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Chemistry - Creatinine, Uric Acid, BUN, and
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Creatinine is formed from the: A. Oxidation of creatine B. Oxidation of protein C. Deamination of dibasic amino acids D. Metabolism of purines - correct answer ✅A. Oxidation of creatine A Creatinine is produced at a rate of approximately 2% daily from the oxidation of creatine mainly in skeletal muscle. Creatine can be converted to creatinine by addition of strong acid or alkali or by the enzyme creatine hydroxylase Creatinine is considered the substance of choice to measure endogenous renal clearance because: A. The rate of formation per day is independent of body size B. It is completely filtered by the glomeruli C. Plasma levels are highly dependent upon diet
Chemistry - Creatinine, Uric Acid, BUN, and
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D. Clearance is the same for both men and women - correct answer ✅B. It is completely filtered by the glomeruli B Creatinine concentration is dependent upon muscle mass, but varies by less than 15% per day. Creatinine is not metabolized by the liver, or dependent on diet, and is 100% filtered by the glomeruli. It is not reabsorbed significantly but is secreted slightly, especially when filtrate flow is slow. Plasma creatinine and cystatin C are the two substances of choice for evaluating the glomerular filtration rate (GFR). Which statement regarding creatinine is true? A. Serum levels are elevated in early renal disease B. High serum levels result from reduced glomerular filtration C. Serum creatine has the same diagnostic utility as serum creatinine D. Serum creatinine is a more sensitive measure of renal function than creatinine clearance -
Chemistry - Creatinine, Uric Acid, BUN, and
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D. Creatinine clearance = U/V X P X 1.73/A - correct answer ✅A. Creatinine clearance = U/P X V X 1.73/A A Clearance is the volume of plasma that contains the same quantity of substance that is excreted in the urine in 1 minute. Creatinine clearance is calculated as the ratio of urine creatinine to plasma creatinine in milligrams per deciliter. This is multiplied by the volume of urine produced per minute and corrected for lean body mass by multiplying by 1.73/A, where A is the patient's body surface area in square meters. Separate reference ranges are needed for males, females, and children because each has a different percentage of lean muscle mass. Which of the following conditions is most likely to cause a falsely high creatinine clearance result? A. The patient uses the midstream void procedure when collecting his or her urine B. The patient adds tap water to the urine container because he or she forgets to save one of the urine samples
Chemistry - Creatinine, Uric Acid, BUN, and
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C. The patient does not empty his or her bladder at the conclusion of the test D. The patient empties his or her bladder at the start of the test and adds the urine to the collection - correct answer ✅D. The patient empties his or her bladder at the start of the test and adds the urine to the collection D Urine in the bladder should be eliminated and not saved at the start of the test because it represents urine formed prior to the test period. The other conditions (choices A-C ) will result in falsely low urine creatinine or volume and, therefore, falsely lower clearance results. Error is introduced by incomplete emptying of the bladder when short times are used to measure clearance. A 24-hour timed urine is the specimen of choice. When filtrate flow falls below 2 mL/min, error is introduced because tubular secretion of creatinine occurs. The patient must be kept well hydrated during the test to prevent this. The modification of diet in renal disease (MDRD) formula for calculating eGFR requires which four parameters?
Chemistry - Creatinine, Uric Acid, BUN, and
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A. Plasma urea B. Cystatin C C. Uric acid D. Potassium - correct answer ✅B. Cystatin C B Although all of the analytes listed are increased in chronic kidney disease as a result of low GFR, potassium, urea, and uric acid may be increased by other mechanisms and therefore, they are not specific for glomerular function. Cystatin C is an inhibitor of cysteine proteases. Being only 13 kilodaltons, it is completely filtered by the glomerulus then reabsorbed by the tubules. The plasma level is highly correlated to GFR because little is eliminated by nonrenal routes. Plasma levels are not influenced by diet, age, gender, or nutritional status. Low GFR causes retention of cystatin C in plasma and levels become abnormally high at clearance rates below 90 mL/min, making the test more sensitive than creatinine. Which of the following enzymes allows creatinine to be measured by coupling the creatinine amidohydrolase (creatininase) reaction to the peroxidase reaction?
Chemistry - Creatinine, Uric Acid, BUN, and
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A. Glucose-6-phosphate dehydrogenase B. Creatinine iminohydrolase C. Sarcosine oxidase D. Creatine kinase - correct answer ✅C. Sarcosine oxidase C The peroxidase-coupled enzymatic assay of creatinine is based upon the conversion of creatinine to creatine by creatinine amidohydrolase (creatininase). The enzyme creatinine amidinohydrolase (creatinase) then hydrolyzes creatine to produce sarcosine and urea. The enzyme sarcosine oxidase converts sarcosine to glycine producing formaldehyde and hydrogen peroxide. Peroxidase then catalyzes the oxidation of a dye (4-aminophenazone and phenol) by the peroxide forming a red- colored product. This method is more specific than the Jaffe reaction, which tends to overestimate creatinine by about 5% in persons with normal renal function. Select the primary reagent used in the Jaffe method for creatinine.
