Predictors of all-cause direct costs after adjusting for baseline characteristics*. Direct costs were calculated as the sum of all-cause medical and pharmacy costs. Outcomes were predicted using a generalized linear model with γ distribution and log link function.
| Predictor | AS, n = 4078, Coefficient Estimate (95% CI)** | p | PsA, n = 10,277, Coefficient Estimate (95% CI)** | p |
|---|---|---|---|---|
| Age, yrs, reference ≥ 65 yrs | ||||
| 18–34 | 1.28 (1.09–1.50) | < 0.01 | 1.18 (1.07–1.31) | < 0.01 |
| 35–44 | 1.18 (1.02–1.36) | < 0.05 | 1.07 (0.98–1.16) | NS |
| 45–54 | 1.20 (1.05–1.38) | < 0.01 | 1.09 (1.01–1.18) | < 0.05 |
| 55–64 | 1.30 (1.13–1.50) | < 0.001 | 1.14 (1.06–1.22) | < 0.001 |
| Sex, reference = female | ||||
| Male | 1.02 (0.94–1.10) | NS | 1.03 (0.99–1.07) | NS |
| Region in the US, reference = West | ||||
| Northeast | 0.98 (0.86–1.11) | NS | 1.03 (0.96–1.11) | NS |
| North Central | 1.01 (0.90–1.12) | NS | 0.88 (0.83–0.94) | < 0.001 |
| South | 0.88 (0.80–0.97) | < 0.05 | 0.93 (0.87–0.98) | < 0.05 |
| Insurance plan*, reference = HMO and POS capitation | ||||
| FFS | 0.91 (0.82–1.10) | ≥ 0.05 | 1.04 (0.98–1.10) | NS |
| Baseline ECI score, reference = 0 | ||||
| 1 | 0.99 (0.87–1.13) | NS | 1.09 (1.04–1.15) | < 0.01 |
| 2 | 1.25 (1.09–1.44) | < 0.01 | 1.21 (1.13–1.28) | < 0.001 |
| 3 | 1.26 (1.07–1.48) | < 0.01 | 1.37 (1.25–1.49) | < 0.001 |
| 4 | 2.12 (1.81–2.48) | < 0.001 | 1.95 (1.78–2.13) | < 0.001 |
| Baseline biologics use, reference = patients with no biologic use | 2.82 (2.61–3.05) | < 0.001 | 3.12 (2.98–3.25) | < 0.001 |
↵* Patients with missing region (n = 42 for AS, < 1%; n = 125 for PsA, < 1%), missing insurance type (n = 157 for AS, < 3%; n = 425 for PsA, < 4%), or zero outcome (n = 11 for AS, < 1%; n = 5 for PsA, < 1%) were excluded from the multivariate analysis.
↵** Coefficient estimates were the exponentials of the original estimates. A coefficient of > 1.0 indicates higher cost and < 1.0 indicates lower costs for a given variable compared with the specified “reference.” AS: ankylosing spondylitis; PsA: psoriatic arthritis; FFS: fee for service; ECI: Elixhauser Comorbidity Index; NS: not significant; HMO: health maintenance organization; POS: point of service.