Serum substance P: an indicator of disease activity and subclinical inflammation in rheumatoid arthritis

The aim of the is study is to examine the role of serum substance P (SP) levels as a simple biomarker for rheumatoid arthritis (RA) disease activity, its correlation with other markers of disease activity, and with selected clinical parameters. The study comprised 90 RA patients and 24 healthy controls. RA activity was assessed by means of the disease activity 28-C-reactive protein (DAS28-CRP) index and ultrasound power Doppler (USPD) by the German ultrasound score based on seven joints. SP serum values were obtained by means of an ELISA commercial kit. Statistics were achieved by the Student's t test and Spearman correlation analysis with Bonferroni correction. As a group, RA patients had significantly increased levels of SP compared with healthy controls (p < 0.0001). SP levels correlated with DAS28-CRP (r = 0.5050, p < 0.0001), number of tender joints (NTJ, r = 0.4668, p < 0.0001), number of swollen joints (NSJ, r = 0.4439, p < 0.0001), visual analogue scale (VAS, r = 0.5131, p < 0.0001). However, SP did not correlate with CRP levels (r = 0.0468, p = 0.6613), nor with the USPD (r = 0.1740, p = 0.1009). Elevated serum SP is a common feature of RA patients, which also appears to correlate with clinical measurements of disease activity and with subjective clinical data (NTJ and VAS). Thus, although SP is higher in RA patients with high disease activity, it also detects subtle RA disease activity even in patients in apparent remission, which suggests its usefulness for therapeutic decisions.