The ICD-10-CM diagnosis code structure will be alphanumeric consisting of three to seven characters. The ICD-10-PCS codes for reporting hospital inpatient procedures will also be alphanumeric with seven characters. What I didn’t realize is that certain alpha characters will be “off limits” in the design of the diagnosis or procedure codes.
According to the ICD-10-PCS coding guidelines:
- One of 34 possible values can be assigned to each axis of classification in the seven-character code, which includes numbers 0 through 9 and the alphabet (except the letters “I” and “O” because they are easily confused with the numbers “1” and “0”).
- “U” is a potential third character describing a root operation called supplement (i.e., putting in/on biological or synthetic material that physically enforces and/or augments the function of a portion of a body part).
According to the ICD-10-CM code descriptions:
- “I” is used to assign diagnosis codes in Chapter 9, Diseases of the Circulatory System (I00–I99)
- “O” is used to assign diagnosis codes in Chapter 15, Pregnancy, Childbirth and the Puerperium (O00–O9a)
- “U” is not being used at this time and is reserved for future diagnosis code expansion
So in summary, use I, O, and U (in the future) to describe diagnosis codes, but never use I and O with procedure codes—use 1, 2, and U instead. Sounds like a jingle just waiting to be written, “I.O.U. and 1.2.U…”