Urine Analysis: Part 2 – Urine formation, and Urine Preservatives

January 6, 2021Lab Tests Urine Analysis

Urine Formation

Urine is formed by the kidneys, passes through the ureters, and is stored in the urinary bladder.
Normal urine specific gravity is around 1.024, and the color is clear.
Urine formation takes place in three steps:
1. Ultrafiltrate formation from the glomeruli from the plasma.
2. Reabsorption of the water and solutes from the ultrafiltrate.
3. Secretion of the extra solutes into the tubular fluid like urine.
Nephron functions are:
1. Ultrafiltration is a protein-free plasma filtrate.
2. Maintain the body fluid volume.
3. Maintain the composition of the electrolytes in balance.
4. Maintain the pH.

Urine tubular reabsorption process:

This is the movement of fluids and substances from the tubular lumen to peritubular capillary plasma.
Tubular Reabsorption:

Mode of transport	Substances	Site of reabsorption
Active transport	Sodium (Na^+₎	Proximal and distal convoluted tubule
	Chloride (Cl^–)	Ascending loop of Henle
	Glucose, salts, and amino acids	Proximal convoluted tubule
Passive transport	Urea	Proximal convoluted tubule and ascending loop of Henle
	Sodium (Na⁺⁾	Ascending loop of Henle
	Water (H2O)	Proximal convoluted tubule, descending loop of Henle, and collecting tubules.

Urine tubular secretion process:

This is the transfer of the substances and fluids from the plasma of peritubular capillaries to the tubular lumen.

Urine tubular and peritubular secretion and reabsorption

Tubular secretion process:

Substances	Site of secretion
Drugs	Proximal convoluted tubules
Organic anions	Proximal convoluted tubule
Organic cations	Proximal convoluted tubule
K⁺	Distal convoluted tubule
NH3⁺	Distal convoluted tubule
Urea	Distal Convoluted tubule
water	Descending loop of Henle, collecting ducts
Hydrogen (H⁺)	Proximal convoluted tubule, Distal convoluted tubule

Chemical substances their excretion and absorption in the renal system:

Chemical substance	After filtration	Reabsorption g/24 hours	Excreted g/24 hours
Sodium	540	537	3.3
Potassium	28	24	3.9
Bicarbonate	300	300	0.3
Chloride	630	625	5.3
Urea	53	28	25
Creatinine	1.4	0.0	1.4
Uric acid	8.5	7.7	0.8
Glucose	140	140	0.0

An average of 115 to 180 L for females and 130 to 200 L of fluids for men filtered through the glomeruli each day.
1. Water 180 L filtered and 178.2 L is reabsorbed, and only 1.5 L is excreted per day.
  Water reabsorption in various part of the kidneys
2. Sodium 25,200 meq/day is filtered, and 25050 is reabsorbed, and only 150 meq is excreted per day.
3. Potassium 720 meq/day is filtered, and 620 is reabsorbed, and only 100 meq is filtered per day.
  Summary of the urine formation
  
  Urine formation various stages in the kidneys

Urine samples Types:

Random Sample This is collected without any precaution and may be collected at any time of the day.
1. This may be collected for the drug abuse analysis.
Morning sample This is the first sample when the patient gets up in the morning.
1. This is the concentrated urine.
2. This is the choice of sample for microscopic examination.
3. This is also the sample choice for the urine constituents to detect protein, HCG, and other substances.
8 hours sample This may be used as alternatives to the first-morning sample because this will also be concentrated in the urine.
24 hours sample This sample is used to quantitate any urine solutes like proteins to avoid that substance’s diurnal variation.
1. Instruct the patient to void the first sample and note the time. After that, collect all the samples until 24 hours are completed.
2. Now void the last sample in the container.
3. 24 hours of samples are used for the substances with diurnal variations like catecholamines, 17-hydroxysteroids, and electrolytes.
Timed Specimen These samples are collected at a predetermined time, like 1,4 or 24 hours.
1. For a collection of times, a sample patient is given special instructions for the diet or the use of drugs.
Clean catch sample, In this case, the patient is instructed to clean the genitalia with water and soap.
1. Then the midstream sample is collected.
2. These are used for the bacteriological culture.
3. Caution should be taken that the container should not touch the genitalia.
Catheter samples These are usually collected from seriously ill patients who are already catheterized.
1. Or can insert the catheter into the urinary bladder to collect the sample.
2. These are used for the microbiological examination in critically ill patients.
Suprapubic sample The sample is directly collected from the urinary bladder with a suprapubic needle inserted into the urinary bladder with complete sterilization.
1. This sample is ideal for the culture, and there is a bypass of the urethra.
2. This will be a good sample of the culture from the urinary bladder.
3. This is usually done from the distended urinary bladder.
Urine from the children To collect the sample from the infants is difficult.
1. There are adhesive bags available to put on the urethra to collect the urine.

