A 27-year-old man has a pre-employment health check in Dubai. He trains with weights 6 days a week, weighs 98 kg with little body fat, eats a high-protein diet and takes creatine monohydrate. He feels well and his blood pressure is normal. Results: enzymatic creatinine 138 µmol/L (1.56 mg/dL) (reference 60-110 µmol/L), giving a creatinine eGFR of 62 mL/min/1.73 m²; urea 8.1 mmol/L (reference 2.5-7.8); urinalysis and urine albumin-to-creatinine ratio normal; cystatin C eGFR 112 mL/min/1.73 m². His creatinine was 135 µmol/L three months ago. The best interpretation is:

Normal GFR; muscle mass and creatine raise creatinine

Prerenal impairment from dehydration and high protein intake

Creatinine eGFR is correct; cystatin C is unreliable in athletes

Chronic kidney disease, as eGFR has stayed low for 3 months

Before replacing its kinetic Jaffe creatinine method with an enzymatic method, a laboratory runs 60 patient serum samples on both. Both methods are calibrated with IDMS-traceable material and agree within 2 µmol/L on aqueous reference solutions. On patient serum, Deming regression gives Jaffe = 1.01 × enzymatic + 22 µmol/L, and the difference is similar at creatinine values of 50 and 800 µmol/L. None of the samples are icteric, hemolysed or lipemic, and none come from patients in ketoacidosis or receiving cephalosporins. Which explanation best fits these data?

Serum proteins react as non-creatinine chromogens in the Jaffe method

Bilirubin is oxidized by the alkaline Jaffe reagent

The enzymatic method also measures creatine in the sample

A calibration slope error in the enzymatic method

Enzymatic creatinine methods are preferred over Jaffe methods in neonates and young children, mainly because:

