Renal Function Tests (Renal Parameters) Part 1
- These tests are done on the serum or plasma of the patient.
- A urine sample may also be needed.
- The specimen is stable for several days at 4 to 6 °C and for at least 2 to 3 months when frozen.
- The creatinine sample is stable when refrigerated for 24 hours.
- Avoid a high concentration of Na fluoride as this inhibits urease.
- Discontinue Vitamin B12 supplements at least 72 hours before sampling.
Purpose of the tests (Indications)
- These tests diagnose any kind of renal disease.
- These tests can tell the prognosis of the patient.
- It can assess the remaining functional capacity of the kidneys.
- Kidneys play an important role in the homeostatic system of the body.
- The kidney is the highly specialized organ in the body for:
- Elimination of the waste products of metabolism.
- It preserves the internal environment of the cells that is it maintains the pH, water balance, ionic equilibrium, and fluid osmotic pressure.
- The impairment of the renal function correlates with morbidity and mortality.
- This is the functional unit of the kidney.
- Each kidney contains 400,000 to 800,000 nephrones.
- Each nephron consists of:
- Proximal Tubule.
- The loop of Henle.
- Distal Tubule.
- Collecting ducts.
- Kidneys biological functions are:
- Homeostatic regulation.
- Endocrine function.
- Urine formation.
- The proximal tubules are most metabolically active and lead to reabsorption of :
- 60 to 90% of the glomerular filtrate.
- 70% of the Na and Cl.
- Other substances are potassium, glucose, bicarbonate, phosphate, and sulfate.
- 90% of H+ is excreted.
- The Distal tubules are involved in excretion and secretion of :
- ADH and aldosterone help in reabsorption.
- K+ secreted in response to aldosterone.
- Sodium, Potassium.
- Chloride and hydrogen.
- H+, ammonia, urea, and uric acid secreted and bicarbonate reabsorbed.
- The Loop of Henle is:
- Selectively permeable to sodium, urea, and water.
- The descending loop is highly permeable to water but not salt.
- Urine becomes concentrated.
- The ascending loop is impermeable to water but actively reabsorbs salts, decreasing the concentration of the urine.
- The collecting system is under the control of ADH.
- Aldosterone regulates NaCl reabsorption.
- K+, H+, and ammonia are reabsorbed.
- Urine formed is carried to the ureter and the urinary bladder.
- Excretory function: This helps the body to get rid of :
- The end product of metabolism.
- Excess of inorganic substances digested in the diet.
- There is the removal of the waste products of metabolism like urea, creatinine, uric acid, and amino acids.
- The removal of the inorganic substances ingested in the diet like sodium, potassium, chloride, calcium, phosphate,
- magnesium, bicarbonate, and sulfate.
- The kidney also removes foreign heavy metals, drugs, and antibiotics.
- Regulatory function: This is a complex system of control in which intrarenal and extrarenal humoral factors are involved.
- Electrolytes homeostasis takes place predominantly in the proximal collecting tubules.
- Where roughly 70% of the sodium, chloride, and water of ultrafiltrate is reabsorbed.
- Other solutes are bicarbonate, phosphate, calcium, and potassium.
- Water homeostasis: Water is reabsorbed in:
- Roughly 70% in the proximal tubule.
- 5% in the loop of Henle.
- 10% in the distal tubule.
- Rest in the collecting ducts.
- Endocrine Function
- The kidney synthesizes hormones and also is the target organ for other site formed hormones.
- The kidney is the site of degradation of hormone-like insulin and aldosterone.
- The kidney produces hormones like :
- Erythropoietin, it stimulates RBCs production.
- Vit. D. It converts 25-hydroxyvitamin D3 to form the biologically active vitamin D.
- Glomerular filtration rate (GFR):
- This is the quantity of blood cleared of substances like creatinine and urea per unit time.
- It is the rate in milliliter (mL) per minute that substances like creatinine and urea filtered through the kidney glomeruli.
- GFR depends upon:
- Plasma concentration of the substance.
- The excretion rate of the kidney, this will reflect:
- Renal plasma flow.
- Glomerular filtration rate.
- This substance used should not be reabsorbed, secreted, synthesized, or degraded in the nephron.
- GFR is the most reliable measure of kidney function.
- GFR may be measured by giving exogenous substances like Inulin clearance.
- GFR may be measured by endogenous substances like creatinine and urea nitrogen.
- GFR depends upon:
- Urine: This is a fluid excreted by the kidneys.
- It is stored in the urinary bladder and discharged through the urethra.
- Normal urine is sterile, clear, and amber color.
- It is slightly acidic and has a characteristic odor.
- Urine contains dissolved compounds, cells, and crystals.
- Urine in a healthy person is clear, pH 5 to 6, and an average specific gravity of 1.024.
- The normal urine output per day is 500 mL.
- Urinary output is described as :
- Anuria < 100 ml/day.
- Oliguria < 400 ml/day.
- Polyuria > 2 L/day.
Routine Renal Function tests are:
- BUN or blood urea.
- Total protein.
- Electrolytes (Sodium and Potassium).
- Chloride in urine and blood.
- Creatinine clearance.
- Creatinine, 24- hours urine.
- Protein in the urine (24 hours urine sample is needed).
- Urine analysis.
Urea nitrogen (Urea N) Concentration:
|Cord||21 to 40|
|Premature (One week)||3 to 25|
|<1 year||4 to 19|
|Infants/child||5 to 18|
|18 to 60 years||6 to 20|
|60 to 90 years||8 to 23|
|>90 years||10 to 31|
|Cord blood||0.6 to 1.2|
|Newborn 1 to 4 days||0.3 to 1.0|
|Infants||0.2 to 0.4|
|Child||0.3 to 0.7|
|Adolescent||0.5 to 1.0|
|18 to 60 years|
|Male||0.9 to 1.3|
|Female||0.6 to 1.1|
|60 to 90 years|
|Male||0.8 to 1.3|
|Female||0.6 to 1.2|
|Male||1.0 to 1.7|
|Female||0.6 to 1.3|
- 5 to 39 mg/dL.
- 7 to 18 mg/dL.
- Male = 0.9 to 1.5 mg/dL.
Female = 0.7 to 1.3 mg/dL.
Normal renal functions tests are:
|Renal parameter||Normal range|
|PSP excretion (phenosulfonaphthalein)||
|Glomerular filtration rate||
Table showing the severity of renal failure:
|The degree of renal failure||GFR mL/min||Creatinine mg/dL||Clinical presentation|
|Mild||30 to 50||1.9||Hypertension|
|Moderate||10 to 30||4.0||Anemia|
|End-stage||< 5||16.9||acidosis, hyperkalemia, coma, and death|
- Note These tests in detail are described under their names. Please see part 2 of the renal function test.