Renal Function Tests, Dialysis, Artificial Kidney

 Properties of urine

  • Volume: 1,000 to 1,500 mL/day
  • Reaction: Slightly acidic with pH of 4.5 to 6
  • Specific gravity: 1.010 to 1.025
  • Osmolarity: 1,200 mOsm/L
  • Colour: Normally, straw coloured
  • Odour: Fresh urine has light aromatic odor. If stored for some time, the odor becomes stronger due to bacterial decomposition

Composition of Urine

  • Urine consists of water and solids. Solids include organic and inorganic substances

Renal Function Tests

Group of tests that are performed to assess the functions of kidney

3 types:

  1. Examination of urine alone
  2. Examination of blood alone
  3. Examination of blood and urine 

1) Examination of Urine – Urine analysis

Done by

  • Physical examination
  • Microscopic examination
  • Chemical analysis

Physical Examination

Volume

  • Increase in urine volume
    • Increased protein catabolism
    • Chronic renal failure
    • Diabetes insipidus
    • Glycosuria

Colour

  • Abnormal colour seen in
    • Jaundice
    • Hematuria
    • Hemoglobinuria
    • Medications
    • Excess urobilinogen
    • Ingestion of coloured food

Appearance

    • Normally urine is clear
    • Becomes turbid in both Physiological and Pathological
    • Physiological conditions
      • Precipitation of crystals, presence of mucus/vaginal discharge
    • Pathological conditions
      • Presence of blood cells, bacteria or yeast

Specific Gravity

  • Low in diabetes insipidus
  • High in diabetes mellitus, acute renal failure and excess medications

Osmolarity

  • Decreases in Diabetes insipidus

pH and Reaction

  • Slightly alkaline in vegetarians and acidic in non-vegetarians
  • pH determines metabolic or respiratory acidosis or alkalosis
  • Decreased in renal diseases

Microscopic examination

Red Blood Cells

  • Presence indicates glomerular disease such as glomerulonephritis

White Blood Cells

  • Normally few cells present, increases in acute glomerulonephritis, infection of urinary tract, vagina or cervix

Epithelial Cells

  • Normally few cells present, increased suggests nephrotic syndrome and tubular necrosis

Casts

  • Appear in glomerulonephritis, pyelonephritis, tubular necrosis

Crystals

  • Abnormal: crystal of cystine and tyrosine appear in liver diseases

Bacteria

  • Few bacteria present, Culture studies necessary to determine type of bacteria

Chemical analysis

Glucose

  • Glycosuria: Appearance of glucose in urine, when blood glucose level increases above 180 mg/dL.
  • First indicator of Diabetes mellitus

Protein

  • Proteinuria (albuminuria) indicates renal diseases, fever and severe exercise

Ketone Bodies

  • Ketonuria occurs in pregnancy, fever, Diabetes mellitus, prolonged starvation and glycogen storage diseases

Bilirubin

  • Bilirubinuria during hepatic and post-hepatic jaundice

Urobilinogen

  • Excess of urobilinogen in urine indicates hemolytic jaundice

Bile salts

  • Bile salts in urine reveals jaundice

Blood

  • Hematuria indicates glomerulonephritis, renal stones, infection or malignancy of urinary tract

Hemoglobin

  • Hemoglobinuria during excess hemolysis

Nitrite

  • Indicates presence of bacteria in urine since some bacteria convert nitrate into nitrite in urine

Examination of Blood

1) Estimation of Plasma Proteins

  • Normal values of plasma proteins:
    • Total proteins: 7.3 g/dL (6.4 to 8.3 g/dL)
    • Serum albumin: 4.7 g/dL
    • Serum globulin: 2.3 g/dL
    • Fibrinogen: 0.3 g/dL

 2) Estimation of Urea, Uric Acid and Creatinine

  • Normal values:
    • Urea: 25 to 40 mg/dL
    • Uric acid: 2.5 mg/dL
    • Creatinine: 0.5 to 1.5 mg/dL

The blood level of these substances increases in renal failure

Examination of Blood and Urine

Plasma Clearance / Renal clearance

  • Amount of plasma that is cleared off a substance in a given unit of time
  • Based on Fick’s principle
  • Determination of clearance value of a substance helps to asses the renal function:
    • Glomerular filtration rate
    • Renal plasma flow
    • Renal blood flow