Chemistry - Creatinine, Uric Acid, BUN, and
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D. Performing a sample blank - correct answer ✅C. Measuring the timed rate of product formation C The Jaffe reaction is nonspecific; proteins and other reducing substances such as pyruvate, protein, and ascorbate cause positive interference. Much of this interference is reduced by using a timed rate reaction. Ketoacids react with alkaline picrate almost immediately, and proteins react slowly. Therefore, reading the absorbance at 20 and 80 seconds and using the absorbance difference minimizes the effects of those compounds. Creatinine can be measured using an amperometric electrode However, this requires the enzymes creatininase, creatinase, and sarcosine oxidase. The last enzyme produces hydrogen peroxide from sarcosine, which is oxidized. This produces current in proportion to creatinine concentration. Performing a sample blank does not correct for interfering substances that react with alkaline picrate. Which of the following statements is true? A. Cystatin C is measured immunochemically
Chemistry - Creatinine, Uric Acid, BUN, and
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B. The calibrator used for cystatin C is traceable to the National Bureau of Standards calibrator C. Cystatin C assays have a lower coefficient of variation than plasma creatinine D. Enzymatic and rate Jaffe reactions for creatinine give comparable results - correct answer ✅A. Cystatin C is measured immunochemically A Cystatin C can be measured by enzyme immunoassay, immunonephelometry, and immunoturbidimetry. However, there is no standardized calibrator as for creatinine, and therefore, results vary considerably from lab to lab. The coefficient of variation for these methods tends to be slightly higher than for creatinine. Since the enzymatic methods are specific, they give lower plasma creatinine results than the Jaffe method in persons with normal renal function. However, they tend to give higher clearance results than for inulin or iohexol clearance because some creatinine is secreted by the renal tubules.
Chemistry - Creatinine, Uric Acid, BUN, and
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for eGFR, it has about the same predictive value as eGFR derived from creatinine. A sample of amniotic fluid collected for fetal lung maturity studies from a woman with a pregnancy compromised by hemolytic disease of the newborn (HDN) has a creatinine of 88 mg/dL. What is the most likely cause of this result? A. The specimen is contaminated with blood B. Bilirubin has interfered with the measurement of creatinine C. A random error occurred when the absorbance signal was being processed by the analyzer D. The fluid is urine from accidental puncture of the urinary bladder
correct answer ✅D. The fluid is urine from accidental puncture of the urinary bladder D Creatinine levels in this range are found only in urine specimens. Adults usually excrete between 1.2 and 1.5 g of creatinine per day. For this reason, creatinine is routinely measured in 24-hour urine
Chemistry - Creatinine, Uric Acid, BUN, and
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samples to determine the completeness of collection. A 24-hour urine with less than 0.8 g/day indicates that some of the urine was probably discarded. Creatinine is also used to evaluate fetal maturity. As gestation progresses, more creatinine is excreted into the amniotic fluid by the fetus. Although a level above 2 mg/dL is not a specific indicator of maturity, a level below 2 mg/dL indicates immaturity Which analyte should be reported as a ratio using creatinine concentration as a reference? A. Urinary microalbumin B. Urinary estriol C. Urinary sodium D. Urinary urea - correct answer ✅A. Urinary microalbumin A Measurement of urinary microalbumin concentration should be reported as a ratio of albumin to creatinine (e.g., mg albumin per g creatinine). This eliminates the need for 24-hour collection in order
Chemistry - Creatinine, Uric Acid, BUN, and
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A Urea is generated by deamination of amino acids. Most is derived from the hepatic catabolism of proteins. Uric acid is produced by the catabolis of purines. Oxidation of pyrimidines produces orotic acid. Urea concentration is calculated from the BUN by multiplying by a factor of: A. 0. B. 2. C. 6. D. 14 - correct answer ✅B. 2. B BUN is multiplied by 2.14 to give the ureaconcentration in mg/dL. BUN (mg/dL) = urea × (% N in urea ÷ 100) Urea = BUN × 1/(% N in urea ÷100) Urea = BUN × (1/0.467) = 2.
Chemistry - Creatinine, Uric Acid, BUN, and
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Which of the statements below about serum urea is true? A. Levels are independent of diet B. Urea is not reabsorbed by the renal tubules C. High BUN levels can result from necrotic liver disease D. BUN is elevated in prerenal as well as renal failure - correct answer ✅D. BUN is elevated in prerenal as well as renal failure D Urea is completely filtered by the glomeruli but reabsorbed by the renal tubules at a rate dependent upon filtrate flow and tubular status. Urea levels are a sensitive indicator of renal disease, becoming elevated by glomerular injury, tubular damage, or poor blood flow to the kidneys (prerenal failure). Serum urea (and BUN) levels are influenced by diet and are low in necrotic liver disease. A patient's BUN is 60 mg/dL and serum creatinine is 3.0 mg/dL. These results suggest:
Chemistry - Creatinine, Uric Acid, BUN, and
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D. All of these options - correct answer ✅C. Nitrogen balance C Because BUN is handled by the tubules, serum levels are not specific for glomerular filtration rate. Urea clearance is influenced by diet and liver function as well as renal function. Protein intake minus excretion determines nitrogen balance. A negative balance (excretion exceeds intake) occurs in stress, starvation, fever, cachexia, and chronic illness. Nitrogen balance = (Protein intake in grams per day ÷ 6.25) - (Urine urea nitrogen in grams per day + 4), where 4 estimates the protein nitrogen lost in the feces per day and dividing by 6.25 converts protein to protein nitrogen BUN is determined electrochemically by coupling the urease reaction to measurement of: A. Potential with a urea-selective electrode B. The timed rate of increase in conductivity C. The oxidation of ammonia
Chemistry - Creatinine, Uric Acid, BUN, and
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D. Carbon dioxide - correct answer ✅B. The timed rate of increase in conductivity B A conductivity electrode is used to measure the increase in conductance of the solution as urea is hydrolyzed by urease in the presence of sodium carbonate. Urea + H2O → 2NH3 + CO 2NH3 + 2H2O + Na2CO3 → 2NH
- CO -2 + 2NaOH Ammonium ions increase the conductance of the solution. The timed rate of current increase is proportional to the BUN concentration. Alternatively, the ammonium ions produced can be measured using an ion-selective electrode In the ultraviolet enzymatic method for BUN, the urease reaction is coupled to a second enzymatic reaction using:
