Intro to Urine

Why is urine a useful diagnostic specimen?

It is readily available and easy to collect. It can be used for evaluation of diseases, screening, aiding in diagnosis, monitoring disease progression, and monitoring therapy.

How does the urine help us evaluate diseases?

It acts as a liquid biopsy of the kidney and metabolism

What is the historic significance of urinalysis?

Urinalysis has been around for a long time. There are reports of urine examination that go back as far as the ancient Egyptians and the Middle Ages. Some of their concepts are still in use today but technology has helped the process evolve.

What is the composition of urine?

Approximately 95% water; urea; creatinine; uric acid; electrolytes (Na+, K+, Cl-); phosphate, sulfate, ammonium, and hydrogen ions; hormones, vitamins, and medications; and formed elements (cells, crystals, casts).

What is the normal urine volume per day?

600–2,000 mL/day. The average is 1,200–1,500 mL/day.

What factors influence urine volume?

Fluid intake, non-renal fluid loss (vomiting, diarrhea), ADH secretion, and the need to excrete increased quantities of dissolved solids (e.g., glucose, salts).

What is oliguria?

Decreased urine volume — less than 400 mL/day. Caused by dehydration (vomiting, diarrhea, sweating, severe burns).

What is anuria?

No urine flow — less than 100 mL/day. Caused by severe kidney damage or decreased blood flow to the kidneys.

What is polyuria?

Increased urine volume — greater than 2.5 L/day. Causes include diabetes mellitus and/or diabetes insipidus, or artificially by diuretics, caffeine, and alcohol (which suppress ADH secretion).

What is nocturia?

Increased urine volume at night.

What are the types of urine collection methods?

Random; clean catch midstream (CCMS); catheterized; pediatric collections; suprapubic aspiration; and drug testing specimen (chain of custody usually required).

What is a clean catch midstream (CCMS) specimen?

A collection method where the patient cleans the urethral area and collects the middle portion of the urine stream, reducing contamination from the skin. Used when a more accurate specimen is needed (e.g., culture).

When is chain of custody required for urine collection?

For drug testing specimens.

What are the different timing types for urine collection?

Random (most common); first morning (best for screening); fasting (e.g., for glucose tolerance testing); and timed collections (24-hour, 12-hour, 2-hour) for substances whose concentration varies (24) or is stable. (12 and 2)

Which urine specimen type is best for screening?

The first morning specimen, because it is the most concentrated.

When is a timed (24-hour) urine collection used?

When the concentration of a substance varies throughout the day and an overall daily amount is needed for accurate measurement.

What are the requirements for a urine specimen container?

Clean, dry, and leak-proof. Must be sterile if the specimen is collected for culture.

What information must be on a urine specimen label?

Patient name, identifier, date, and time of collection.

How quickly should a urine specimen be transported and tested?

Within 2 hours of collection. Prompt delivery is important.

What are the methods of urine specimen preservation?

Refrigeration (but specimen must return to room temperature before analysis) and chemical preservatives, which ideally are bactericidal, inhibit urease, preserve formed elements, and do not interfere with chemical tests.

Examples include formalin and boric acid. There is no "one-size-fits-all" preservative.

When should refrigeration be used?

Should be the first action if the sample can’t be tested right away.

What three categories of changes does preservation prevent?

Physical changes (color, clarity, odor); chemical changes (increases or decreases in analytes); and microscopic changes (disintegration of formed elements).

What analytes/components INCREASE if a urine specimen is not preserved within 2 hours?

pH (urea converts to ammonia (NH3) via bacteria); nitrite (nitrate reduced by bacteria); bacteria (multiply); turbidity (bacterial growth, amorphous material); odor (bacterial growth and urea breakdown); and color.

What analytes/components DECREASE if a urine specimen is not preserved within 2 hours?

Glucose (glycolysis and bacterial use since bacteria eat glucose); ketones (volatilization and bacterial metabolism); bilirubin (photosensitive from light exposure); urobilinogen (oxidizes to urobilin); and formed elements (RBCs and casts, which disintegrate).

What are the major structural components of the renal system?

Kidneys (composed of cortex, medulla, and pelvis), ureters, bladder, and urethra. The medulla has the pyramids and calyces

What are the functions of the kidneys?

Filtering waste; pH regulation (basic homeostasis); water and electrolyte balance; regulating blood pressure and blood volume; endocrine functions; and vitamin D metabolism (key important for regulating phosphates).

What is the nephron?

The functional unit of the kidney. Located between the renal cortex and medulla. Each kidney has approximately 1–1.5 million nephrons. Blood must pass through the nephron for filtration (secretion and absorption) to occur.

What is the pathway of blood to filtration in the kidney?

Abdominal aorta (the pipeline in the center of you) → renal artery (blood enters kidney here) → afferent arterioles → glomerulus (dumps the big stuff out here) → efferent arterioles (smaller diameter than afferent) → peritubular capillaries → venules → renal vein → inferior vena cava (then goes back into the body)

Why are efferent arterioles important to filtration?

They have a smaller diameter than afferent arterioles, which increases pressure in the glomerulus and drives ultrafiltration.

What is the pathway of filtrate (nothing big going through filtrate) through the nephron?

Glomerulus (glomerular filtrate) → proximal convoluted tubule (PCT) → loop of Henle → distal convoluted tubule (DCT) → collecting ducts → out to the ureters.

What is the function of the glomerulus?

Ultrafiltration — it is a non-selective filter (big stuff goes back into circulation or stays in the glomerulus) for all particles up to approximately 67,000 MW (Think big cells). It occurs because of pressure, and it is extremely permeable to water and low-molecular-weight particles. Almost everything passes through except plasma proteins (albumins), protein-bound substances, and cells (blood cells).

