Urine Chemistry Flashcards

chemical examination of the urine

What is the correct dipping procedure for applying urine to a chemical reagent strip?

Dip the reagent strip briefly into a well-mixed urine specimen at room temperature.

How should excess urine be removed from a reagent strip after dipping?

Touch the edge of the strip to the container rim as it is withdrawn, then blot the edge on an absorbent pad.

In what physical orientation should a reagent strip be held while comparing pad colors to the color chart?

Horizontally (to prevent reagent runover between adjacent pads).

Why does dipping an unmixed urine specimen lead to inaccurate leukocyte and blood results?

Red blood cells (RBCs) and white blood cells (WBCs) sink to the bottom of unmixed urine, leaving the upper sample depleted of cellular elements.

Why must un-refrigerated or room-temperature urine specimens be used for reagent strip testing?

Enzymatic reactions on the strip pads are temperature-dependent and calibrated for room temperature.

What error occurs when a reagent strip is left submerged in urine for too long?

Reagents leach off the absorbent pads into the urine.

What technical error causes cross-contamination of reagents between neighboring pads on a urine strip?

Failing to blot excess urine from the strip edge, leading to reagent runover.

Which reagent strip test pad requires the longest incubation time before reading?

Leukocyte Esterase (LE), which requires exactly 2 minutes (120 seconds).

Why must urine reagent strips be stored in an opaque container with a desiccant?

Reagent pads are highly hygroscopic and degrade rapidly when exposed to ambient moisture and light.

What are the temperature storage requirements for unopened and opened reagent strip bottles?

Store below 30 °C; do not freeze.

How frequently must quality control (QC) testing using positive and negative controls be performed on urine reagent strips?

At least once every 24 hours, or on each shift, as well as whenever a new bottle is opened or results are questionable.

Why is distilled water unacceptable as a negative quality control for urine reagent strips?

Reagent strip chemical reactions are designed specifically for the ionic concentration and ionic strength of human urine.

What is the acceptable variance for positive quality control readings when evaluating reagent strips against manufacturer values?

Control results must agree with published target values within $\pm 1$ color block.

What indicators make up the double-indicator system used for measuring urine pH on reagent strips?

Methyl red (pH 4–6: red/orange to yellow) and Bromthymol blue (pH 6–9: green to blue).

What is the physiological range of pH in normal fresh urine?

4.5 to 8.0 (first morning urine is typically acidic at 5.0–6.0).

A fresh urine specimen yields a pH reading of 9.0. What does this result signify?

An improperly stored, old, or bacteria-contaminated specimen (normal fresh urine cannot reach pH 9.0).

What dietary factors cause urine to become more acidic versus more alkaline?

Meat diets and cranberry juice produce acidic urine; vegetable/citrus diets produce alkaline urine.

What technical reagent strip interference can cause an erroneously low (acidic) pH reading on an alkaline urine sample?

Runover of acid buffer from the adjacent protein pad.

Which protein analyte is the primary target detected on standard reagent strip protein pads?

Albumin.

What chemical principle underlies the protein test pad on urine reagent strips?

Protein-error-of-indicators (at a constant buffered pH of 3.0, indicator color changes proportional to protein concentration).

What active chromogenic indicator chemicals are used on reagent strip protein pads?

Tetrabromophenol blue or tetrachlorophenol tetrabromsulfonphthalein (in an acid buffer).

What cut-off level defines clinical proteinuria in a 24-hour urine collection or random strip estimate?

$30\text{ mg/dL}$ (or $300\text{ mg/24 hr}$), compared to normal values of $<10\text{ mg/dL}$.

Which condition is a major source of false-positive protein readings on reagent strips?

Highly buffered alkaline urine, which overrides the acid buffer system of the pad.

What type of urine protein is missed (false negative) by standard reagent strip protein pads?

Non-albumin proteins (e.g., Bence Jones immunoglobulins, Tamm-Horsfall mucoprotein, microglobulins).

