We have previously shown that markers of tubule cell injury,1 fibrosis,2 abnormal acid-base homeostasis,3 and diminished proximal tubule reabsorptive capacity4 are associated with more rapid chronic kidney disease (CKD) progression, independent of eGFR and albuminuria. Another key function of kidney tubules is secreting many toxins and medications. Whether secretory capacity gives insights into kidney tubule health is uncertain.