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glomerular filtration rate (GFR)

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The glomerulus produces a selective ultrafiltrate of the blood. The rate of ultrafiltration is called the glomerular filtration rate which is about 120 ml/min or 170 litres per day. This is written 120 ml/min/1.73m2, to emphasise the fact that the rate is closely related to the body surface area.

The glomerular filtration rate increases from birth - in the neonate it is about one millilitre per minute.

In children the GFR may be estimated from the plasma creatinine as follows:

GFR = height (in cm) x 40 / plasma creatinine

In adults, there are various methods that may be used to estimate GFR:

Cockcroft-Gault equation is often used as a method of estimating GFR (although it was developed as a method of predicting creatinine clearance) from knowledge of serum creatinine, age and weight:

  • creatinine clearance = (((140 - age in years) x (wt in kg)) x 1.23) / (serum creatinine in micromol/l)

For women multiply the result of calculation by 0.85.The calculation is unreliable if the patient has unstable renal function, is very obese, or is oedematous.

An alternative is the 4-variable Modification of Diet in Renal Disease (MDRD) equation (1):

NICE suggest that clinicians should use the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine equation to estimate GFRcreatinine, using creatinine assays with calibration traceable to standardised reference material:

consider using eGFRcystatinC at initial diagnosis to confirm or rule out CKD in people with:

  • an eGFRcreatinine of 45-59 ml/min/1.73 m2, sustained for at least 90 days and
  • no proteinuria (albumin:creatinine ratio [ACR] less than 3 mg/mmol) or other marker of kidney disease

Notes:

  • the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) have developed equations to calculate eGFR based on serum creatinine (referred to as CKD-EPI2009Scr) and serum creatinine plus serum cystatin C (CKD-EPI2012Scr-cys)
  • in patients with near-normal kidney function, the Modification of Diet in Renal Disease (MDRD) equations underestimate GFR
    • the CKD-EPI2009Scr equation partly overcomes the major limitation of the MDRD equation
    • the CKD-EPI working group has two newer CKD-EPI equations:
      • one using cystatin C concentration (CKD-EPI2012cys) and the other using both cystatin C and serum creatinine concentrations (CKD-EPI2012Scr-cys)
      • they validate the new equations represent an advance over currently available equations across the range of GFR and in relevant subgroups
        • the advance even holds true among participants with an extreme body-mass index of less than twenty. The two new equations even have been recommended by KDIGO 2012Clinical Practice Guidelines for the Evaluation and Management of CKD (3)
        • there has been data that the CKD-EPI2012Scr-cys is superior to calculations based on serum creatinine alone (4,5,6)
          • a study in Chines population ((n = 788; median age, 54 [range, 19-94] years)) concluded (7) '..CKD-EPI2012Scr-cys appeared less biased and more accurate in overall participants. Neither of the new CKD-EPI equations achieved ideal accuracy in senior participants with moderately-severely injured GFR...'
        • a review states that overall, the CKD-EPICR-CYS equation had better precision and accuracy than that based on creatinine alone or CysC alone (8)
  • GFR estimates between 60 and 89 mL/min/1.73 m2 do not indicate chronic kidney disease unless there is other laboratory/clinical evidence of disease
  • there is no need to collect 24 h urine samples to measure creatinine clearance in primary care
  • in cases where there are extremes of muscle mass - for example, in bodybuilders, amputees or people with muscle wasting disorders - interpret the eGFR with caution
    • reduced muscle mass will lead to overestimation and increased muscle mass to underestimation of the GFR
  • advise people not to eat any meat in the 12 hours before having a blood test for GFR estimation. Avoid delaying the despatch of blood samples to ensure that they are received and processed by the laboratory within 12 hours of venepuncture

Reference:

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