Urinalysis 1

MLTD 1207 Clinical Chemistry I: Introduction to Urinalysis Notes

Lesson Outcomes

  • describe how the body maintains homeostasis.

  • describe the function of the kidney and the role it plays in urine production.

  • identify urine specimen collection procedures.

  • perform Urinalysis describing the physical examination and chemical properties using test strips on Quality Control and patient samples.

Homeostasis

  • Definition: The maintenance of equilibrium of the internal body functions in response to external changes.

  • Importance: The body attempts to maintain homeostasis, leading to an intricate balance between body water and specific chemical constituents of cells - namely, electrolytes.

Water Balance

  • Significance: Water is the most abundant component of the human body, constituting about 60% of body weight.

  • Regulation:

    • Water intake is primarily regulated by thirst, which is controlled by the thirst center in the hypothalamus.

    • The kidneys contribute to water balance through water loss, which is under the hormonal control of antidiuretic hormone (ADH) from the hypothalamus.

Electrolyte Balance

  • Definition: Electrolytes are substances whose molecules dissociate into ions when placed in solution.

  • Types of Electrolytes:

    • Cations: Positively charged electrolytes (e.g., Na+, K+, Ca++, Mg++).

    • Anions: Negatively charged electrolytes (e.g., Cl-, HCO3-, HPO4--, SO4--, organic acids like lactate, and proteins).

Maintaining Acid-Base Homeostasis

  • Process: Each day acid waste products are produced by normal metabolism; if not disposed of efficiently, they can cause cellular damage.

  • Types of Acids: Carbonic acid, lactic acid, keto acids, among others that must be continually transported in the plasma and excreted.

  • Kidney Role: The kidneys help control pH (or maintain acid-base homeostasis) by conserving bicarbonate ions and removing metabolic acids.

Kidney Anatomy

  • Location: Kidneys are located on each side of the spinal column in the back of the abdominal cavity.

  • Appearance:

    • They are bean-shaped, purplish-brown, and surrounded by fatty tissue.

    • The top of the kidneys is located just under the bottom-most rib, and the lower ends are opposite the 3rd lumbar vertebra.

    • The right kidney is slightly lower than the left due to the liver’s position above it.

Nephrons: The Functional Unit of the Kidney

  • Quantity: Each kidney contains approximately 1-1.5 million microscopic tubular structures called nephrons.

  • Location of Nephrons: Most nephrons are located in the renal cortex.

Urine Production

  • Initiation: Urine production begins in nephrons, specifically the following structures:

    1. Renal corpuscle (Bowman's capsule and glomerulus)

    2. Proximal convoluted tubule

    3. Loop of Henle

    4. Distal convoluted tubule

    5. Collecting duct

Functions of Nephrons

  • Waste Clearance: Controls the selective clearing of waste products.

  • Balances Water and Electrolytes: Maintains essential water and electrolyte balance via:

    • Renal blood flow

    • Glomerular filtration

    • Tubular reabsorption

    • Tubular secretion

Glomerular Function

  • Blood Flow: About 1200-1500 mL of blood flows through the kidneys each minute, approximately ¼ of total cardiac output.

  • Filtration Process: The glomerulus is the first part of the nephron to receive incoming blood and functions to filter this blood, producing a filtrate that is protein-free and cell-free fluid.

Factors Facilitating Filtration

  1. High Pressure: Unusually high pressure in the glomerular capillaries, approximately 3x greater than in other capillaries.

  2. Semipermeable Glomerular Basement Membrane: This structure has a molecular cutoff of approximately 66,000 daltons, which includes the molecular size of albumin.

Glomerular Filtration

  • Filtrate Production: The average adult forms about 180 L of filtrate per day, with normal urine output approximately 0.6 – 2.0 L/day. Most of this filtrate is reabsorbed.

Proximal Convoluted Tubule

  • Function: Returns most valuable substances back to the body (reabsorption).

    • Reabsorbs about 85% of bicarbonate, 60-75% of sodium, chloride, potassium, and water, 100% of glucose (up to renal threshold), and most amino acids.

    • Varying extents of reabsorption also occur for urea, phosphate, magnesium, low molecular weight proteins (albumin), urate, potassium, and calcium.

Proximal Tubule Secretion

  • Waste Removal: A variety of organic acids and bases, hydrogen ions, and ammonia are secreted as waste into the tubular fluid, where hydrogen ions are exchanged for sodium ions throughout the tubule.

Loop of Henle

  • Structure: Forms a hairpin-like loop between the proximal tubule and distal convoluted tubule.

  • Reabsorption Characteristics: Reabsorbs water in the descending loop and sodium and chloride in the ascending loop.

Countercurrent Mechanism in Loop of Henle

  • Concentration Changes:

    • In the descending loop, the urine becomes more concentrated as water passively moves from the tubular fluid into the renal medulla.

