Urine Concentration

Question 1

Normally, urine consists of_____% water and ____% solutes. The concentration varies depending on diet, physical activity, and health status.

Answer 1

94

6

Question 2

refers to the amount of solutes present in the volume of water excreted.

Answer 2

Urine concentration

Question 3

Normally, urine consists of_____ water and _____ solutes.

Answer 3

94% water and 6% solutes

Question 4

The concentration varies depending on (3)

Answer 4

diet
physical activity
health status

Question 5

• ______: Fewer solute particles per unit volume of water → Lighter color or even colorless

• ______: More solute particles per unit volume of water → Darker yellow

Answer 5

Dilute urine

Concentrated urine

Question 6

provides a crude indicator of urine concentration.

Answer 6

color

Question 7

In a routine urinalysis test, urine concentration is easily and quickly determined using specific gravity (SG) measurement.

The two most commonly used methods are (2)

Note that when a more accurate assessment of urine concentration is needed, the______ is determined.

Answer 7

refractometry and reagent strips.

urine osmolality

Question 8

Specific gravity is an expression of urine concentration in terms of______ (i.e., the mass of solutes present per volume of solution).

Answer 8

density

Question 9

SG

It is a ratio of _____ to _____under specific conditions.

As a density measurement, the number of solutes in the urine, as well as their molecular size, affects SG.

Answer 9

urine density to the density of an equal volume of pure water

Question 10

The greater the urine density, the larger the SG value.

It is physiologically impossible for the body to excrete_____, and the lowest urine SG obtainable is approximately______.

Answer 10

pure water
(1.000)

1.002

Question 11

The maximum SG that urine can attain is a value equal to that of the hyperosmotic renal medulla, which is approximately______.

Answer 11

1.040

Question 12

In clinical laboratories, indirect measurements of urine SG such as (2) method are used.

It is important to be aware of the differences between these two indirect SG methods.

Answer 12

refractometry and the reagent strip chemical

Question 13

Uncharged large-molecular-weight solutes are not detected by the______, whereas these solutes are measured when using_____.

Answer 13

reagent strip method; refractometry

Question 14

Renal concentrating ability predominantly involves the exchange of countless numbers of ________and not the exchange of________

Answer 14

low-molecular-weight ionic solutes (i.e., electrolytes)

uncharged large-molecular-weight solutes (e.g., glucose, protein)

Question 15

After the intravenous administration of a large-molecular-weight solute such as (2), a urine specimen should not be obtained until after a suitable time (_____) has passed to allow for complete elimination of the infused agent.

Answer 15

radiographic media or mannitol

~8 hours

Question 16

______, an indirect measure of SG, is based on the refractive index of light.

When light passes from air into a solution at an angle, the direction of the light beam is refracted and its speed is decreased.

Answer 16

Refractometry

Question 17

___________is called the refractive index.

Answer 17

The ratio of light refraction in the two differing media

Question 18

______ is the refractive index of air, which by convention equals 1.0;

______ is the refractive index of the solution being measured;

______is the velocity of light in air;

______ is the velocity of light in the solution;

_______ is the angle of the incident beam of light; and

_______is the angle of the refracted beam of light. Although the velocity or the angles of refraction can be used to determine the refractive index, measurement of angles is the principle routinely used by refractometers.

Answer 18

n1
n2

V1
V2

sin 01
sin 02

Question 19

refractive index (n)

Question 20

Three factors affect the refractive index of a solution:

Answer 20

(1) the wavelength of light used
(2) the temperature of the solution,(3) the concentration of the solution.

Question 21

Methods of Measuring Specific Gravity

1. _______(Measures all solutes, including proteins and glucose)
2. _______(Measures only ionic solutes, ignoring large molecules like glucose and protein)

Answer 21

Refractometry

Reagent strip method

Question 22

How a Refractometer Works
•______ is passed through a prism inside the refractometer.
• The prism isolates a single wavelength of light (commonly_____).
• Light passes through the urine sample.
• The angle of refraction is measured and converted into an SG value.
• The refractometer automatically compensates for temperature variations (_____).
• Results are displayed on a calibrated scale.