What confirmatory test precipitates all urine proteins without requiring heat?

Sulfosalicylic Acid (SSA) precipitation test.

Substances such as radiographic contrast dye, tolbutamide, and certain antibiotics cause false-positive results in which protein assay?

The Sulfosalicylic Acid (SSA) precipitation test.

What is the primary clinical utility of microalbumin testing in urine?

Early detection of diabetic nephropathy and evaluation of cardiovascular disease risk.

What chemical principle is utilized by the creatinine pad on microalbumin combination reagent strips?

Pseudoperoxidase activity of copper-creatinine complexes (copper sulfate, TMB, and DBDH).

What range of albumin-to-creatinine ratio (A:C ratio) signifies microalbuminuria?

30 to $300\text{ mg/g}$ ($3.4\text{ to }33.9\text{ mg/mol}$).

What is the enzymatic reaction sequence utilized on reagent strip glucose pads?

1. $\text{Glucose} + O_2 \xrightarrow{\text{Glucose Oxidase}} \text{Gluconic Acid} + H_2O_2$ 2. $H_2O_2 + \text{Chromogen} \xrightarrow{\text{Peroxidase}} \text{Oxidized Chromogen} + H_2O$

Why is the glucose reagent strip specific ONLY for glucose?

The enzyme glucose oxidase reacts exclusively with $\beta$-D-glucose and does not catalyze other sugars.

What chemical method is used by Clinitest tablets to measure reducing sugars?

Benedict's copper reduction test (cupric sulfate reduced by sugars to cuprous oxide in an alkaline environment).

In pediatrics, why is a Clinitest performed on urine from infants even if the glucose reagent strip is negative?

To screen for inborn errors of metabolism involving non-glucose reducing sugars, specifically galactose (galactosemia).

What phenomenon occurs during a Clinitest procedure when extremely high levels of reducing sugars cause the color to cycle from bright orange back to greenish-brown?

The "pass-through" phenomenon (remedied by using the 2-drop Clinitest method).

A urine test yields a negative glucose strip result and a positive Clinitest. What is the clinical interpretation?

A non-glucose reducing substance (e.g., galactose, fructose, lactose, maltose) or an interfering reducing compound is present.

What three ketone bodies can be produced during accelerated fat metabolism?

Acetone (2%), Acetoacetic acid (20%), and Beta-hydroxybutyric acid (78%).

Which ketone body is primarily detected by reagent strips and Acetest tablets?

Acetoacetic acid (via reaction with sodium nitroprusside).

What active chemical agent reacts with acetoacetic acid on ketone test pads to yield a purple color?

Sodium nitroprusside (nitroferricyanide).

What additive ingredient in Acetest tablets allows the detection of acetone in addition to acetoacetic acid?

Glycine.

What chemical principle powers the blood reagent strip pad for detecting RBCs, hemoglobin, and myoglobin?

The pseudoperoxidase activity of hemoglobin catalyzing oxidation of tetramethylbenzidine by hydrogen peroxide ($H_2O_2$).

How does the appearance of the blood test pad differ between intact RBCs and free hemoglobin/myoglobin?

Intact RBCs lyse on contact with the pad to produce a speckled pattern; free hemoglobin or myoglobin produces a uniform green color.

How do hematuria, hemoglobinuria, and myoglobinuria differ regarding urine clarity and patient plasma color?

• Hematuria: cloudy red urine, clear plasma • Hemoglobinuria: clear red urine, red/pink plasma • Myoglobinuria: clear red-brown urine, clear plasma (with elevated serum CK)

Which test differentiates hemoglobinuria from myoglobinuria by precipitating hemoglobin out of solution?

The Ammonium Sulfate precipitation test (hemoglobin precipitates out leaving a clear supernatant; myoglobin remains in red supernatant).

How does high ascorbic acid ($>25\text{ mg/dL}$) interfere with blood and glucose reagent strip tests?

