Table 2 Correlation of the three factors resulting from the principal components analysis with the variables in ISSAS database (results for candidate index A, including all items selected in the Delphi exercise excluding BASFI)

	Factors
	1 Patient-reported outcomes	2 Peripheral activity	3 Laboratory
Back pain at night	0.84	0.10	0.05
Total spinal pain	0.88	0.15	0.02
Patient global assessment	0.82	0.21	0.15
BASDAI question 1	0.73	0.24	0.00
BASDAI question 2	0.86	0.11	0.05
BASDAI question 3	0.39	0.66	0.11
BASDAI question 6	0.66	0.11	0.09
Swollen joint count	−0.05	0.86	0.17
CRP	0.07	0.06	0.89
Tender entheses count	0.31	0.57	−0.03
ESR	0.09	0.14	0.88

BASDAI, Bath Ankylosing Spondylitis Disease Activity Index; BASFI, Bath Ankylosing Spondylitis Functional Index; CRP, C-reactive protein; ESR, erythrocyte sedimentation rate; ISSAS, International Study on Starting TNF blocking agents in Ankylosing Spondylitis; TNF, tumour necrosis factor.