Wiki Diagnosis codes in a nursing home/ rehab

[Natashia01]

I work in a nursing home/ rehab. For the diagnosis codes their must be a primary and admission diagnosis. This billing office is saying we can not use 800 codes, they are using V codes as the primary and/or admission code, and/or using muscle weakness and pain as primary and/or admission. Also, using a lot of aftercare codes. Can someone who also works in a nursing home shine some light on this?

A lot of codes I'm also seeing for example, constipation, pain, and nausea, should these be used as diagnosis codes?

Thanks