Assessment of glomerular function - GFR

• Creatinine still widely used but variably useful
  • Creatinine isn't sensitive to changes in GFR until GFR is quite low
  • Serum creatinine reflects the amount of creatinine produced per day which confounds measurement
    • Key confounders - muscle mass, diet
• Historically used to measure urine creatinine clearance which is sensitive to changes in GFR at higher GFR levels
  • Volume of blood 'cleared' of creatinine per minute by glomerular filtration
  • Measure creatinine in blood and, 24-hour urine collection and volume of urine
  • Calculation: [urine creatinine conc. x urine volume] DIVIDED BY [serum creatinine conc. x duration of collection)
• eGFR takes some of the confounders of serum creatinine into account and is easier than urine creatinine
  • Age, sex, and ethnicity are used as proxies for muscle mass and diet in order to estimate GFR more accurately
• But even eGFR is not very accurate at higher GFR (hence reported as ≳60 ml/min)
• If accurate assessment of GFR is essential, 51Cr-EDTA is the validated reference standard

Classification of CKD based on eGFR

Assessment of glomerular function - proteinuria

• Plasma proteins are normally retained by the glomeruli

• Can estimate by doing a 24-hour collection

• Protein >150mg/day indicates significant glomerular damage

• Protein/creatinine ratio (spot sample) also helpful

• There are different types of proteinuria - overflow and glomerular are the two situations which result in significant proteinuria, with glomerular being the most common

  

Ways of expressing proteinuria

• Microalbuminuria refers to excretion of albumin in abnormal quantities but still below the limit of protein detection by dipstick
  • Earliest expression of diabetic nephropathy - standard part of routine diabetic assessment

Assessment of tubular function

• Urine osmolality used to assess tubular function
  • If urine osmolality is exactly the same as serum osmolality, this indicates the tubules are not functioning
  • If urine osmolality is very different to serum osmolality, this indicates the tubules are functioning