Mapping the relationship between clinical and quality-of-life outcomes in patients with ankylosing spondylitis

ABSTRACT

Aim: To map health-related quality of life (Qol) with clinical parameters BASFI and ASDAS-CRP measure, and other covariates.

Methods: Our prospective multicenter non-interventional observation study of ankylosing spondylitis (AS) collected data about QoL and clinical outcomes on the initial and four subsequent visits. We employed simple linear regression analysis of a cross-sectional dataset, and fixed effect, random effect and pooled linear regression of a longitudinal dataset.

Results: We showed that BASFI and ASDAS-CRP are very strong, robust predictors of EQ-5D utilities in all regression specifications together with sex (female), invalidity, and activity impairment. Additionally, the longitudinal regression analysis showed that a fixed effect model may be a viable alternative to the most commonly used random effect model or pooled linear regression due to the nature of our dataset.

Conclusion: This is one of the first studies using a fixed effect model in longitudinal patient-level data, although, this method has been widely used in economics.

KEYWORDS:

• Ankylosing spondylitis
• mapping
• outcomes research
• health-related quality of life
• fixed effect regression
• longitudinal analysis
• regression analysis

Acknowledgements

The authors acknowledge careful proofreading and insightful suggestions from Klara Kruntradova MSc. and Martina Mazalova MSc. (both from iHETA). The authors would like to thank three anonymous reviewers whose revisions markedly increased the overall quality of our paper.

Declaration of interest

L Sedova, J Stolfa, M Urbanova and K Pavelka were supported by the Czech Ministry of Health IGA MZ CR: No. 000 000 23,728. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Notes

1. Stage I: Grade II or higher bilateral radiographic sacroiliitis; Stage II: minor radiographic evidence of spinal involvement; Stage III: moderate radiographic evidence of spinal involvement; Stage IV: radiographic evidence of spinal involvement; Stage V: Widespread fusion of the spine [16].

2. Impairment of leisure time activities is in the WPAI questionnaire labeled as ‘activity impairment due to problem’, henceforth and throughout the text only activity impairment.

3. Working productivity and thus overall work impairment was proportionally decreased according to the stage of invalidity since invalidity is in the Czech Republic defined in stages: 1st stage is defined by law as a decrease in working productivity by 35–49%, 2nd stage by 50–69%, and 3rd stage by 70–100%.

4. In one regression specification, we included also age and time from diagnosis to assess the impact of these outcomes on other parameters’ values although these were statistically highly insignificant.

5. Specifically, in the cross-section data set, seven patients did not have completely filled ASDAS-CRP, two patients had missing BASFI, then there was one missing value of weight, time from diagnosis and EQ-VAS.

6. We did not test for the statistical significance between different AS forms, since only 32 patients had 2 forms of AS (see Table 2).

7. BASFI can be also explained by ASDAS-CRP when using linear regression, regression line: BASFI = −0.0897 + 1.4502 × ASDAS-CRP, while ASDAS-CRP was statistically significant with a p value < 0.001.

Additional information

Funding

This work was supported by the Ministerstvo Zdravotnictví Ceské Republiky: [Grant Number IGA MZ CR: No. 000 000 23728].