New terminology for the renogram's three segments is recommended, namely, tracer appearance, blood flow, and drainage, to replace the original terms, vascular, tubular, and excretion. The first, or tracer appearance segment, is no longer considered a useful index of renal vascular capacity because activity of blood within the kidney contributes in only a minor way to the height of the first segment. The slope of the second segment is an index of renal blood flow because tracer is retained in the kidney during this time and the extraction efficiency of the kidney for tracer quantities of sodium iodohippurate is approximately 92%. The third segment indicates the state of mechanical drainage from the kidney. The term "renal transit time" is added as another parameter of the renocystogram and provides an index of individual urine flow and has special significance in hypertension due to renal artery occlusive disease.
Dore EK, Taplin GV, Johnson DLE. Current Interpretation of the Sodium Iodohippurate I131 Renocystogram. JAMA. 1963;185(12):925-932. doi:10.1001/jama.1963.03060120035018