Common compound words (not hyphenated)
Some combinations of words are read together as a unit and are so commonly recognized as a unit, they are not hyphenated. As these words come into common use, the hyphen tends to be eliminated without sacrifice of clarity.
When in doubt, refer to accepted current dictionaries or request addition in the MT Desk Style Guide.
When no confusion is likely, do not hyphenate. If there is a possibility of confusion, hyphenate.
A short list of common compound words follows.
amino acid levels
birth control methods
bone marrow biopsy
deep vein thrombosis
foreign body infiltrate
health care system
inner ear disorder
lower extremity amputation
medical school student
natural killer cell
open heart surgery
peer review process
primary care physician
public health official
small cell carcinoma
tertiary care center
Wiki