How much filtrate does the glomerulus produce per minute?

Approximately 120 mL/min.

What is the specific gravity of filtrate leaving the glomerulus?

Approximately 1.010.

What is a tubular function?

what happens after the filtrate leaves the glomerulus

What are the three main tubular functions after filtrate leaves the glomerulus?

Reabsorption (water and solutes return to the bloodstream), transport, and secretion (materials pass from blood in the peritubular capillaries into the tubular filtrate for excretion, e.g., H+ ions, waste, drugs).

What are the two types of transport in tubular function?

Active and Passive

What is active transport in tubular reabsorption?

Materials to be reabsorbed must combine with a carrier protein in the renal tubular epithelium (RTE) membrane, which provides the energy to transport them back across the membrane.

What is passive transport in tubular reabsorption?

Movement of materials across the membrane due to differences in concentration or electrical potential — no energy or carrier protein required.

What is vasopressin (ADH) and what does it do?

Vasopressin, also known as anti-diuretic hormone (ADH), is secreted by the pituitary gland. It controls the reabsorption of water by controlling the permeability of the collecting ducts.

What is permeability

Whether something holds water or not

What is aldosterone and what does it do?

A hormone produced in the adrenal cortex. Its production is stimulated by the renin-angiotensin-aldosterone system (RAAS) and by increases in plasma K+. It responds to changes in blood pressure and regulates reabsorption of Na+ in the distal convoluted tubule (DCT).

What is renin and what stimulates its release?

Is an enzyme produced and stored by juxtaglomerular cells. It is released in response to either decreased blood volume, decreased arterial pressure, decreased sodium, or increased potassium. Its secretion results in the formation of angiotensin and the secretion of aldosterone which helps hold onto water in tubules.

What is reabsorbed or secreted in the Glomerulus

nothing is

What is reabsorbed and secreted at the proximal convoluted tubule (PCT)?

Reabsorbed: sodium, chloride, amino acids, bicarbonate, glucose, uric acid, phosphate. Secreted: hydrogen ions.

What is reabsorbed or secreted at the loop of Henle?

Reabsorbed: Water (descending limb); sodium and chloride (ascending limb).

Secreted: nothing

What is reabsorbed and secreted at the distal convoluted tubule (DCT)?

Reabsorbed: sodium (requires aldosterone) and water.

Secreted: potassium, ammonia, and hydrogen ions.

What is reabsorbed and secreted at the collecting ducts?

Reabsorbed: sodium and water (water reabsorption requires ADH).

Secreted: hydrogen ions.

What tests are used to assess renal function?

Routine urinalysis; urine volume monitoring; clearance testing for glomerular filtration rate (GFR); estimated GFR (eGFR); and tubular function tests (specific gravity and osmolality) for early renal disease.

What does clearance testing measure?

The filtering capacity of the glomerulus — how many nephrons are functioning and how well they are filtering. It measures how many mL of plasma can be cleared of a substance per minute (mL/min).

What is the formula for clearance?

Clearance = Urine (mg/dL) × Volume (mL/min) ÷ Plasma (mg/dL). May need to correct for body size by multiplying by 1.73/BSA (body surface area).

What are the clinical uses of clearance testing?

To determine the extent of nephron damage; monitor known cases of renal disease; evaluate effectiveness of treatment; prevent further damage; and determine the safety of medications.

What is EGFR and what are its advantages?

Estimated GFR — allows an estimated GFR calculation without collecting a 24-hour urine. It uses serum creatinine, age, gender, and weight to estimate GFR. It is good for mass screening and monitoring patients, easily programmed into instruments, and reported as >60 mL/min or as a numerical value if below 60.

What is the ideal characteristic of a substance used for clearance testing?

It should be neither secreted nor reabsorbed by the tubules, so that only glomerular filtration is measured.

What substances are used in clearance testing?

Creatinine; radioisotopes; cystatin C; beta-2 microglobulin (may be an issue if another malignancy is present); and inulin (no longer used).

Creatinine Clearance

What is creatinine and why is it used in clearance testing?

A waste product of muscle metabolism, excreted at a relatively constant level. It is freely filtered by the glomeruli and not reabsorbed or metabolized by the renal tubules.

What is the advantage to creatine clearance?

It is easily tested using reliable methodology.

What are the disadvantages of creatinine clearance?

Some creatinine is secreted by the renal tubules (slightly overestimates GFR); bacteria can break it down; a diet heavy in meat may influence results; it is unreliable for patients with muscle wasting diseases or athletes; and some medications may inhibit tubular secretion.

What is cystatin C and why is it useful as a GFR marker?

A small protein (MW 13,250 Daltons) produced at a constant rate by all nucleated cells. It is freely filtered by the glomerulus, then reabsorbed and broken down (not returned to the bloodstream) with no secretion. Blood levels are not greatly influenced by age, gender, body size, or muscle mass, making it a good alternative marker of GFR.

How does cystatin C differ from creatinine as a GFR marker?

Unlike creatinine, cystatin C blood levels are not significantly affected by age, gender, body size, or muscle mass. It is not secreted by tubules and is completely reabsorbed and broken down (not returned to bloodstream), making it a more accurate and unbiased GFR marker.

What is radioisotope clearance testing and when is it used?

Performed by nuclear medicine. Radioisotopes are injected and measured in the body via their disappearance from plasma. It enables visualization of filtration in one or both kidneys individually, making it especially useful for transplant evaluation. It is a good method but is inconvenient and c