MMP9 and chronic kidney disease: It is known that reactive oxygen and nitrogen species can interact with both TIMP-1 and MMP-9, blocking endogenous MMP-9 inhibition and activating MMP-9 constitutively [13] and, therefore, assessment of the MMP-9/TIMP-1 ratio as an indicator of MMP-9 activation is not recommended [18], especially in conditions of oxidative stress such as CKD or ESRD.