2 to 3 vertebral segments; it sounds like you are only doing two segments; this may be correct, depending on your primary procedure.  If there is an artificial disk removal be sure to report 22864 for anterior approach, single, if applicable.
Without knowing the primary (parent code) procedure, I can only assume your primary code is listed among the following codes according to AMA guidelines, (Use 22845 in conjunction with 22100-22102, 22110-22114, 22206, 22207, 22210-22214, 22220-22224, 22310-22327, 22532, 22533, 22548-22558, 22590-22612, 22630, 22633, 22634, 22800-22812, 63001-63030, 63040-63042, 63045-63047, 63050-63056, 63064, 63075, 63077, 63081, 63085, 63087, 63090, 63101, 63102, 63170-63290, 63300-63307).
Perhaps you can glean some information from the Tufts health plan and what is required for preauthorization on some of their spine procedures or contract your payer for their requirements.   
https://www1.radmd.com/media/841303/tufts-2019-spine-surgery-authorization-and-coding-reference-guide-020119.pdf