Arthritis is the inflammation or swelling of one or more joints with symptoms of joint pain and stiffness. It also affects the connective tissues.
Coders can find information about arthritis coding in Chapter 13: Diseases of the Musculoskeletal System and Connective Tissue (M00-M99). Osteoarthritis is in the M15-M19 sub-section.
OS coding depends on specificity
Arthritis NOS and osteoarthritis NOS both code to M19.90 (Unspecified osteoarthritis, unspecified site). Coders should review the record for type and site specificity. In home health, M19.90 is an unacceptable principal diagnosis under PDGM ─ but it is acceptable as a secondary diagnosis.
Additional knowledge needed to code OA properly is the understanding of generalized versus localized and primary versus secondary.
Generalized vs. localized
Generalized osteoarthritis affects many joints, while localized osteoarthritis affects the joints of one site. Bilateral OA of the same site is considered localized. Localized osteoarthritis can be further broken down into two other categories: primary and secondary.
Primary vs. secondary
Primary OA NOS — also called idiopathic osteopathy — codes to M19.90 (Unspecified osteoarthritis, unspecified site). Secondary OA (M19.2-; M89.4) refers to degenerative disease of the joint and has a specific cause, such as an injury, an effect of obesity, genetics, inactivity, or other diseases.
Code guidelines explained
Most of the codes within Chapter 13 have site and laterality designations. The site represents the bone, joint, or muscle involved. For some conditions where more than one bone, joint, or muscle is usually involved, such as osteoarthritis, a “multiple sites” code is available.
For categories where no multiple site code is provided and more than one bone, joint or muscle is involved, use multiple codes to indicate the different sites involved. This guideline means that when a multiple site code is available for a condition within Chapter 13 and multiple sites are specified, assign the multiple site code instead of codes for multiple individual sites.
Coding note
Code primary OA of the joint if the documentation does not state primary, secondary, or generalized. OA, primary is the default. For instance, for Bilateral OA of knee, code M17.0. If unilateral, remember to identify laterality, represented in the 5th character — for instance, Unilateral OA of the right knee, code M17.11.