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Chemistry - Nonprotein Nitro Compounds
Terms in this set (28)
D. Constituents in the plasma that contain the element nitrogen are categorized as being protein- or nonprotein-nitrogen compounds. The principal substances included among the nonprotein-nitrogen compounds are urea, amino acids, uric acid, creatinine, creatine, and ammonia.
Of these compounds, urea is present in the plasma in the greatest concentration, comprising approximately 45% of the nonprotein-nitrogen fraction.
117. What is the compound that comprises the majority of the nonprotein-nitrogen fractions in serum?
A. Uric acid
D. urea nitrogen x 2.14 = urea
Because the substances classified as nonprotein-nitrogen (NPN) compounds were quantified
by assaying for their nitrogen content, it became customary to express urea as urea nitrogen. When urea was expressed as urea nitrogen,
a comparison could be made between the concentration
of urea and the concentration of other
NPN compounds. When it is necessary to convert urea nitrogen values to urea, the concentration
may be calculated easily by multiplying the urea nitrogen value by 2.14. This factor is derived
from the molecular mass of urea (60 daltons) and the molecular weight of its two nitrogen atoms (28):
Express 30 mg/dL of urea nitrogen as urea.
A. 14 mg/dL
B. 20 mg/dL
C. 50 mg/dL
D. 64 mg/dL
B. In addition to the fact that sodium fluoride is a weak anticoagulant, it also functions as an
antiglycolytic agent and is used as a preservative for glucose in blood specimens. With the urease
reagent systems for the quantification of urea, the use of sodium fluoride must be avoided because of its inhibitory effect on this system.
Additionally, contamination from the use of ammonium oxalate and ammonium heparin
must be avoided, because urease catalyzes the production of ammonium carbonate from urea.
In several methods, the ammonium ion formed reacts proportionally to the amount of urea originally
present in the sample. Anticoagulants containing ammonium would contribute falsely to
the urea result.
In the urea method, the enzymatic action of urease is inhibited when blood for analysis is drawn in a tube containing what anticoagulant?
A. Sodium heparin
B. Sodium fluoride
C. Sodium oxalate
D. Ethylenediaminetetra-acetic acid
B. In the diacetyl method, acidic diacetyl reacts directly with urea to form a yellow-diazine
derivative. Thiosemicarbazide and ferric ions are reagents used to intensify the color of the reaction.
Because urea is quantified directly, the method does not suffer from interferences from ammonia contamination, as do some of the urea
In the diacetyl method, what does diacetyl react with to form a yellow product?
C. Uric acid
A. Adequate specificity is generally obtained when using the urease/glutamate dehydrogenase
method. Because urease hydrolyzes urea to ammonia and water, a positive interference from
endogenous ammonia will occur with elevated blood levels of ammonia. Such interference may
occur from use of aged blood specimens and in certain metabolic diseases.
What endogenous substance may cause a positive interference in the urease/glutamate
B. An enzymatic method for quantifying urea
employs urease and glutamate dehydrogenase
(GLDH) in a coupled enzymatic reaction. Urease
catalyzes the production of ammonium carbonate
from urea. The ammonium ion produced
reacts with 2-oxoglutarate and NADH in the
presence of GLDH with the formation of NAD+
and glutamate. The decrease in absorbance, as
NADH is oxidized to NAD+, is followed kinetically
at 340 nm using a spectrophotometer. In
the conductimetric method, the formation of
ammonium ions and carbonate ions, from the
ammonium carbonate, causes a change in conductivity
that is related to the amount of urea
present in the sample.
Which of the following methods utilizes urease and glutamate dehydrogenase for the quantification of serum urea?
B. Coupled enzymatic
D. Indicator dye
D. Ion-exchange, ion-selective electrode, and
enzymatic methods have been employed for the
analysis of ammonia in plasma specimens.
Because the enzymatic method is a direct assay,
prior separation of ammonium ions is not
required. The enzymatic reaction catalyzed by
glutamate dehydrogenase follows:
2-Oxoglutarate + NHj + NADPH ^±
Glutamate + NADP+ + H2O
The rate of oxidation of NADPH to NADP+ is
followed as a decreasing change in absorbance
at 340 nm.
Which of the following statements can be associated with the enzymatic assay of ammonia?
A. Increase in absorbance monitored at 340 nm
B. Nicotinamide-adenine dinucleotide
(NAD+) required as a cofactor
C. Ammonium ion isolated from
specimen before the enzymatic step
D. Reaction catalyzed by glutamate
C. The Berthelot reaction is based on the production of a blue-indophenol compound when ammonia reacts in an alkaline medium with phenol
and sodium hypochlorite. This basic colorimetric reaction can be used to quantify both urea and blood ammonia levels. Therefore, any
ammonia contamination (i.e., in the distilled water used to make reagents for the urea procedure
and on glassware) must be avoided so that falsely elevated urea values will not be obtained.
