6.10 Clinical Chemistry Problem-Solving

Question 1

Which is the best procedure to measure proportional error in a new method for glucose?
A. Compare the standard deviation of 40 patient samples to the hexokinase method
B. Measure a mixture made from equal parts of normal and high-QC sera
C. Add increasing amounts of glucose to a sample of known concentration and measure
D. Compare the mean of 40 normal samples with the hexokinase method

Answer 1

C. Add increasing amounts of glucose to a sample of known concentration and measure

Question 2

Which of two instruments can be assumed to have the narrower bandpass? Assume that wavelength is accurately calibrated.
A. The instrument giving the highest absorbance for a solution of 0.1 mmol/L NADH at 340 nm
B. The instrument giving the lowest %T for a solution of nickel sulfate at 700 nm
C. The instrument giving the highest %T reading for 1.0% volume per volume (v/v) HCl at 350 nm
D. The instrument giving the most linear plot of absorbance versus concentration

Answer 2

A. The instrument giving the highest absorbance for a solution of 0.1 mmol/L NADH at 340 nm

Question 3

A lipemic sample gives a sodium of 130 mmol/L on an analyzer that uses a 1:50 dilution of serum or plasma before introducing it to the ion-selective electrodes. The same sample gives a sodium of 142 mmol/L using a direct (undiluted) ion-selective electrode. Assuming acceptable quality control, which of the following is the most appropriate course of action?
A. Report a sodium result of 136 mmol/L
B. Ultracentrifuge the sample and repeat by ISE
C. Dilute the sample 1:4 and repeat by ISE
D. Report a sodium result of 142 mmol/L

Answer 3

D. Report a sodium result of 142 mmol/L

Question 4

SITUATION: A 22S QC error occurs for serum calcium by atomic absorption. Fresh standards prepared in 5% w/v albumin are found to be linear, but repeating the controls with fresh material does not improve the QC results. Select the most likely cause of this problem.
A. Matrix effect caused by a viscosity difference between the standards and QC sera
B. Chemical interference caused incomplete atomization
C. Incomplete deconjugation of protein-bound calcium
D. Ionization interference caused by excessive heat

Answer 4

B. Chemical interference caused incomplete atomization

Question 5

SITUATION: A serum osmolality measured in the emergency department is 326 mOsm/kg. Two hours later, chemistry results are:
Na = 135 mmol/L
glucose = 72 mg/dL
BUN = 18 mg/dL
measured osmolality = 318 mOsm/kg
What do these results suggest?
A. Laboratory error in electrolyte or glucose measurement
B. Drug or alcohol intoxication
C. Specimen misidentification
D. Successful rehydration of the patient

Answer 5

B. Drug or alcohol intoxication

Question 6

When calibrating a pH meter, unstable readings occur for both pH 7.00 and 4.00 calibrators, although both can be set to within 0.1 pH unit. Select the most appropriate course of action.
A. Measure the pH of the sample and report to the nearest 0.1 pH
B. Replace both calibrators with unopened buffers and recalibrate
C. Examine the reference electrode junction for salt crystals
D. Move the electrodes to another pH meter and calibrate

Answer 6

C. Examine the reference electrode junction for salt crystals

Question 7

A method calls for extracting an acidic drug from urine with an anion exchange column. The pKa of the drug is 6.5. Extraction is enhanced by adjusting the sample pH to:
A. 8.5
B. 6.5
C. 5.5
D. 4.5

Answer 7

A. 8.5

Question 8

SITUATION: A patient who has a positive urinalysis result for glucose and ketones has a glycated Hgb of 4.0%. A fasting glucose performed the previous day was 180 mg/dL. Assuming acceptable QC, you would:
A. Report the glycosylated Hgb
B. Request a new specimen and repeat the glycosylated Hgb
C. Perform a Hgb electrophoresis on the sample
D. Perform a glucose measurement on the sample

Answer 8

B. Request a new specimen and repeat the glycosylated Hgb

Question 9

Quality control results for uric acid are as follows:
Run 1 Run 2 Run 3 Run 4 Mean s
QC1 3.5 3.8 4.1 4.2 3.6 0.40
QC2 6.8 7.2 7.4 7.5 7.0 0.25
Results should be reported from:
A. Run 1 only
B. Runs 1 and 2
C. Runs 1, 2, and 3
D. Runs 1, 2, 3, and 4

Answer 9

C. Runs 1, 2, and 3

Question 10

SITUATION: A peak blood level for gentamicin administered intramuscularly (therapeutic range 5–10 μg/mL) is 0.7 μg/mL. The preceding trough level was 0.5
μg/mL. What is the most likely explanation of these results?
A. Laboratory error made on peak measurement
B. Specimen for peak level was collected from wrong patient
C. Blood for peak level was drawn too soon
D. Elimination rate has reached maximum

