[The clinical significance of kidney function tests]

Abstract

Clearance studies have lost much of their importance in clinical nephrology during the last two decades, having been replaced by percutaneous renal biopsy for diagnostic purposes. Serum creatinine, if increased, is a rather reliable index for glomerular filtration rate (GFR) provided nonrenal factors influencing creatinine values are taken into account. Thus, the necessity of performing endogenous creatinine clearance to assess GFR is confined to the clearance range between 50-120 ml/min. For long-term observation of progressive renal failure the reciprocal of serum creatinine with time has become popular, the relationship usually being linear if obstruction or infection do not disturb the course. Kinking of the straight line towards the baseline means acceleration of progression, while kinking away from the abscissa expresses retardation of the process. Physiologic aging of the kidneys includes a decrease in glomerular filtration rate starting after the age of 20. In a healthy population of 80-years-olds mean GFR is about 50 ml/min.