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Physiology

Renal

Question 94 of 180

Glomerular filtration rate can be calculated using any substance that:

Answer:

Clearance of a substance can provide an accurate estimate of the glomerular filtration rate (GFR) provided that the substance is:
  1. freely filtered
  2. not reabsorbed in the nephron
  3. not secreted in the nephron
  4. not synthesised or metabolised by the kidney

Creatinine Clearance

Physiology / Renal / Mechanism of Filtration

Last Updated: 10th March 2019

Clearance is defined as the volume of plasma that is cleared of a substance per unit time.

Clearance of substance = (Urine concentration of substance x Urine flow rate)/Plasma concentration of substance.

Clearance of a substance can provide an accurate estimate of the glomerular filtration rate (GFR) provided that the substance is:

  1. freely filtered
  2. not reabsorbed in the nephron
  3. not secreted in the nephron
  4. not synthesised or metabolised by the kidney

Inulin is such a substance but measurement of inulin clearance is complicated and not used in routine clinical practice.

Creatinine (which is steadily released from skeletal muscle) clearance can be used because it is freely filtered and not reabsorbed, and there is very little secretion, allowing relatively accurate approximation of the GFR (except when plasma creatinine or GFR is abnormally low).

In practice, GFR is usually estimated from the plasma creatinine using a formula e.g. the MDRD equation making an adjustment for age, sex and race.

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  • Biochemistry
  • Blood Gases
  • Haematology
Biochemistry Normal Value
Sodium 135 – 145 mmol/l
Potassium 3.0 – 4.5 mmol/l
Urea 2.5 – 7.5 mmol/l
Glucose 3.5 – 5.0 mmol/l
Creatinine 35 – 135 μmol/l
Alanine Aminotransferase (ALT) 5 – 35 U/l
Gamma-glutamyl Transferase (GGT) < 65 U/l
Alkaline Phosphatase (ALP) 30 – 135 U/l
Aspartate Aminotransferase (AST) < 40 U/l
Total Protein 60 – 80 g/l
Albumin 35 – 50 g/l
Globulin 2.4 – 3.5 g/dl
Amylase < 70 U/l
Total Bilirubin 3 – 17 μmol/l
Calcium 2.1 – 2.5 mmol/l
Chloride 95 – 105 mmol/l
Phosphate 0.8 – 1.4 mmol/l
Haematology Normal Value
Haemoglobin 11.5 – 16.6 g/dl
White Blood Cells 4.0 – 11.0 x 109/l
Platelets 150 – 450 x 109/l
MCV 80 – 96 fl
MCHC 32 – 36 g/dl
Neutrophils 2.0 – 7.5 x 109/l
Lymphocytes 1.5 – 4.0 x 109/l
Monocytes 0.3 – 1.0 x 109/l
Eosinophils 0.1 – 0.5 x 109/l
Basophils < 0.2 x 109/l
Reticulocytes < 2%
Haematocrit 0.35 – 0.49
Red Cell Distribution Width 11 – 15%
Blood Gases Normal Value
pH 7.35 – 7.45
pO2 11 – 14 kPa
pCO2 4.5 – 6.0 kPa
Base Excess -2 – +2 mmol/l
Bicarbonate 24 – 30 mmol/l
Lactate < 2 mmol/l

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