Answer 22

White light

589 nm

15°C – 38°C

Question 23

Daily calibration is done using solutions of known SG values.
• Calibration solutions used:

SG Value Solution
_________ Distilled water
_________ 0.513 mol/L (3% w/v) NaCl
_________ 0.856 mol/L (5% w/v) NaCl
_________ 0.263 mol/L (9% w/v) Sucrose

Answer 23

1.000

1.015

1.022

1.034

Question 24

Advantages of Refractometry

Answer 24

1. Requires a small urine sample (1–2 drops)
2. Automatically adjusts for temperature
3. Measures all solutes present, including glucose and proteins
4. Used in both manual and automated urine analyzers

Question 25

Presence of high-molecular-weight solutes (glucose, protein, radiographic contrast media, mannitol) increases SG readings. • Correction factors: • Protein: SG increases by______ • Glucose: SG increases by______

Answer 25

0.003 per g/dL 0.002 per g/dL

Question 26

Extremely high SG (_______) is physiologically impossible and suggests radiographic contrast media or mannitol infusion. • Recommendation: Delay urine collection for ______administration of these agents.

Answer 26

>1.050 8 hours after

Question 27

The _______is an ***indirect colorimetric estimation of urine density*** based on the ***quantity of ionic or charged solutes (Nat, Cl, K+, NH+) present.***

Answer 27

reagent strip SG method

Question 28

Note that non-ionic solutes are not measured. Hence this method determines the ionic specific gravity (SGionic.)

Answer 28

Reagent strip method

Question 29

In health, the ability of the kidneys to selectively reabsorb and secrete ionic solutes and water determines the ______of the urine excreted. In contrast, excretion of nonionic solutes such as (4)does not reflect the status of this renal function. Glucose in urine usually indicates a metabolic disorder (diabetes mellitus), whereas protein often indicates a renal condition such as a change in the glomerular filtration barrier (glomerulonephri-tis, nephrotic syndrome).

Answer 29

concentration (density) urea, glucose, protein, or radiographic media

Question 30

No other SG method is able to eliminate the effects of nonionic large-molecular-weight solutes on SG results. Therefore reagent strip SG results are uniquely valuable in assessing the ability of the kidneys to handle water and ionic solutes when glucose or protein is also present in the urine. In summary, although reagent strip SG results may not indicate the true density of urine when nonionic solutes are present, they do reflect the renal concentrating ability to selectively handle ionic solutes and water.

Question 31

Note, however, that reagent strip SG measurements are affected by urine pH, with the most accurate results obtained when the urine pH is 7.0 to 7.5.4 Acid urine causes falsely increased SG results, whereas more alkaline urine causes falsely decreased results. When reagent strips are read by a reagent strip reader, the SG reading is automatically corrected by the instrument.

Question 32

This chemical SG method consists of a reagent test pad adhered to an inert plastic strip. The test pad is impregnated with a______, a pH indicator, and is maintained at an alkaline pH. When the strip is immersed in urine, the pK (which is the negative logarithm of the ionization constant of an acid) of the polyelectrolyte decreases proportionately to the ionic concentration of the specimen. As the pH of the test pad decreases, the bromothymol blue indicator changes color from dark_____(SGionic 1.000) to yellow-green (SGionic 1.030). Stated another way, as the number of ions present in the urine is increased, more protons are released from the polyelectrolyte, resulting in a decrease in the test pad pH and a color change in the indicator

Answer 32

polyelectrolyte blue-green

Question 33

Specific Gravity Result Discrepancies Between Reagent Strip and Refractometry Strip and Refractometry Because different methods can be used to determine SG, it is imperative that the limitations of each method are known to ensure proper interpretation of results. Similarly, the range of SG values that are physiologically possible must be recognized (i.e., 1.002 to 1.040). When urine produces an abnormally high SG value (> 1.040) by refractometry but a normal SG result by reagent strip, radiographic contrast media or another large-molecular-weight solute (e.g., man-nitol) should be suspected. Note that because the kidneys cannot produce urine with an SG greater than 1.040, when observed the excretion of an iatrogenic substance is indicated. A discrepancy (i.e., a difference > 0.005) between these SG methods alerts the laboratorian to the presence of a nonionic solute that is detected by refractometry but not by the reagent strip method. In these cases, the reagent strip method provides a more accurate assessment of the ability of the kidney to concentrate the urine. After intravenous administration