MLT GlobeChemistry: Renal function (urea, creatinine) – page 2
54 Chemistry MCQs on Renal function (urea, creatinine) with answers and explanations.
A fructose homopolymer that is freely filtered and neither reabsorbed nor secreted, making it the reference marker for GFR, is:
Inulin is a polyfructose; its clearance equals glomerular filtration rate. Dextran, glycogen and cellulose are all glucose polymers.
In the water deprivation–desmopressin (ADH) test, which laboratory measurement is used to judge the kidney's concentrating ability?
Serial urine osmolality (with plasma osmolality) shows whether urine concentrates with water deprivation and after desmopressin. Specific gravity is a cruder, older surrogate.
Why is urea clearance no longer used to measure GFR?
About 40–50% of filtered urea diffuses back, depending on urine flow, so urea clearance is well below GFR. Creatinine, cystatin C or eGFR equations are used instead.
Which substance is a well-known positive interferent in the alkaline picrate (Jaffe) creatinine method?
Ketoacids such as acetoacetate form a coloured product with alkaline picrate and falsely raise Jaffe creatinine, e.g. in diabetic ketoacidosis. Bilirubin tends to cause negative, not positive, interference.
Why is creatinine calibration traceable to isotope dilution mass spectrometry (IDMS) important?
Current eGFR equations such as CKD-EPI were derived using IDMS-traceable creatinine, so non-standardised results give biased eGFR. Calibration cannot correct biological factors such as muscle mass or tubular secretion.
A creatinine clearance is reported as 25 mL/min in a healthy young athlete with normal serum creatinine. The most likely cause is:
An under-collected urine lowers the urine creatinine total and falsely reduces clearance. Checking 24-hour creatinine excretion against expected output (based on body weight) detects this. High muscle mass would not lower clearance.
Which variables are required for the 2021 CKD-EPI creatinine equation?
The 2021 CKD-EPI creatinine equation uses serum creatinine, age and sex; the race coefficient was removed. Weight is used by Cockcroft-Gault, not CKD-EPI.
A patient with upper gastrointestinal bleeding has serum urea (BUN) 20 mmol/L (56 mg/dL) and creatinine 110 µmol/L (1.2 mg/dL). The high BUN/creatinine ratio is best explained by:
Blood in the upper GI tract is digested and absorbed as protein, increasing urea production, so BUN rises more than creatinine (ratio well above 20:1). Liver failure and low protein intake lower urea.
In the urease–glutamate dehydrogenase method for urea, the reaction is followed by measuring:
Urease produces ammonia, which GLDH uses with 2-oxoglutarate and NADH; NADH oxidation is seen as falling absorbance at 340 nm. Conductivity-based urea methods exist, but the coupled GLDH method is read at 340 nm.
Cystatin C is useful for estimating GFR mainly because it:
Cystatin C is made by all nucleated cells at a fairly constant rate and is freely filtered, so it is less affected by muscle mass than creatinine. However, it is affected by thyroid status and corticosteroids.
An elderly woman with very low muscle mass after amputation has serum creatinine 50 µmol/L (0.57 mg/dL). Which test gives a more reliable GFR estimate?
Low muscle mass lowers creatinine production, so creatinine-based eGFR overestimates GFR. KDIGO suggests confirming with cystatin C (combined equation) in such cases.
According to KDIGO, a urine albumin-to-creatinine ratio (ACR) of 45 mg/g (4.5 mg/mmol) is classified as:
KDIGO categories: A1 <30 mg/g (<3 mg/mmol), A2 30–300 mg/g (3–30 mg/mmol), A3 >300 mg/g (>30 mg/mmol). 45 mg/g falls in A2, formerly called microalbuminuria.
A raised urine ACR is found on a single sample collected after heavy exercise. Before diagnosing CKD, the correct next step is to:
Exercise, fever, infection and heart failure can raise albumin excretion temporarily. Albuminuria should be confirmed in at least 2 of 3 samples over 3 to 6 months, as CKD requires persistence over 3 months.
Serum creatinine is a relatively late marker of reduced kidney function because:
Because of the hyperbolic relationship between creatinine and GFR, and some tubular secretion, creatinine may remain 'normal' until substantial GFR loss. Creatinine is not reabsorbed.
A BUN result is 28 mg/dL. What is the equivalent urea concentration in SI units (mmol/L)?
BUN (mg/dL) × 0.357 = urea (mmol/L); 28 × 0.357 = 10.0 mmol/L. Multiplying by 2.14 instead converts BUN to urea in mg/dL, not mmol/L.
Why does creatinine clearance tend to overestimate the true glomerular filtration rate?
Besides glomerular filtration, some creatinine is secreted by proximal tubules, so more appears in urine than is filtered. Reabsorption would cause underestimation, which is the problem with urea, not creatinine.
According to KDIGO, which change in serum creatinine meets the definition of acute kidney injury?
KDIGO defines AKI as a rise of ≥0.3 mg/dL (≥26.5 µmol/L) within 48 hours, or ≥1.5 times baseline within 7 days, or low urine output. A single value above range does not define AKI.
Blood for urea by the urease method was collected into an ammonium heparin tube. What effect is expected?
Urease methods measure ammonia released from urea, so ammonium ions from the anticoagulant add to the signal and falsely raise urea. Lithium heparin should be used instead.
In the fully enzymatic creatinine method, which enzyme performs the first step, converting creatinine to creatine?
Creatininase converts creatinine to creatine; creatinase then forms sarcosine, and sarcosine oxidase produces hydrogen peroxide for a Trinder-type color. Creatine kinase is not part of this assay.
An ICU patient's serum creatinine rises daily: 90, 150, then 230 µmol/L. Why should an eGFR from creatinine not be relied on at this time?
eGFR equations assume stable creatinine production and excretion. In AKI, creatinine is still rising toward a new level, so eGFR overestimates true GFR.