PON1 and rheumatoid arthritis: Of note, also after excluding the four largest studies (accounting for ~50% of the overall sample size), PON-1 activity persisted to be significantly lower in RA compared to controls (SMD = −1.22, 95% CI −1.83 to −1.60, p  =  0.001; I2  =  93.9%, p  <  0.001).