In the Berthelot reaction, what contaminant will cause the urea level to be falsely elevated?
A. Sodium fluoride
A. The catabolism of some amino acids involves a transamination reaction in which the a-amino
group of the amino acid is enzymatically removed.
After its removal, the a-amino group is transferred to an a-keto acid (a-ketoglutarate) with the formation
of L-glutamate. Glutamate, which is the common product formed by most transaminase reactions, then may undergo oxidative deamination
in the liver mitochondria with the formation of ammonia. The ammonia thus formed leaves the mitochondria as the amino group of citrulline.
Citrulline, in turn, condenses with aspartate, which contains the second amino group needed for urea synthesis, forming argininosuccinate,
which ultimately leads to the formation of urea.
Therefore, the formation of urea and its excretion in the urine provide the principal means by which the body is able to free itself of excess ammonia.
To maintain acid-base balance, it is
necessary that the blood ammonia level be kept within narrow limits. This is accomplished primarily by which of the following?
A. Synthesis of urea from ammonia
B. Synthesis of glutamine from ammonia
C. Excretion of ammonia in the bile
D. Excretion of ammonia in the stools
D. It is necessary that certain precautions in specimen handling be exercised because the
enzymatic process of deamination of amides continues at room temperature after a blood
sample is drawn. When blood is drawn for ammonia analysis, it is critical that any in vitro ammonia formation be prevented. It is recommended that the tube containing the blood specimen
be placed in an ice bath immediately after the blood is drawn, because the cold environment will help retard metabolic processes. It is
also important that the chemical analysis of the specimen be started within 20 minutes of drawing
When a blood ammonia determination is performed, the blood specimen must be treated in a manner that will ensure that
A. The deamination process continues
B. Glutamine formation in vitro is
C. The transamination process continues
D. Ammonia formation in vitro is avoided
C. Plasma is the specimen of choice for ammonia analysis. Ethylenediaminetetra-acetic acid
(EDTA) and heparin (not the ammonium salt) are acceptable anticoagulants. Because exposure of blood to air is contraindicated, the evacuated blood collection tube should be filled completely.
The blood specimen should be placed on ice immediately and centrifuged as soon as possible to inhibit deamination of amino acids. Because
the concentration of ammonia in red blood cells is approximately three times greater than in plasma, the analysis should be performed on a
nonhemolyzed specimen. Because of the false increase in ammonia levels caused by smoking, patients should be instructed to refrain from
smoking for 8 hours before blood collection.
Which of the following does not need to be done when collecting, handling, and using a specimen for ammonia analysis?
A. Avoid using a hemolyzed specimen.
B. Collect blood in EDTA or heparin
C. Place specimen in a 37°C water bath
D. Advise patient not to smoke for
8 hours before blood collection.
D. The gastrointestinal tract is the primary source of blood ammonia. With normal liver function, ammonia is metabolized to urea for urinary
excretion. When blood ammonia levels become elevated, toxicity of the central nervous system occurs. Diseases associated with elevated
blood ammonia levels include Reye syndrome, renal failure, chronic liver failure, cirrhosis, and hepatic encephalopathy.
Which of the following disorders is not
associated with an elevated blood
A. Reye syndrome
B. Renal failure
C. Chronic liver failure
D. Diabetes mellitus
A. Creatinine is a waste product of muscle metabolism and as such its production is rather constant on a daily basis. Creatinine is freely filtered
by the glomerulus, with only a very small amount secreted by the proximal tubule. Thus, measurement of creatinine is a reflection of
glomerular filtration. An increase in the serum creatinine level would be indicative of decreased glomerular filtration. Although uric acid, urea, and
ammonia levels may be increased with decreased glomerular filtration, increased levels of these analytes
are associated with a number of specific metabolic diseases and, therefore, they are not used as indicators of the glomerular filtration rate.
An increased serum level of which of the following analytes is most commonly associated with decreased glomerular filtration?
B. Uric acid
D. Serum urea nitrogen and creatinine levels are frequently requested together so that their ratio
can be evaluated. The normal ratio of serum urea nitrogen to creatinine ranges between 10:1 and 20:1. Abnormal values obtained when kidney function tests are performed may be the result of a prerenal, renal, or postrenal malfunction. The
ratio of urea nitrogen to creatinine is sometimes used as an index in the assessment of kidney function
and as a means of differentiating the source of the malfunction.
A serum creatinine was found to be
6.0 mg/dL. Which of the following urea
nitrogen serum results would support the same pathological condition?
A. 6 mg/dL
B. 20 mg/dL
C. 35 mg/dL
D. 70 mg/dL
C. Creatine is synthesized from the amino acids arginine, glycine, and methionine. In tissues that
include the kidneys, small intestinal mucosa, pancreas, and liver, arginine and glycine form guanidoacetate through a transaminidase reaction.
The guanidoacetate is transported in the blood to the liver, where it reacts with S-adenosylmethionine
through a transmethylase reaction to
form creatine. Creatine is transported in the blood to muscle tissue. Creatine in the form of phosphocreatine
is a high-energy storage compound that provides the phosphate needed to produce adenosine
triphosphate (ATP) for muscle metabolism. When ATP is formed from phosphocreatine, free creatine is also released. Creatine, through a
spontaneous and irreversible reaction, forms creatinine.
Creatinine serves no functional metabolic role. It is excreted in the urine as a waste product of creatine.
From what precursor is creatinine formed?
D. Uric acid
D. The Jaffe reaction, which was described in 1886, is still used for creatinine analysis. The Jaffe
reaction employs the use of an alkaline picrate solution that reacts with creatinine to form a bright
orange-red complex. A drawback to this procedure is its lack of specificity for creatinine, because noncreatinine chromogens, glucose, and
proteins are also able to react with alkaline picrate.
What analyte is measured using the Jaffe reaction?
B. Uric acid
A. Because protein will interfere with the Jaffe
reaction, serum for a manual creatinine analysis is
treated with sodium tungstate and sulfuric acid to
precipitate the proteins. The use of tungstic acid to
make a protein-free filtrate is known as the Folin-
Wu method. The protein-free filtrate, which still
contains creatinine and other reducing substances,
is then mixed with alkaline picrate reagent to yield
the characteristic Jaffe reaction. Automated
methods have replaced manual methods. These
kinetic methods using the alkaline picrate reagent
system have been adapted to use small volumes of
serum and have readings taken within a short
interval of 25-60 sec following initiation of the
reaction. Because of the speed at which the analysis
is performed and the small serum sample
requirement, serum may be used directly, alleviating
the need for a protein-free filtrate.
When the Jaffe reaction is employed as a kinetic assay to quantify serum creatinine, which of the following is used in the analysis?
A. Serum sample used directly
B. Folin-Wu filtrate
C. Somogyi-Nelson filtrate
D. Trichloroacetic acid filtrate
D. The creatinine clearance test is used to
assess the glomerular filtration rate. An accurately
timed 24-hour urine specimen and a blood
sample, drawn in the middle of the 24-hour urine
collection, are required. The creatinine concentrations
of the urine specimen and the plasma are
determined, and these values, along with the
urine volume, are used to determine the creatinine
clearance. The body surface area will not be
used in the calculation because the clearance is
being done on an average-size adult. The following
general mathematical formula is used to calculate
U / P x V = Creatinine clearance (mL/min)
where U = urine creatinine concentration in
milligrams per deciliter, P = plasma creatinine
concentration in milligrams per deciliter, and V =
volume of urine per minute, with volume
expressed in milliliters and 24 hours expressed as
1440 minutes. Applying this formula to the problem
presented in the question:
120 mg/dL 1520 mL/24 hr
1.2 mg/dL 1440 min/24 hr
= 106 mL/min
It should be noted that both the size of the kidney
and the body surface area of an individual influence
the creatinine clearance rate. Because normal
values for creatinine clearance are based on the
average adult body surface area, it is necessary that
the clearance rate be adjusted when the body surface
area of the individual being tested differs
significantly from the average adult area. This type
of adjustment is especially critical if the individual
is an infant, a young child, or an adolescent.
The creatinine clearance test is routinely used to assess the glomerular filtration rate. Given the following information for an average-size adult, calculate a creatinine clearance.
Urine creatinine—120 mg/dL
Plasma creatinine—1.2 mg/dL
Urine volume for 24 hours—1520 mL
A. 11 mL/min
B. 63 mL/min
C. 95 mL/min
D. 106 mL/min
C. Creatinine assays are preferably performed on fresh urine specimens. If an acid urine specimen
is kept for a time, any creatine in the urine will be converted to creatinine. In alkaline urine an equilibrium situation will occur between the
creatine and creatinine present in the specimen. To avoid either of these situations, it is recommended
that the urine be adjusted to pH 7.0 and that the specimen be frozen. It is thought that at a
neutral pH, the integrity of the urine specimen will be maintained because it will require days or even weeks for equilibrium to occur between
the two compounds.
When it is not possible to perform a
creatinine assay on a fresh urine specimen, to what pH level should the urine be adjusted?
D. Creatine is predominantly found in muscle
cells, where the quantity of creatine is proportional
to muscle mass. As muscle metabolism
proceeds, creatine is freed from its high-energy
phosphate form, and the creatine, thus liberated,
forms the anhydride creatinine. The quantity of
creatinine formed daily is a relatively constant
amount because it is related to muscle mass.
Therefore, it has been customary to quantify the
creatinine present in a 24-hour urine specimen as
an index of the completeness of the collection.
What compound normally found in urine may be used to assess the completeness of
a 24-hour urine collection?
B. Uric acid
A. In addition to the endpoint and kinetic methods, which use the Jaffe reaction (picric
acid), several methods have been developed that
use coupled enzymatic reactions for the quantification
of creatinine. In one such method, creatinine
amidohydrolase (creatininase) catalyzes the
conversion of creatinine to creatine and subsequently
to sarcosine and urea. Sarcosine oxidase
catalyzes the oxidation of sarcosine to glycine,
formaldehyde, and hydrogen peroxide. The
hydrogen peroxide reacts with the reduced form
of a chromogenic dye in the presence of peroxidase
to form an oxidized colored dye product
that is read spectrophotometrically.
Which of the following reagents is not
utilized in a coupled enzymatic reaction
method to quantify serum creatinine?
A. Picric acid
B. Chromogenic dye
C. Creatinine amidohydrolase
D. Sarcosine oxidase
C. Creatinine is an endogenous substance that
is filtered by the glomeruli and normally is neither
reabsorbed nor secreted by the tubules.
When plasma levels of creatinine rise, some
secretion of creatinine by the tubules will occur.
The filtration properties of creatinine and the
fact that it is a substance normally present in
blood make the creatinine clearance test the
method of choice for assessing the glomerular
An endogenous substance assayed to
assess the glomerular filtration rate may
be described as being filtered by the
glomeruli, not reabsorbed by the tubules,
and only secreted by the tubules when
plasma levels become elevated. What is
this frequently assayed substance?
B. Uric acid
B. Through a sequence of enzymatic reactions,
the purine nucleosides, adenosine and guanosine,
are catabolized to the waste product uric acid. The
catabolism of purines occurs primarily in the liver,
with the majority of uric acid being excreted as a
urinary waste product. The remaining amount of
uric acid is excreted in the biliary, pancreatic, and
gastrointestinal secretions through the gastrointestinal
tract. In the large intestine, uric acid is further
degraded by bacteria and excreted in the stool.
What is the end product of purine
catabolism in humans?
B. Uric acid
D. Uric acid may be quantified by reacting it with phosphotungstic acid reagent in alkaline solution. In this reaction, uric acid is oxidized to
allantoin and the phosphotungstic acid is reduced,
forming a tungsten blue complex. The intensity of the tungsten blue complex is proportional to the
concentration of uric acid in the specimen.
When mixed with phosphotungstic acid, what compound causes the reduction of the former to a tungsten blue complex?
D. Uric acid
B. Uric acid absorbs light in the ultraviolet region of 290-293 nm. When uricase is added to
a uric acid mixture, uricase destroys uric acid by
catalyzing its degradation to allantoin and carbon
dioxide. On the basis of these two characteristics,
differential spectrophotometry has been applied
to the quantification of uric acid. This type of
method is used on analyzers that are capable of
monitoring the decrease in absorbance as uric
acid is destroyed by uricase. The decrease in
absorbance is proportional to the concentration
of uric acid in the specimen.
In the ultraviolet procedure for
quantifying uric acid, what does the
reaction between uric acid and uricase
A. Production of reduced nicotinamideadenine
B. The formation of allantoin
C. An increase in absorbance
D. A reduction of phosphotungstic acid
A. As renal function continues to be lost over time, chronic renal failure develops. Chronic renal
failure is manifested by loss of excretory function,
inability to regulate water and electrolyte balance,
and increased production of parathyroid hormone,
all of which contribute to the abnormal laboratory
findings. The decreased production of erythropoietin causes anemia to develop.
Which of the following disorders is best characterized by laboratory findings that include increased serum levels of inorganic phosphorus, magnesium, potassium, uric
acid, urea, and creatinine and decreased serum calcium and erythropoietin levels?
A. Chronic renal failure
B. Renal tubular disease
C. Nephrotic syndrome
D. Acute glomerulonephritis
D. Gout is a pathological condition that may be caused by a malfunction of purine metabolism
or a depression in the renal excretion of uric acid. Two of the major characteristics of gout are
hyperuricemia and a deposition of uric acid as monosodium urate crystals in joints, periarticular
cartilage, bone, bursae, and subcutaneous tissue.
Such a deposition of urate crystals causes inflammation of the affected area and precipitates
an arthritic attack.
In gout, what analyte deposits in joints and other body tissues?
D. Uric acid
A. An increase in serum uric acid levels may be seen during chemotherapy for leukemia. The
cause of this is the accelerated breakdown of cell nuclei in response to the chemotherapy. Other proliferative disorders that may respond similarly are lymphoma, multiple myeloma, and polycythemia.
It is important that serum uric acid be monitored during chemotherapy to avoid nephrotoxicity.
During chemotherapy for leukemia, which of the following analytes would most likely be elevated in the blood?
A. Uric acid
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