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.
NICE state (2):
- measuring kidney function - creatinine-based estimate of glomerular filtration rate
- creatinine-based estimate of glomerular filtration rate
- whenever a request for serum creatinine measurement is made, clinical laboratories should report an estimate of (eGFRcreatinine) using a prediction
equation in addition to reporting the serum creatinine result
- eGFRcreatinine may be less reliable in certain situations (for example, acute kidney injury, pregnancy, oedematous states, muscle wasting disorders, and in adults who are malnourished, who have higher muscle mass or use protein supplements, or who have had an amputation) and has not been well validated in certain ethnic groups (for example, black, Asian and other minority ethnic groups with CKD living in the UK)
- interpret eGFRcreatinine with caution in adults with extremes of muscle mass, for example, in bodybuilders, people who have had an amputation or people with muscle wasting disorders. (Reduced muscle mass will lead to overestimation and increased muscle mass to underestimation of the GFR.)
- advise adults not to eat any meat in the 12 hours before having a blood test for eGFRcreatinine. Avoid delaying the despatch of blood samples to ensure that they are received and processed by the laboratory within 12 hours of venepuncture
- clinical laboratories should:
- use the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine equation to estimate GFRcreatinine for adults, using creatinine assays with calibration traceable to standardised reference material
- use creatinine assays that are specific (for example, enzymatic assays) and zero-biased compared with isotope dilution mass spectrometry (IDMS)
- reporting and interpreting GFR values
- clinical laboratories should report eGFR either as a whole number if it is 90 ml/min/1.73 m2 or less, or as 'greater than 90 ml/min/1.73 m2'
- if eGFR is greater than 90 ml/min/1.73 m2, use an increase in serum creatinine concentration of more than 20% to infer significant reduction in kidney function
- interpret eGFR values of 60 ml/min/1.73 m2 or more with caution, bearing in mind that estimates of GFR become less accurate as the true GFR increases
- confirm an eGFR result of less than 60 ml/min/1.73 m2 in an adult not previously tested by repeating the test within 2 weeks. Allow for biological and analytical variability of serum creatinine (+/-5%) when interpreting changes in eGFR
- when highly accurate measures of GFR are needed
- if a highly accurate measure of GFR is needed, for example, during monitoring of chemotherapy and in the evaluation of kidney function in potential living donors, consider a reference standard measure (inulin, 51Cr-EDTA, 125I-iothalamate or iohexol)
Notes:
- 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
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