One can approach differentiating between these via diagnostic codes, supplemented with the joint counts. The diagnostic codes that directly refer to the polyarticular status and can be used as a proxy pcJIA are:
ICD9:
- 714.30: Polyarticular juvenile rheumatoid arthritis, chronic or unspecified
- 714.31: Polyarticular juvenile rheumatoid arthritis, acute
- ICD10: M08.3 seronegative polyarthritis
- The above ICD10 code is the only one with “polyarticular” information. Another possible approach with ICD10 is to include all juvenile arthritis M08.* and then exclude the following (which are evidently not polyarticular JIA):
- (Exclude) M08.1 Juvenile ankylosing spondylitis
- (Exclude) M08.2* Systemic JIA
- (Exclude) M08.4* Pauciarticular JIA
Both codes could be supplemented with a joint count-based criterion. Specifically, one can additionally require the first swollen joint count in 28 joints (SJC_28) or tender joint count in 28 joints (TJC_28) > 4 for the first available measurement. Strictly speaking, it should be > 4 joints within 6 months of the onset, but this may be too strict considering the somewhat limited availability of joint counts in routine care data. Instead, using the first joint count within 12 months of the first JIA code might be more appropriate.
The codes below can be used as a proxy as following:
- ICD-9 did not have specific codes for systemic JIA.
- ICD-10 M08.2* codes are specific to “JIA with systemic onset”.
- If a given patient meets both criteria for polyarticular JIA and systemic JIA, the patient should either be excluded or classified as a systemic JIA patient.