Structure of an ICD-10-CM Code

Letter Number or Letter
1st Digit 2nd Digit 3rd Digit   4th Digit 5th Digit 6th Digit 7th Digit
C 5 0 . 5 1 2*  
Category   Etiology, anatomic site, severity, laterality Added code extensions for obstetric, injuries and external causes of injury
* C50.512: Malignant neoplasm of lower-outer quadrant of left female breast

Length 3-7 digits
Character 1 Letter only (all letters except U are used)
Character 2 Number only
Characters 3-7 Numbers or letter
Decimal After 3rd character (not counted)
Placeholder Use of “x” as a dummy placeholder
Letter Format Letters are case-sensitive

Updated: December 3, 2018