Interferents cause a larger relative error at low creatinine

Children's creatinine reacts more slowly with picrate

Jaffe reagents need more sample than children can give

Enzymatic methods correct for the child's muscle mass

Renal function: urea, creatinine, eGFR and clearance

25 DOH Abu Dhabi (HAAD) lab licensing practice questions on Renal function: urea, creatinine, eGFR and clearance, part of Clinical Chemistry. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A 27-year-old man has a pre-employment health check in Dubai. He trains with weights 6 days a week, weighs 98 kg with little body fat, eats a high-protein diet and takes creatine monohydrate. He feels well and his blood pressure is normal. Results: enzymatic creatinine 138 µmol/L (1.56 mg/dL) (reference 60-110 µmol/L), giving a creatinine eGFR of 62 mL/min/1.73 m²; urea 8.1 mmol/L (reference 2.5-7.8); urinalysis and urine albumin-to-creatinine ratio normal; cystatin C eGFR 112 mL/min/1.73 m². His creatinine was 135 µmol/L three months ago. The best interpretation is:
1. APrerenal impairment from dehydration and high protein intake
2. BCreatinine eGFR is correct; cystatin C is unreliable in athletes
3. CChronic kidney disease, as eGFR has stayed low for 3 months
4. DNormal GFR; muscle mass and creatine raise creatinine
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Answer: D · Normal GFR; muscle mass and creatine raise creatinine
Serum creatinine depends on muscle mass as well as GFR, and creatine supplements add to it, because creatine converts to creatinine in the body. Cystatin C is made by all nucleated cells and depends little on muscle, so its eGFR of 112 mL/min/1.73 m² is the better estimate here, as KDIGO advises when creatinine is likely to mislead. Chronic kidney disease needs a GFR below 60 or markers of kidney damage for more than 3 months; his urinalysis and albuminuria are normal, and his cystatin C eGFR is normal. The urea/creatinine ratio is 8.1 ÷ 0.138 = 59, well below the value of about 100 that suggests a prerenal cause, and the slightly raised urea fits his protein intake. The report can note that creatinine-based eGFR is unreliable at extremes of muscle mass.
- A: His urea/creatinine ratio is about 59, and urea is only slightly raised by protein intake.
- B: Cystatin C depends little on muscle mass, so it is the more reliable marker here.
- C: CKD needs eGFR below 60 or kidney damage markers; his cystatin C eGFR and urine tests are normal.
Question 2 · Hard
A 46-year-old man is sent home with a container to collect a 24-hour urine for creatinine clearance. The returned label reads: started 06:00 Sunday (first void discarded), finished 12:00 Monday (final void included). Volume 2,400 mL; urine creatinine 7.5 mmol/L (85 mg/dL). Plasma creatinine, drawn at noon on Monday, is 110 µmol/L (1.24 mg/dL). His body surface area is 2.10 m², and the clinician asks for the clearance corrected to 1.73 m². The collection is otherwise complete, with no missed voids or spills. What is the creatinine clearance corrected to 1.73 m²?
1. A91 mL/min/1.73 m²
2. B75 mL/min/1.73 m²
3. C110 mL/min/1.73 m²
4. D94 mL/min/1.73 m²
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Answer: B · 75 mL/min/1.73 m²
The collection ran from 06:00 Sunday to 12:00 Monday, which is 30 hours (1,800 minutes), not 24 hours. Clearance = (U × V) ÷ (P × t) = (7.5 mmol/L × 2,400 mL) ÷ (0.110 mmol/L × 1,800 min) = 91 mL/min, after plasma creatinine is converted from µmol/L to mmol/L. Corrected to 1.73 m²: 91 × 1.73 ÷ 2.10 = 75 mL/min/1.73 m². Assuming 24 hours gives 114 mL/min, or 94 after correction, and multiplying by 2.10 ÷ 1.73 instead gives 110. The laboratory should calculate with the real collection time on the label and state it on the report.
- A: 91 mL/min is the uncorrected clearance; it still needs scaling to 1.73 m².
- C: 110 comes from multiplying by 2.10 ÷ 1.73, which inverts the correction.
- D: 94 assumes a 24-hour collection, but the label shows 30 hours.
Question 3 · Hard
Before replacing its kinetic Jaffe creatinine method with an enzymatic method, a laboratory runs 60 patient serum samples on both. Both methods are calibrated with IDMS-traceable material and agree within 2 µmol/L on aqueous reference solutions. On patient serum, Deming regression gives Jaffe = 1.01 × enzymatic + 22 µmol/L, and the difference is similar at creatinine values of 50 and 800 µmol/L. None of the samples are icteric, hemolysed or lipemic, and none come from patients in ketoacidosis or receiving cephalosporins. Which explanation best fits these data?
1. ABilirubin is oxidized by the alkaline Jaffe reagent
2. BSerum proteins react as non-creatinine chromogens in the Jaffe method
3. CThe enzymatic method also measures creatine in the sample
4. DA calibration slope error in the enzymatic method
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Answer: B · Serum proteins react as non-creatinine chromogens in the Jaffe method