Various Urine samples and their indications:

Type of urine sample	Indications
Random urine	For routine examination
First-morning sample	For routine examination For pregnancy test Orthostatic protein test
2 hours postprandial sample	Diabetes mellitus monitoring
3- hours sample	For prostatic infection
24 hours sample	For quantitation of the solutes
Mid-clean-catch sample	For bacterial examination and culture
Midstream sample	Routine examination For bacterial culture
Catheter sample	For microbiological examination
Suprapubic sample	Bladder urine for bacterial culture For cytology
Adhesive bag sample	In children for examination For culture

Urine preservation and type of preservatives:

Role of urine preservatives:

1. These preservative are added to:
  1. Reduce bacterial growth.
  2. Decrease the decomposition of the chemicals.
  3. Keep the substance in solute form.
  4. Decreases the atmospheric oxidation of unstable compounds.
2. Refrigeration is the most useful for the collection, and its utility increases with the preservatives.
3. There are tablets commercially available are:
  1. Potassium acid phosphate.
  2. Sodium benzoate.
  3. Benzoic acid.
  4. Methenamine.
  5. Sodium bicarbonate.
  6. Red mercuric acid.
4. Advantage and disadvantages of preservatives:
  1. These tablets act mainly by lowering the pH of urine and releasing formaldehyde.
  2. Formalin is used as the preservative, but it will precipitate urea and inhibit some chemical reaction when it is in a large amount.
  3. Acidification of the urine below pH 3 is widely recommended. This is widely used to preserve the urine for 24 hours and particularly useful for calcium, steroids, and vanillylmandelic acid determination. HCL 10 mL, 6 mol/L per 24 hours urine is used.
  4. Sulfamic acid is used as 10 g/L to reduce the pH.
  5. Boric acid is used as 5 mg/30 mL, but it causes precipitation of the urates.
  6. Toluene is only an organic preservative and still used; this will not prevent anaerobic microorganisms’ growth.
  7. Sodium carbonate is used 5 g/for 24 hours urine sample to preserve the porphyrins and urobilinogen.

Various urine preservatives are:

Refrigeration: When there is a delay in examining urine, refrigerate at 2 C to 8 C.
1. Urine can be freezed at -24 C to -16 C.
2. If there is a delay of more than 2 hours of urine collection for the examination, it needs preservatives.
3. When urine is refrigerated, then:
  1. 1. This will not interfere with the urine chemical tests.
    2. It prevents bacterial growth.
    3. The disadvantage is there is an increase in the specific gravity.
    4. Precipitate amorphous phosphates and urates.
4. For albumin, aldosterone, amylase, chloride, cortisol, estradiol, FSH, lipase, oxalate, protein, uric acid = Refrigerate.

HCl For Aldosterone, calcium, creatinine, cystine, homovanillic acid, 17-ketosteroids, oxalate, VMA, = Add 6 mL HCl.
Boric acid willPreserves protein.
1. Preserves formed elements.
2. It will not interfere with the routine examination, except the pH.
3. This is bacteriostatic.
4. It interferes with drugs and hormones.
5. The disadvantage is that it may precipitate crystals when used in large amounts.
6. For albumin, cortisol, estrogen, aldosterone, amino acid, chloride, HCG, citrate, cortisol, creatine, DHEA, FSH, glucose, phosphate, 17- Ketosteroids, protein, uric acid, and zinc = Add Boric acid.
Formalin (formaldehyde) is the best for urine sediments.
1. It acts as a reducing agent.
2. It interferes with chemical tests like glucose, esterase, blood, and white blood cells.
3. Rinse the container with formalin to preserve the cells and casts.
Sodium fluoride prevents glycolysis.
1. This is a good preservative for the analysis of the drug.
2. It will interfere with test strips for glucose, white blood cells, and red blood cells.
Phenol Does not interfere with routine tests.
1. It changes the odor.
2. Use one drop of phenol per ounce of urine.
Toluene does not interfere with routine tests.
1. The disadvantages are that it floats over the surface of the urine and may block the pipette.
Thymol is an excellent preservative for glucose and sediments.
1. It interferes with acid precipitation for the protein.
Commercial preservative tablets are easy to use when refrigeration is not available.
1. Check the composition of the tablets to find out any effect on the test.
2. These tablets contain the above preservatives and sodium fluoride.