    • In the ascending loop, urine becomes more dilute by retaining water and removing Na and Cl.

Distal Convoluted Tubule

  • Function: Makes small adjustments to achieve electrolyte and acid-base homeostasis.

    • Aldosterone from the adrenal cortex stimulates sodium reabsorption.

    • Anti-diuretic hormone (ADH or vasopressin) from the hypothalamus promotes water retention (reabsorption).

Collecting Duct

  • Final Site: The collecting duct is the final site for either concentrating urine or diluting it.

    • The upper portion is under the influence of aldosterone, which stimulates Na reabsorption, and ADH which stimulates water reabsorption.

Overview of Water Balance

  • Reabsorption Rates:

    • About 70% of the water content in tubular fluid is reabsorbed in the proximal tubule.

    • 5% in the loop of Henle, 10% in the distal tubule, and the remainder in the collecting duct.

    • During states of dehydration or water surplus, the renal tubules can reabsorb water at maximum or minimal rates for precise control of fluid balance.

Urine Testing

  • Historical Context: Urine testing has been performed since Egyptian times, with chemical testing for albumin beginning in 1694.

  • Importance: Urinalysis serves as a fluid “biopsy” of the kidney and urinary system; it is easy to collect and most tests are inexpensive.

Urine Composition

  • Water Content: 95% of urine is water.

  • Solutes: 5% solutes include urea, creatinine, uric acid, chloride, sodium, potassium, hormones, vitamins, and medications. The composition is influenced by dietary intake.

Urine Volume

  • Factors: Depends on fluid intake, non-renal loss, ADH variations, and excretion of dissolved solids.

  • Definitions:

    • Polyuria: Increased urine excretion (> 2500 mL/24 hrs).

    • Oliguria: Decreased urine excretion (< 400 mL/24 hrs).

    • Anuria: No urine output due to renal failure.

Causes of Polyuria

  • Conditions causing polyuria include diabetes mellitus, diabetes insipidus, ingestion of alcohol and caffeine, motion sickness drugs, diuretics, and chronic nephritis (kidney unable to concentrate urine).

  • Polyuria is often associated with polydipsia.

Causes of Oliguria

  • Possible causes include dehydration, voluntary fluid intake restriction, increased salt ingestion, excessive sweating (fever), severe vomiting, severe diarrhea, mechanical obstruction to urine flow (e.g., stones), cardiac insufficiency, acute nephritis, end-stage renal disease, severe burns.

Causes of Anuria

  • Possible causes include acute renal failure (due to shock, poisons), acute glomerulonephritis, urinary tract obstruction, and major hemolytic transfusion reaction resulting in renal shutdown.

Nocturia

  • Definition: Normally, kidneys secrete 2-3 times more urine during day than at night; nocturia refers to an increase in nocturnal urine excretion.

Specimen Collection for Urine Procedures

  • Biohazardous Samples: Collection of biological samples must follow safety regulations.

  • Containers: Use wide-mouthed, screw-top, sterile containers for cultures.

  • Labels and Requisitions: Specimens must be labeled correctly and requisition forms attached. Samples should be delivered to the laboratory and tested within 2 hours. If not, specimens should be refrigerated (up to 24 hours).

Specimen Identification

  • Quality Document: Every lab follows a quality document with specimen acceptance criteria, including labeling requirements:

    • Patient's name, Health Care Number (HCN), date of birth (DOB), date and time of collection.

    • Labels must be attached to the specimen container, not the lid.

Specimen Rejection

  • Policy: Each hospital has a specimen acceptance and rejection policy. Specimens may be rejected for:

    • Lack of proper labeling, nonmatching specimens and requisitions, contamination, leaking specimens, insufficient quantity, improper transportation.

  • Communication: In case of rejection, the physician or floor technician must be contacted. For out-patient specimens, the office will ensure the patient is contacted directly.

Physical Examination of Urine

  • Assess the following parameters:

    • Color

    • Clarity

    • Odor

Urine Color

  • Range: Can vary from near-colorless to black. Normal variations include:

    • Pale yellow, yellow, dark yellow, amber.

    • Color intensity correlates with urine concentration.

  • Urochrome: The yellow color is a pigment produced endogenously and increases with time at room temperature.

Abnormal Urine Colors

  • Dark Yellow/Amber/Orange: Caused by bilirubin (detected chemically), phenazopyridine (Pyridium) leading to thick orange, which interferes with color reactions.

  • Red/Pink/Brown:

    • Uroerythrin can cause a pink color in refrigerated urines.

    • Other causes include blood, beets, and certain pH levels indicating specific conditions.

  • Blue/Green: Associated with bacterial infections (such as Pseudomonas sp.) or medications (e.g., methylene blue).