A slope of 1.01 with an intercept of 22 µmol/L is a constant systematic difference, the same at low and high creatinine. Because the methods agree on aqueous solutions but not on serum, the cause lies in the serum matrix rather than in calibration. In alkaline picrate, proteins and other non-creatinine chromogens add color, giving a positive bias of roughly 20 µmol/L in serum; 'compensated' Jaffe methods subtract a fixed amount for this reason. A calibration error would change the slope, the enzymatic method removes creatine in a pre-incubation step (and measuring creatine would raise the enzymatic result, not the Jaffe), and bilirubin gives a negative bias and was excluded. After the switch, creatinine falls and eGFR rises for many patients, so clinicians should be told before the new method goes live.
- A: Bilirubin gives a negative Jaffe bias, and no samples were icteric.
- C: Creatine is removed before the enzymatic reaction, and measuring it would raise the enzymatic result, not the Jaffe.
- D: A slope error gives a proportional difference, but the slope here is 1.01.
Question 4 · Medium
Enzymatic creatinine methods are preferred over Jaffe methods in neonates and young children, mainly because:
1. AChildren's creatinine reacts more slowly with picrate
2. BJaffe reagents need more sample than children can give
3. CInterferents cause a larger relative error at low creatinine
4. DEnzymatic methods correct for the child's muscle mass
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Answer: C · Interferents cause a larger relative error at low creatinine
Jaffe non-creatinine chromogens, such as proteins, glucose and ketones, add a roughly constant amount to every result. In young children true creatinine may be only 20-40 µmol/L, so this bias becomes a large share of the result, and neonatal hyperbilirubinemia adds a negative error. Enzymatic methods are more specific at low concentrations. The bedside Schwartz equation was derived with enzymatic, IDMS-traceable creatinine.
- A: Creatinine reacts the same way whatever the patient's age.
- B: Jaffe and enzymatic methods use similar, small sample volumes.
- D: No creatinine method corrects for muscle mass; equations do that only roughly.
Question 5 · Medium
To estimate GFR in a 6-year-old, the bedside Schwartz equation combines serum creatinine with the child's:
1. ABody weight
2. BAge
3. CBody surface area
4. DHeight
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Answer: D · Height
The bedside Schwartz equation is eGFR (mL/min/1.73 m²) = 0.413 × height (cm) / creatinine (mg/dL), or about 36.5 × height (cm) / creatinine (µmol/L). Height stands in for muscle mass in a growing child. Adult equations such as CKD-EPI are not used for young children. The equation assumes an enzymatic, IDMS-traceable creatinine.
- A: Body weight is used in the Cockcroft-Gault equation, not in the bedside Schwartz equation.
- B: Age is not an input of the bedside Schwartz equation.
- C: The result is already indexed to 1.73 m², but BSA is not an input.
Question 6 · Easy
In the Jaffe method, creatinine forms an orange-red complex with:
1. ASodium nitroprusside
2. BPhenol and hypochlorite
3. CDiacetyl monoxime
4. DAlkaline picrate
Answer and explanation in LearnFree account
Question 7 · Hard
A woman at 30 weeks of pregnancy has creatinine 95 µmol/L (1.07 mg/dL). The non-pregnant reference interval is 45-90 µmol/L, and in normal late pregnancy creatinine is usually 35-75 µmol/L. How should this result be interpreted?
1. AExpected, as pregnancy raises creatinine production
2. BAcceptable, as a pregnancy-adjusted eGFR would be normal
3. CFalsely high, caused by hemodilution in late pregnancy
4. DHigh for pregnancy, suggesting reduced kidney function
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Question 8 · Medium
Why can serum creatinine be about 15% higher than usual in a sample drawn 2 hours after a large cooked-meat meal?
1. AUrea from the meal reacts with picrate in the Jaffe method
2. BA protein meal lowers GFR for several hours afterwards
3. CThe liver converts dietary amino acids directly to creatinine
4. DCooking turns meat creatine into creatinine, which is absorbed
Answer and explanation in LearnFree account
Question 9 · Medium
A first eGFR of 52 mL/min/1.73 m² is found in an outpatient with a normal urinalysis. Under KDIGO, the finding that would establish chronic kidney disease is:
1. AeGFR below 60 persisting for more than 3 months
2. BA 24-hour creatinine clearance below 60 mL/min
3. CA repeat eGFR below 60 on the next day
4. DA plasma urea above its reference range
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Question 10 · Hard
A 78-year-old woman has had diarrhea and vomiting for 4 days. Urea 21.0 mmol/L (reference 2.5-7.8), creatinine 140 µmol/L (45-90). The laboratory rule: urea (mmol/L) / creatinine (mmol/L) above 100 suggests a prerenal cause. What is the ratio, and which interpretation fits best?
1. A150: prerenal, from volume depletion
2. B0.15: intrinsic renal damage
3. C6.7: low urea production
4. D150: intrinsic renal damage
Answer and explanation in LearnFree account
Question 11 · Medium
Which patient is most likely to have a raised plasma urea while GFR is normal?
1. AA patient on a very low-protein diet
2. BA woman in the third trimester of pregnancy
3. CA patient with acute liver failure
4. DA patient on high-dose corticosteroids
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Question 12 · Hard
A patient receiving high-dose intravenous cefoxitin has creatinine 210 µmol/L by a Jaffe method; yesterday it was 85 µmol/L, and urea and urine output are normal. An enzymatic method on the same sample gives 88 µmol/L. The best explanation is:
1. ACefoxitin inhibits creatininase, falsely lowering the enzymatic result
2. BCefoxitin reacts with alkaline picrate, falsely raising the Jaffe result
3. CEarly acute kidney injury that the enzymatic method cannot yet detect
4. DCefoxitin blocks tubular creatinine secretion, raising true creatinine
Answer and explanation in LearnFree account
Question 13 · Hard
Cockcroft-Gault in SI units: creatinine clearance (mL/min) = (140 - age) × weight (kg) × 1.23 / creatinine (µmol/L), multiplied by 0.85 for women. A 72-year-old woman weighs 55 kg and has creatinine 120 µmol/L (1.36 mg/dL). What is her estimated creatinine clearance?
1. A28 mL/min
2. B45 mL/min
3. C38 mL/min
4. D33 mL/min
Answer and explanation in LearnFree account
Question 14 · Hard
An overnight timed urine is collected from 22:00 to 10:00: volume 540 mL, urine creatinine 7.2 mmol/L. Plasma creatinine is 120 µmol/L (1.36 mg/dL). What is the creatinine clearance?
1. A45 mL/min
2. B23 mL/min
3. C2,700 mL/min
4. D0.05 mL/min
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Question 15 · Hard
A 19-year-old in diabetic ketoacidosis has glucose 28 mmol/L (505 mg/dL), creatinine by Jaffe 186 µmol/L (2.10 mg/dL), creatinine by an enzymatic method 104 µmol/L (1.18 mg/dL) and urea 7.9 mmol/L. Which compound mainly explains the gap between the two creatinine results?
1. ALactate
2. BAcetoacetate
3. CFree fatty acids
4. DBeta-hydroxybutyrate
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Question 16 · Hard
A 58-year-old man with cirrhosis and marked muscle wasting has creatinine 52 µmol/L (0.59 mg/dL), giving a creatinine eGFR above 90 mL/min/1.73 m². His cystatin C eGFR is 41 mL/min/1.73 m². Which conclusion is best?
1. ABoth are valid; they reflect tubular and glomerular function
2. BCreatinine eGFR is reliable, as liver disease does not alter creatinine
3. CCreatinine eGFR overestimates GFR; cystatin C is more reliable
4. DCystatin C eGFR is falsely low because the liver clears cystatin C
Answer and explanation in LearnFree account
Question 17 · Easy
Apart from GFR, the factor that most determines a person's serum creatinine is:
1. AMuscle mass
2. BDaily fluid intake
3. CDietary carbohydrate intake
4. DUrine flow rate
Answer and explanation in LearnFree account
Question 18 · Medium
The 2021 CKD-EPI creatinine equation differs from the 2009 version in that it:
1. ANo longer uses the patient's age
2. BRequires cystatin C as well as creatinine
3. CUses body weight in place of sex
4. DNo longer includes a race coefficient
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Question 19 · Hard
Ten days after starting trimethoprim-sulfamethoxazole, an outpatient's enzymatic creatinine has risen from 82 to 101 µmol/L (0.93 to 1.14 mg/dL). Cystatin C, urea and urinalysis are unchanged. The most likely explanation is:
1. ASulfamethoxazole is interfering with the enzymatic assay
2. BThe antibiotic is increasing creatine release from muscle
3. CDrug-induced interstitial nephritis is lowering the GFR
4. DTrimethoprim is blocking tubular secretion of creatinine
Answer and explanation in LearnFree account
Question 20 · Hard
24-hour urine: volume 1,440 mL; urine creatinine 9.6 mmol/L. Plasma creatinine 160 µmol/L (1.81 mg/dL). Body surface area 1.50 m². What is the creatinine clearance corrected to 1.73 m²?
1. A0.07 mL/min/1.73 m²
2. B52 mL/min/1.73 m²
3. C60 mL/min/1.73 m²
4. D69 mL/min/1.73 m²
Answer and explanation in LearnFree account
Question 21 · Medium
After cardiac surgery, a patient's creatinine rises from 75 to 150 µmol/L (0.85 to 1.70 mg/dL) within 36 hours, and the laboratory reports an eGFR of 42 mL/min/1.73 m². How should this eGFR be interpreted?
1. AToo high: creatinine has not yet reached a steady state
2. BReliable: the equation already adjusts for age and sex
3. CReliable once indexed to the patient's body surface area
4. DToo low: surgery has increased creatinine production
Answer and explanation in LearnFree account
Question 22 · Easy
Which GFR marker is made at a steady rate by all nucleated cells and depends little on muscle mass?
1. AInulin
2. BCreatinine
3. CCystatin C
4. DUrea
Answer and explanation in LearnFree account
Question 23 · Medium
Marked hyperbilirubinemia, such as total bilirubin 450 µmol/L (26 mg/dL), usually makes a Jaffe creatinine result:
1. AFalsely high, as bilirubin's yellow color adds to the absorbance
2. BUnaffected, as the kinetic read cancels the bilirubin color
3. CFalsely low, as bilirubin is oxidized in the alkaline reagent
4. DFalsely high, as bilirubin reacts like creatinine with picrate
Answer and explanation in LearnFree account
Question 24 · Medium
An eGFR of 50 mL/min/1.73 m² must be converted to an absolute GFR in mL/min for drug dosing in a patient whose body surface area is 2.40 m². The correct calculation and result are:
1. A50 / 2.40 = 21 mL/min
2. BUnchanged at 50 mL/min
3. C50 × 1.73/2.40 = 36 mL/min
4. D50 × 2.40/1.73 = 69 mL/min
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Question 25 · Easy
Which equation is widely recommended to estimate GFR from serum creatinine, age and sex?
1. AFriedewald
2. BCKD-EPI
3. CBeer-Lambert
4. DHenderson-Hasselbalch
Answer and explanation in LearnFree account

