The [TF/P]x ratio refers to the ratio between the concentration of a substance "x" in the tubular fluid (TF) and the plasma (P) in a nephron. This ratio provides information about the reabsorption or secretion of a substance in the nephron.
The [TF/P]inulin ratio is a measurement of glomerular filtration rate (GFR), which is a measure of how well the kidneys are functioning. Inulin is a polysaccharide that is freely filtered by the glomerulus and is neither reabsorbed nor secreted by the tubules.
Both [TF/P]x ratio and [TF/P]inulin measurements can provide valuable information on renal function.