Formula to calculate clearance value

Measurement of Glomerular Filtration Rate

  • A substance that is completely filtered but neither reabsorbed nor secreted should be used to measure glomerular filtration rate (GFR)
  • Ideal substance is Inulin
  • So inulin clearance indicates GFR
  • Creatinine clearance is also used to measure GFR accurately as it is already present in the body fluids

Measurement of Renal Plasma Flow

  • A substance, which is filtered and secreted but not reabsorbed is used to measure Renal Plasma Flow
  • Paraaminohippuric acid (PAH) clearance indicates plasma flow through the kidney

Measurement of Renal Blood Flow

  • Factors necessary to determine renal blood flow are:
    • Renal plasma flow
    • Percentage of plasma volume in blood (100 - PCV)

Urea Clearance Test

  • Urea is a waste product formed during protein metabolism and excreted in urine
  • Determination of urea clearance forms a specific test to assess renal function

Dialysis and Artificial Kidney

  • Procedure to remove waste and toxic substances and to restore normal volume and composition of body fluid in severe renal failure cases, also called hemodialysis
  • Artificial kidney
    • Machine that is used to carry out dialysis during renal failure (acute and chronic)
  • Principle involved
    • Diffusion of solutes from an area of higher concentration to the area of lower concentration, through a semipermeable membrane

Procedure:

  • Patient’s arterial blood is passed continuously or intermittently through the artificial kidney and then back to the body through the vein
  • In the machine, blood passes through a dialyzer called hemofilter, which contains minute channels interposed between two cellophane membranes.
  • The cellophane membranes are porous in nature
  • The outer surface of these membranes is bathed in the dialyzing fluid called dialysate. The used dialysate in the artificial kidney is constantly replaced by fresh dialysate
  • Almost all the substances, except plasma proteins are exchanged between the blood and dialysate through the cellophane membranes
  • In addition to the dialyzer, the dialysis machine has several blood pumps with pressure monitors, which enable easy flow of blood from the patient to the machine and back to the patient
  • It also has pumps for flow of fresh dialysate and for drainage of used dialysate.
  • Total amount of blood in the dialysis machine at a time: 500 mL
  • Rate of blood flow through the dialysis machine: 200-300 mL/min.
  • Rate of dialysate flow: 500 mL/min.

Frequency and duration of dialysis

  • depends upon the severity of renal dysfunction
  • usually done thrice a week in severe uremia
  • each time, artificial kidney used for 6 hours

Dialysate

  • Concentration  of various substances in the dialysate is adjusted in accordance with the needs of the patient’s body
  • The fluid does not contain urea, urate, sulfate, phosphate or creatinine, so that, these substances move from the blood to the dialysate
  • The fluid has low concentration of sodium, potassium and chloride ions and high concentration of glucose, bicarbonate and calcium ion than in the uremic blood

Peritoneal Dialysis

  • Technique in which peritoneal membrane is used as a semipermeable membrane
  • Used to treat the patients suffering from renal failure

Procedure

  • A catheter is inserted into the peritoneal cavity through anterior abdominal wall and sutured
  • Dialysate is passed through this catheter under gravity
  • Required electrolytes from dialysate pass through vascular peritoneum into blood vessels of abdominal cavity
  • Urea, creatinine, phosphate and other unwanted substances diffuse from blood vessels into dialysate
  • Dialysate is drained from peritoneal cavity by gravity

Peritoneal dialysis vs Artificial Kidney

  • Simple, convenient and less expensive compared to hemodialysis
  • Patients themselves can change the fluid on an outpatient basis
  • Drawbacks
    • Less efficient in removing some of the toxic substances
    • May lead to complications by infections

Uremia

  • Is a condition characterized by excess accumulation of end products of protein metabolism such as urea, nitrogen and creatinine in blood and toxic substances like organic acids and phenols
  • Because of the failure of kidney to excrete the metabolic end products and toxic substances
  • Common features:
    • Anorexia
    • Lethargy
    • Pigmentation of skin
    • Muscular twitching
    • Confusion and mental deterioration
    • Coma

Complications of Dialysis

  • Depend upon the patient’s condition, age, existence of diseases other than renal failure and many other factors
  • Common complications having only renal dysfunction are:
    • Sleep disorders
    • Anxiety
    • Depression