Background/Purpose:

Recently, ASDAS has been developed as a disease activity measuring tool for ankylosing spondylitis (AS), using a similar methodology to that used for the development of the Disease Activity Score (DAS) in rheumatoid arthritis. Two versions of ASDAS have been proposed, one based on CRP (preferred) and the other based on ESR (alternative).In the light of recent reports showing substantial difference between the two versions of DAS28, it is of interest to assess the agreement between the two ASDAS versions at individual level. Our aim is to assess the agreement between the ESR and CRP-based ASDAS scores in AS patients.

Methods:

Data were obtained from the local clinical database which contains slightly over 500 AS patients .Patients with full data at baseline were included in this analysis. Mean ASDAS-CRP and ASDAS-ESR values were compared by Spearman correlation and scatter plot with linear regression analysis. Bland-Altman analysis and kappa statistics were used to assess the agreement between the two ASDAS definitions in the whole group as well as in different gender and age groups.

Results:

396 patients (291 M; 44 ±12.0 years) were identified with complete data at baseline for this analysis. Mean (±SD) disease duration was 9.4 (±8.2) years.  Mean (±SD) BASDAI, BASFI and BASMI scores were 3.5 (±2.2), 2.9 (±2.6), 3.9 (±1.9), respectively.  HLA B27 was positive in 65% of the patients of whom 83.7% were using NSAIDs and 20.7% were using TNF inhibitors. Mean (±SD) ASDAS scores, based on CRP and ESR were 2.9 (± 1.1), and 2.8 (± 1.0), respectively. There was a strong correlation between the two definitions by Spearman’s correlation test (r=0.9, p<0.001) and linear regression analysis (R²=0.82, p<0.001). The agreement  was good for both genders  and all age groups with weighted kappa values ranging from  0.680  to 0.876 (Table 1). Similar number of patients was classified into the defined categories of disease activity with ASDAS-CRP and ASDAS-ESR(Table 2). Bland-Altman analysis showed excellent agreement between the two scores (ASDAS-CRP – ASDAS-ESR) with a mean difference (bias) of −0.0 ± 0.48 (95% CI −0.04, −0.047). Upper and lower limits of agreement were 0.95 (95% CI 0.87, 1.03) and −0.95 (95% CI −1.03, −0.87), respectively. Mean difference was 0.20 ± 0.49 in females and -0.10 ± 0.44 in males.

Conclusion:

The results suggest a good agreement between ASDAS-ESR and ASDAS-CRP for both genders and all age groups. 

Table 1: The correlation and agreement between ASDAS-CRP and ASDAS-ESR values in different gender and age groups

Patient group	Mean ASDAS-CRP	Mean ASDAS-ESR	Spearman correlation coeficient		Agreement rate	Kappa value	Weighted kappa value
			r	P
Females (n=104)	2.8±1.1	3.1±1.0	0.906	<0.001	64.4%	0.455	0.680
Males (n=292)	2.9±1.1	2.7±1.0	0.923	<0.001	73.6%	0.618	0.803
≤40 years  old (n=163)	3.0±1.1	2.9±1.0	0.918	<0.001	69.7%	0.549	0.790
41-60 years old (n=193)	2.7±1.1	2.8±1.0	0.888	<0.001	70.5%	0.561	0.740
≥61 years old (n=40)	3.0±1.2	3.0±1.0	0.954	<0.001	74.4%	0.672	0.876
Overall (n=396 )	2.9 ±1.1	2.8 ±1.0	0.906	<0.001	70.0%	0.567	0.760

Table 2: Agreement between ASDAS-CRP and ASDAS-ESR on the classification of the patients into different categories of disease activity

		ASDAS CRP
ASDAS ESR		<1.3	1.3-2.0	2.1-3.5	>3.5	Total
	<1.3	13	5	2	0	20
	1.3-2.0	17	40	19	0	76
	2.1-3.5	6	19	137	25	187
	>3.5	0	0	22	91	113
	Total	36	64	180	116	396

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/good-agreement-between-the-ankylosing-spondylitis-disease-activity-scores-based-on-c-reactive-protein-and-erythrocyte-sedimentation-rate/