M85.8 vs Q77.4
M85.8 and Q77.4 can never be reported together. ICD-10-CM declares them mutually exclusive with an Excludes1 note, meaning the two conditions cannot occur in the same patient — so exactly one of them is correct for any given encounter.
The official note, declared on M85.8
osteosclerosis congenita (Q77.4)
M85.8
Other specified disorders of bone density and structure
Not billable
- Chapter 13
- Diseases of the musculoskeletal system and connective tissue
- Section
- M80-M85
Use when the record says
- Hyperostosis of bones, except skull
- Osteosclerosis, acquired
Indexed under
- Osteopenia
- Borderline › osteopenia
- Osteopenia › borderline
- Osteosclerosis › acquired
- Disorder › bone › density and structure › specified type NEC (of)
Q77.4
Achondroplasia
Billable
- Chapter 17
- Congenital malformations, deformations, chromosomal abnormalities, and genetic disorders
- Section
- Q65-Q79
Use when the record says
- Hypochondroplasia
- Osteosclerosis congenita
Indexed under
- Achondroplasia (osteosclerosis congenita)
- Chondrogenesis imperfecta
- Hypochondroplasia
- Chondrodysplasia › fetalis
- Dwarfism › achondroplastic
These two codes sit in different chapters of the Tabular List, which is usually the clearest signal that they describe different underlying processes rather than different degrees of the same one.
Excludes1 is not always absolute: where two conditions covered by the note are genuinely unrelated, CMS guidance permits reporting both. Document the reasoning rather than assuming it. More on Excludes1 vs Excludes2 · FY2027