GFR and what it really means?

Estimating Glomerular Filtration Rate (GFR) and Creatinine Clearance (CrCl) in Health and Disease: Using Popular Formulae (September 2009) 

This discussion piece reviews common methods for estimating kidney function. The reader may find it helpful to become re-acquainted with basic renal physiology as that information will not be covered here. 

It is considered good Radiologic practice to screen patients for renal dysfunction prior to the administration of any iodinated or gadolinium-based contrast medium. The American College of Radiology recommends that all practitioners screen patients for renal dysfunction; this can easily be done by questioning the patient or by chart review, looking for a history of renal disease/transplant, diabetes mellitus, long-standing hypertension, diuretics or nephrotoxic drugs, multiple myeloma, hyperuricemia, or advanced age (≥70 years). If renal dysfunction is present or suspected, then laboratory measurements of renal function should be obtained. The US FDA recommends that patient renal function be assessed either by history or laboratory measurements prior to the administration of gadolinium-based contrast agents.

Renal function can be evaluated by obtaining laboratory measurements of blood urea nitrogen (BUN) and serum creatinine (SCr), but neither are sensitive measures of renal function. SCr is influenced by the patient’s gender, muscle mass, nutritional status, and age. GFR is traditionally considered the best overall index of renal function in health and disease. Direct measurement of GFR (or CrCl using the compound inulin) would be the most accurate method of defining renal function, but this requires timed urine collections or radioisotope elimination methods, and thus is difficult to measure in clinical practice. Therefore, most people estimate the GFR from the SCr using specially derived predictive equations.                                                 The GFR may be estimated using several formulae, but the most common are:

• Cockcroft-Gault formula, estimates CrCl in adults
• Schwartz formula, estimates GFR in children under 12 years
• Modification of Diet in Renal Disease, estimates GFR in adult.                                                                               Cockcroft-Gault equation for estimating GFR (CrCl):
  eGFR = [(140-age) X body weight (kg) X 0.85 (if female)] ÷ [72 X SCr (mg/dl)]