Renal function: urea, creatinine, eGFR and clearance

Enzymatic creatinine methods are preferred over Jaffe methods in neonates and young children, mainly because:

To estimate GFR in a 6-year-old, the bedside Schwartz equation combines serum creatinine with the child's:

In the Jaffe method, creatinine forms an orange-red complex with:

A woman at 30 weeks of pregnancy has creatinine 95 µmol/L (1.07 mg/dL). The non-pregnant reference interval is 45-90 µmol/L, and in normal late pregnancy creatinine is usually 35-75 µmol/L. How should this result be interpreted?

Why can serum creatinine be about 15% higher than usual in a sample drawn 2 hours after a large cooked-meat meal?

A first eGFR of 52 mL/min/1.73 m² is found in an outpatient with a normal urinalysis. Under KDIGO, the finding that would establish chronic kidney disease is:

A 78-year-old woman has had diarrhea and vomiting for 4 days. Urea 21.0 mmol/L (reference 2.5-7.8), creatinine 140 µmol/L (45-90). The laboratory rule: urea (mmol/L) / creatinine (mmol/L) above 100 suggests a prerenal cause. What is the ratio, and which interpretation fits best?

Which patient is most likely to have a raised plasma urea while GFR is normal?

A patient receiving high-dose intravenous cefoxitin has creatinine 210 µmol/L by a Jaffe method; yesterday it was 85 µmol/L, and urea and urine output are normal. An enzymatic method on the same sample gives 88 µmol/L. The best explanation is:

Cockcroft-Gault in SI units: creatinine clearance (mL/min) = (140 - age) × weight (kg) × 1.23 / creatinine (µmol/L), multiplied by 0.85 for women. A 72-year-old woman weighs 55 kg and has creatinine 120 µmol/L (1.36 mg/dL). What is her estimated creatinine clearance?

An overnight timed urine is collected from 22:00 to 10:00: volume 540 mL, urine creatinine 7.2 mmol/L. Plasma creatinine is 120 µmol/L (1.36 mg/dL). What is the creatinine clearance?

A 19-year-old in diabetic ketoacidosis has glucose 28 mmol/L (505 mg/dL), creatinine by Jaffe 186 µmol/L (2.10 mg/dL), creatinine by an enzymatic method 104 µmol/L (1.18 mg/dL) and urea 7.9 mmol/L. Which compound mainly explains the gap between the two creatinine results?

A 58-year-old man with cirrhosis and marked muscle wasting has creatinine 52 µmol/L (0.59 mg/dL), giving a creatinine eGFR above 90 mL/min/1.73 m². His cystatin C eGFR is 41 mL/min/1.73 m². Which conclusion is best?

Apart from GFR, the factor that most determines a person's serum creatinine is:

The 2021 CKD-EPI creatinine equation differs from the 2009 version in that it:

Ten days after starting trimethoprim-sulfamethoxazole, an outpatient's enzymatic creatinine has risen from 82 to 101 µmol/L (0.93 to 1.14 mg/dL). Cystatin C, urea and urinalysis are unchanged. The most likely explanation is:

24-hour urine: volume 1,440 mL; urine creatinine 9.6 mmol/L. Plasma creatinine 160 µmol/L (1.81 mg/dL). Body surface area 1.50 m². What is the creatinine clearance corrected to 1.73 m²?

After cardiac surgery, a patient's creatinine rises from 75 to 150 µmol/L (0.85 to 1.70 mg/dL) within 36 hours, and the laboratory reports an eGFR of 42 mL/min/1.73 m². How should this eGFR be interpreted?

Which GFR marker is made at a steady rate by all nucleated cells and depends little on muscle mass?

Marked hyperbilirubinemia, such as total bilirubin 450 µmol/L (26 mg/dL), usually makes a Jaffe creatinine result:

An eGFR of 50 mL/min/1.73 m² must be converted to an absolute GFR in mL/min for drug dosing in a patient whose body surface area is 2.40 m². The correct calculation and result are:

Which equation is widely recommended to estimate GFR from serum creatinine, age and sex?