MLT GlobeChemistry: Renal function (urea, creatinine) – page 3
54 Chemistry MCQs on Renal function (urea, creatinine) with answers and explanations.
A hemodialysis patient has pre-dialysis urea 25 mmol/L and post-dialysis urea 8 mmol/L. What is the urea reduction ratio (URR)?
URR = (pre − post)/pre × 100 = (25 − 8)/25 × 100 = 68%. A URR of at least 65% is commonly used as a target for adequate dialysis. 32% is the post/pre ratio, not the reduction.
An outpatient has creatinine drawn 2 hours after a large meal of cooked meat. What effect is expected?
Cooking converts muscle creatine into creatinine, which is absorbed and can temporarily raise serum creatinine. Fasting or avoiding cooked meat before testing reduces this effect.
Urea clearance is a poor measure of GFR mainly because urea is:
Urea is freely filtered but 40–60% diffuses back from the tubules, more when urine flow is low, so urea clearance underestimates GFR. Secretion is the problem with creatinine, not urea.
A dehydrated patient has BUN 60 mg/dL (urea 21 mmol/L) and serum creatinine 1.5 mg/dL (133 µmol/L). The BUN/creatinine ratio of 40 most suggests:
A BUN/creatinine ratio above about 20 suggests a prerenal cause such as dehydration, because urea is reabsorbed more when tubular flow is low. Liver failure and low protein intake lower urea.
A patient's serum creatinine rises from 1.0 to 2.0 mg/dL (88 to 177 µmol/L) with no change in muscle mass. The GFR has most likely:
Serum creatinine is roughly inversely proportional to GFR at steady state, so a doubling of creatinine indicates that GFR has fallen by about 50%.
Which factor can raise serum urea without any change in kidney function?
More dietary protein means more amino acid nitrogen converted to urea. Liver failure and overhydration lower urea, and muscle mass affects creatinine rather than urea.
A 24-hour urine volume is 1440 mL, urine creatinine 8.0 mmol/L and serum creatinine 100 µmol/L. What is the creatinine clearance?
Clearance = (U × V) / P. Urine flow = 1440/1440 = 1.0 mL/min; U = 8000 µmol/L; P = 100 µmol/L. So (8000 × 1.0)/100 = 80 mL/min. Forgetting to convert mmol to µmol gives a wrong answer.
Which pattern best suggests post-renal azotemia from bilateral ureteral obstruction early in its course?
Early obstruction increases urea back-diffusion, so the ratio is usually raised as in prerenal states. A low ratio with casts suggests intrinsic renal injury such as acute tubular necrosis.
The uricase method for uric acid measures a decrease in absorbance at 293 nm because:
Uricase converts uric acid, which absorbs UV at about 293 nm, into allantoin, which does not, so absorbance falls. Many analysers instead couple the H2O2 formed to a peroxidase colour reaction.
Rasburicase therapy can cause falsely low uric acid results if the blood sample is:
Rasburicase is a recombinant uricase that continues to degrade uric acid in the tube. Samples should be collected into pre-chilled tubes, kept on ice and analysed promptly.
A patient starts trimethoprim. Serum creatinine rises by 15% but measured GFR by iohexol clearance is unchanged and cystatin C is stable. The most likely explanation is:
Trimethoprim (like cimetidine) blocks tubular creatinine secretion, raising serum creatinine without true loss of GFR. Stable cystatin C and measured GFR argue against real kidney injury.
Oliguric patient: urine Na 10 mmol/L, plasma Na 140 mmol/L, urine creatinine 8000 µmol/L, plasma creatinine 200 µmol/L. What is the fractional excretion of sodium (FENa)?
FENa = (UNa × PCr)/(PNa × UCr) × 100 = (10 × 200)/(140 × 8000) × 100 = 0.18%. A value below 1% shows avid sodium retention, typical of prerenal azotemia; ATN usually gives above 2%.
A patient with oliguria has been given furosemide. Which index is more reliable than FENa to separate prerenal azotemia from tubular injury?
Loop diuretics increase sodium excretion and falsely raise FENa. Urea handling is less affected, so a fractional excretion of urea below about 35% still suggests prerenal azotemia.
A child is 120 cm tall with serum creatinine 0.6 mg/dL (53 µmol/L). Using the bedside Schwartz equation (eGFR = 0.413 × height in cm ÷ creatinine in mg/dL), the eGFR is about:
0.413 × 120 = 49.6; 49.6 ÷ 0.6 = 82.6, about 83 mL/min/1.73 m². Forgetting to divide by creatinine gives about 50, a common error.