Clinical Chemistry (Nitrogenous Compounds) Flashcards
117 - 144
What is the compound that comprises the majority of the nonprotein-nitrogen fractions in serum?
A. Uric acid
B. Creatinine
C. Ammonia
D. Urea
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.
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
D
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): 60/28 = 2.14
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 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 diacetyl method, what does diacetyl react with to form a yellow product?
A. Ammonia
B. Urea
C. Uric acid
D. Nitrogen
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 methods
What endogenous substance may cause a positive interference in the urease/glutamate dehydrogenase assay?
A. Ammonia
B. Creatinine
C. Glucose
D. Cholesterol
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
Which of the following methods utilizes urease and glutamate dehydrogenase for the quantification of serum urea?
A. Berthelot
B. Coupled enzymatic
C. Conductimetric
D. Indicator dye
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
In the Berthelot reaction, what contaminant will cause the urea level to be falsely elevated?
A. Sodium fluoride
B. Protein
C. Ammonia
D. Bacteria
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.
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
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.
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 in vitro
B. Glutamine formation in vitro is avoided
C. The transamination process continues in vitro
D. Ammonia formation in vitro is avoided
D
It is necessary that certain precautions in specimen handling be exercised because the nzymatic process of deamination of amides
continues at room temperature after a blood ample 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 the specimen.
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 evacuated tubes.
C. Place specimen in a 37°C water bath immediately.
D. Advise patient not to smoke for 8 hours before blood collection.
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 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 dehydrogenase
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 + NH+4 + 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 disorders is not associated with an elevated blood ammonia level?
A. Reye syndrome
B. Renal failure
C. Chronic liver failure
D. Diabetes mellitus
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
An increased serum level of which of the following analytes is most commonly associated with decreased glomerular filtration?
A. Creatinine
B. Uric acid
C. Urea
D. Ammonia
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
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
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.
From what precursor is creatinine formed?
A. Urea
B. Glucose
C. Creatine
D. Uric acid
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.
