ICD-10-CM guide
ICD-10-CM Laterality: Right, Left, Bilateral, and Unspecified
How ICD-10-CM laterality works: when to use right, left, or bilateral codes, when separate side codes apply, and why unspecified side should be rare.
Reviewed 2026-09-29 | 2 min read
Many ICD-10-CM codes carry the side of the body in one of their characters. Knee pain, knee osteoarthritis, and a knee prosthesis each have separate codes for right and left, and some conditions have a bilateral code as well. Picking the wrong side produces a valid-looking code that describes the wrong patient. Laterality is also one of the most common reasons a practice answer is almost right.
Read the side from the record
When a code specifies laterality, it tells you whether the condition is on the left, on the right, or bilateral. Code the side documented in the record. Right knee pain is M25.561, Pain in right knee. Left knee pain is M25.562. The two codes are equally specific; only the side differs.
Bilateral conditions
- When a bilateral code exists and the condition affects both sides, assign the bilateral code. M17.0, Bilateral primary osteoarthritis of knee, is one example.
- When no bilateral code is provided and the condition is bilateral, assign separate codes for the left and the right side.
- When each side is treated at a separate encounter, the first encounter still gets the bilateral code, because the condition still exists on both sides.
- At a later encounter, after one side was treated and the condition no longer exists there, assign the unilateral code for the side that still has it. If the first treatment did not resolve the condition, the bilateral code is still appropriate.
Unspecified side should be rare
If the side is not identified in the medical record, the guidelines direct you to the code for the unspecified side. They also say those codes should rarely be used. Laterality documented by other clinicians in the record, not only the treating provider, can be used to assign the side. When the documentation conflicts, the provider should be queried.
Different conditions, different sides
A patient can have a condition on one side and a status on the other. Left knee osteoarthritis in a patient with a prior right knee replacement is M17.12, Unilateral primary osteoarthritis, left knee, with Z96.651, Presence of right artificial knee joint. Each code carries its own side, and mixing them up changes the meaning of both.
A laterality check for every code
- Does this code have a character for side? Confirm it in the Tabular List, not from memory.
- Does the documented side match the character you chose?
- Is the condition on both sides? Look for a bilateral code before reporting two unilateral codes.
- Did you default to unspecified? Search the record again, including other clinicians' notes, before accepting it.
Questions coders ask
- What if the provider does not document the side?
- Laterality documented by other clinicians in the record may be used. If the record still does not identify the side, assign the unspecified side code. If the documentation conflicts, query the provider.
- Do I report two codes for a bilateral condition?
- Only when no bilateral code is provided. When a bilateral code exists, assign it instead of separate left and right codes.
- Is an unspecified side code wrong?
- It is valid, but the guidelines say it should rarely be used, only when the documentation cannot establish the side and clarification is not possible.
Practice these rules
Sources
ICD-10-CM Official Guidelines for Coding and Reporting FY 2027
Section I.B.13, Laterality
ICD-10-CM Official Guidelines for Coding and Reporting FY 2027
Section I.B.12, Reporting Same Diagnosis Code More than Once
FY 2027 ICD-10-CM Code Descriptions in Tabular Order
Codes M25.561, M25.562, M17.0, M17.11, M17.12, Z96.651