Key points

• Glomerular filtration rate (GFR) refers to the ability of the glomeruli to filter plasma and form ultrafiltrate.
• GFR is a tool used in the diagnosis, staging, and management of chronic kidney disease (CKD), as well as to calculate drug doses and establish a prognosis or predict mortality.
• The normal range of GFR in healthy patients is from 100 to 125 mL/min per 1.73 m². While CKD is considered when we find a GFR level < 60 mL/min per 1.73 m², or evidence of kidney damage (albuminuria) for more than 3 months.
• Direct measurement methods are expensive, invasive and not available to everyone, so indirect methods or those estimated from the concentration of endogenous substances are recommended in clinical practice. Of these, the most used are creatinine and Cystatin C.
• Creatinine:
  • Advantage: low cost, easy to obtain and perform test, short time, minimally invasive.
  • Disadvantages: tends to overestimate the GFR (the lower the level, the higher the overestimation). Factors that can alter your result: health status, body composition (muscle mass), diet, physical activity, medications.
• Cystatin C:
  • Advantage: higher accuracy than creatinine. Less influenced by external factors such as age, gender and race. Factors that can alter its result: inflammation, obesity, thyroid disorders.
  • Disadvantages: Higher cost.
• The recommendation is that the most used equations to estimate GFR from creatinine are the CKD-PPE (Chronic Kidney Disease Epidemiology Collaboration) in adults and the CKiD (CKD in Children Schwartz “bedside”) in children.
  • Cockcroft-Gault: underestimates GFR, is unreliable in older adults, patients with obesity, and edematous states.
  • MDRD formula: underestimates GFR, the higher the GFR, the more it underestimates it.
• When a confirmatory test is required, it is suggested to use a 24-hour urine collection to measure creatinine, urea, and albumin clearance.