  • Brown/Black: Caused by melanin (from malignant melanoma) or homogentisic acid and some medications (e.g., Flagyl).

Urine Clarity

  • Normal Clarity: Freshly voided urine is typically clear.

  • Possible Causes of Haze: Squamous epithelial cells, mucus, and other substances can result in normal hazy urine. Storage can lead to turbidity from amorphous phosphates and urates.

  • Pathological Causes: Red blood cells (RBCs), white blood cells (WBCs), bacteria, yeast, abnormal crystals, lipids, and abnormal epithelial cells.

Odor of Urine

  • Normal Odor: Mild and inoffensive.

  • Indicators of Conditions: Ammonia-like odor suggests urea-splitting bacteria, fruity/sweet odor indicates ketones, and strong, unpleasant smells may suggest bacterial infections.

  • Metabolic Disorders: Conditions like maple sugar disease create strong odors, as do certain foods (e.g., garlic, asparagus).

Urinalysis Overview

  • Significance: The most frequently performed urine test, urinalysis may reveal the earliest detectable signs of disease.

Specimen for Urinalysis

  • Optimal Collection: The first-morning voided urine is most desirable as it is concentrated and provides the most accurate picture of the body's physiology. Patients should be instructed to collect on waking and bring the sample to the lab within 2 hours.

Clean Catch Urine Collection

  • Method: Midstream collection helps avoid contamination from the distal urethra. The patient discards an initial volume, collects the next portion of urine, and discards the last portion.

Specimen Collection and Handling

  • Container Preparation: Collect specimens in clean, well-rinsed, disposable containers.

  • Chemical Analysis Preparation: Ensure specimens are well-mixed, at room temperature, and tested as soon as possible. Refrigeration is necessary if testing cannot occur within 2 hours of collection.

Urinalysis Testing: Reagent Test Strips

  • Structure: Consists of reagent-impregnated strips with mesh-laminated absorbent pads and plastic backing; multi-parameter strips are available.

Chemical Examination of Urine

  • Parameters tested may include pH, specific gravity, protein, glucose, ketones, blood, nitrite, leukocyte esterase, bilirubin, and urobilinogen.

Visual Observation of Test Strips

  • Strips can be read visually with no additional lab equipment needed for testing. They are semi-quantitative.

Major Reagent Strip Brands

  1. Multistix (Bayer)

  2. Chemstrip (Roche)

Mechanism of Reagent Strips

  • Functionality: A color-producing chemical reaction occurs when the absorbent pad comes into contact with urine.

  • Reading Time Variability: Timing for reading strips varies by test and manufacturer, from immediate reactions for pH to up to 120 seconds for leukocyte esterase testing.

  • Temperature Sensitivity: Specimens that have been refrigerated must be warmed to room temperature for proper enzymatic reactions on test strips.

Testing Procedure

  1. Collect fresh urine specimen in a clean, dry container; mix well without centrifuging.

  2. Remove one strip at a time and replace the cap. Briefly immerse all reagent areas in the specimen and remove the strip immediately.

  3. Run the edge of the strip against the rim of the urine container to remove excess urine. Hold the strip horizontally.

  4. For visual reading, compare test areas to the corresponding color chart on the bottle label at specified times and record results.

Reference Ranges

  • Substances Normally Not Detected in Urine: Glucose, ketones, blood, protein, nitrite, leukocyte esterase.

  • Specific Gravity Range: 1.005 to 1.035.

  • pH Range: First-morning specimen's pH is typically between 5.0 and 6.0; it may range from 4.5 to 8.0.

Delays in Urinalysis Testing: Stability of Constituents

  • Constituents may undergo changes leading to misleading results if urine stands too long:

    • Glucose levels decrease.

    • Bilirubin and urobilinogen oxidize (must use fresh specimens).

    • RBCs and WBCs lyse in alkaline urine.

    • Casts disintegrate.

    • pH increases, and acetoacetic acid converts to acetone.

    • Bacteria proliferate, leading to increased nitrite levels.

Other Sources of Error

  • Errors can arise from interfering substances, technical carelessness, incorrect timing, and using expired strips.

  • Factors such as color blindness can also impact interpretation.

Care of Reagent Strips

  1. Store in a cool (< 30°C), dry place.

  2. Use opaque container to protect from light.

  3. Protect from moisture (use desiccant) and excessive heat.

  4. Cap should remain tightly on the container except when removing strips for testing.

  5. Avoid touching impregnated areas with fingers.

Quality Control in Testing

  • Importance of Controls: Perform positive and negative controls daily or at the beginning of each shift; when opening a new bottle of reagent strips; and if results are questionable.

References

  • Textbook Reference: Urinalysis and Body Fluids, 6th Edition. Susan K. Strasinger, Marjorie S. DiLorenzo. F.A. Davis Company, 2008.

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