Ascorbic acid acts as a strong reducing agent that scavenges peroxide intermediates, producing false-negative results.

Why does only conjugated bilirubin appear in urine during clinical disease states?

Conjugated bilirubin is water-soluble; unconjugated bilirubin is water-insoluble and bound to albumin, preventing renal filtration.

What chemical reaction is utilized by reagent strips to measure urine bilirubin?

A diazo reaction (bilirubin glucuronide + diazonium salt in acid medium $\rightarrow$ azodye).

What primary confirmatory test is used to verify urine bilirubin results?

The Ictotest tablet test (more sensitive at $0.05\text{--}0.10\text{ mg/dL}$).

What color reaction on the special mat indicates a positive result in an Ictotest?

Blue to purple color on the absorbent mat around the tablet within 60 seconds (ignore pink/red).

Why does unpreserved urine exposed to light yield false-negative bilirubin results?

Bilirubin undergoes photo-oxidation to biliverdin, which does not react with diazonium salts.

How do urine bilirubin and urobilinogen levels differ between bile duct obstruction and hemolytic disease?

• Bile duct obstruction: Urine Bilirubin = Positive ($+++$); Urobilinogen = Normal • Hemolytic disease: Urine Bilirubin = Negative; Urobilinogen = Elevated ($+++$)

What active chemical reagent is used on Multistix strips for detecting urobilinogen?

Ehrlich's reagent ($p$-dimethylaminobenzaldehyde).

Why can a standard reagent strip never report a true "negative" urobilinogen result?

Normal urine naturally contains small amounts of urobilinogen ($<1\text{ mg/dL}$ or $0.2\text{--}1.0\text{ EU}$), which is the baseline sensitivity lower limit of the strip.

What chemical reaction is used on nitrite reagent strip pads to screen for bacterial urinary tract infections?

The Greiss reaction (nitrite reacts with an aromatic amine/sulfanilamide to form a diazonium salt, which couples with tetrahydrobenzoquinoline to form a pink azodye).

What bacterial concentration threshold is required to produce a positive nitrite reagent strip reaction?

At least 100,000 ($10^5$) organisms/mL.

Why can a patient with a bacterial UTI have a false-negative urine nitrite test?

Non-nitrate reducing bacteria (e.g., Gram-positive organisms/yeasts), insufficient bladder incubation time ($<4\text{ hours}$), or lack of dietary nitrate.

How should spotting or pink edges on a nitrite test pad be interpreted?

As a negative result.

What enzyme detected by the leukocyte esterase (LE) pad indicates the presence of granulocytic white blood cells?

Leukocyte esterase (present in azurophilic granules of neutrophils, eosinophils, basophils, and monocytes).

What chemical principle powers the Leukocyte Esterase (LE) test pad?

Leukocyte esterase hydrolyzes indoxylcarbonic acid ester to indoxyl, which reacts with a diazonium salt to form a purple azodye.

Why is the LE reagent strip capable of detecting white blood cells that have already lysed in dilute or alkaline urine?

The test measures esterase enzymes released into the urine, not intact cellular membranes.

What chemical principle is used by reagent strips to estimate urine specific gravity (SG)?

Change in $pK_a$ (dissociation constant) of a polyelectrolyte relative to ionic concentration, releasing $H^+$ ions to lower pH.

Which pH indicator is present on the specific gravity test pad to track $H^+$ ion release?

Bromthymol blue (changes from blue/alkaline through green to yellow/acidic).

What adjustment must be made to reagent strip specific gravity readings if the urine pH is 6.5 or higher?

Add 0.005 to the specific gravity reading (alkaline urine neutralizes $H^+$ ions, causing a false low reading).

Why does high concentration of glucose or radiographic contrast media NOT interfere with reagent strip specific gravity?

Reagent strips measure ionic concentration only, whereas refractometers measure total dissolved solutes (including non-ionized glucose and dye molecules).