transport maximum, the highest rate in milligrams per minute at which the renal tubules can transfer a substance either from the tubular luminal fluid to the interstitial fluid or from the interstitial fluid to the tubular luminal fluid, beyond which it may be excreted in the urine. In kidney function tests, it is expressed as Tm with inferior letters representing the substance used in the test, such as TmPAH (transport maximum for p-aminohippuric acid). Also called tubular maximum.