Answer 10

C. Blood for peak level was drawn too soon

Question 11

SITUATION: A patient breathing room air has the following arterial blood gas and electrolyte results:
pH = 7.54 PCO2 = 18.5 mm Hg PO2 = 145 mm Hg
HCO3 = Na = 135 mmol/L K = 4.6 mmol/L
18 mmol/L
Cl = 98 mmol/L TCO2 = 20 mmol/L
The best explanation for these results is:
A. Blood for electrolytes was drawn above an IV site
B. Serum sample was hemolyzed
C. Venous blood was sampled for arterial blood gases
D. Blood gas sample was exposed to air

Answer 11

D. Blood gas sample was exposed to air

Question 12

SITUATION: The following laboratory results are reported. Which result is most likely to be erroneous?
Arterial blood gases:
pH = 7.42 PO2 = 90 mm Hg
PCO2 = 38.0 mm Hg HCO3= 24 mmol/L
Plasma electrolytes:
Na = 135 mmol/L Cl = 98 mmol/L
K = 4.6 mmol/L TCO2 = 33 mmol/L
A. pH
B. Na
C. K
D. TCO2

Answer 12

D. TCO2

Question 13

SITUATION: Laboratory results on a patient from the ED are:
Glucose = Na = 155 mmol/L K = 1.2 mmol/L
1,100 mg/dL
Cl = 115 mmol/L TCO2 = 3.0 mmol/L
What is the most likely explanation of these results?
A. Sample drawn above an IV site
B. Metabolic acidosis with increased anion gap
C. Diabetic ketoacidosis
D. Laboratory error measuring electrolytes caused by hyperglycemia

Answer 13

A. Sample drawn above an IV site

Question 14

SITUATION: A plasma sample from an adult male in a coma as a result of an automobile accident gave the following results:
Total CK 480 IU/L CK-MB 8 μg/L
Myoglobin 800 μg/L Troponin I 2.1 ng/L
What is the best interpretation of these results?
A. The person had a heart attack that caused the accident
B. The accident caused traumatic injury, but no heart attack had occurred
C. A heart attack had occurred in addition to a stroke
D. It is not possible to tell whether a heart attack had occurred because of the extensive trauma

Answer 14

B. The accident caused traumatic injury, but no heart attack had occurred

Question 15

SITUATION: A patient has the following electrolyte results:
Na = 130 mmol/L K = 4.8 mmol/L
Cl = 105 mmol/L TCO2 = 26 mmol/L
Assuming acceptable QC, select the best course of action.
A. Report these results
B. Check the albumin, total protein, calcium, phosphorus, and magnesium results; if normal, repeat the sodium test
C. Request a new sample
D. Recalibrate and repeat the potassium test

Answer 15

B. Check the albumin, total protein, calcium, phosphorus, and magnesium results; if normal, repeat the sodium test

Question 16

A stat plasma lithium determined by using an ISE is measured at 14.0 mmol/L. Select the most appropriate course of action.
A. Immediately report this result
B. Check sample for hemolysis
C. Call for a new specimen
D. Rerun the lithium calibrators

Answer 16

C. Call for a new specimen

Question 17

A chromatogram for blood alcohol (GC) gives broad trailing peaks and increased retention times for ethanol and internal standard. This is most likely caused by:
A. A contaminated injection syringe
B. Water contamination of the column packing
C. Carrier gas flow rate that is too fast
D. Oven temperature that is too high

Answer 17

B. Water contamination of the column packing

Question 18

SITUATION: The amylase result is 550 units/L. A 1:4 dilution of the specimen in NaCl gives 180 units/L (before mathematical correction for dilution). The dilution is repeated with the same results. Select the best course of action.
A. Report the amylase as 550 units/L
B. Report the amylase as 720 units/L
C. Report the amylase as 900 units/L
D. Dilute the sample 1:10 in distilled water and repeat

Answer 18

B. Report the amylase as 720 units/L

Question 19

SITUATION: A patient’s biochemistry results are:
ALT = 55 IU/L AST = 165 IU/L
Glucose = 87 mg/dL LD = 340 IU/L
Na = 142 mmol/L K = 6.8 mmol/L
Ca = 8.4 mg/dL Pi = 7.2 mg/dL
Select the best course of action.
A. Report results along with an estimate of the degree of hemolysis
B. Repeat LD, but report all other results
C. Request a new sample
D. Dilute the serum 1:2, and repeat AST and LD

Answer 19

A. Report results along with an estimate of the degree of hemolysis

Question 20

A blood sample is left on a phlebotomy tray for 4.5 hours before it is delivered to the laboratory. Which group of tests could be performed?
A. Glucose, Na, K, Cl, TCO2
B. Uric acid, BUN, creatinine
C. Total and direct bilirubin
D. CK, ALT, ALP, AST

Answer 20

B. Uric acid, BUN, creatinine

Question 21

An HPLC assay for procainamide gives an internal standard peak that is 15% greater in area and height for sample 1 compared with sample 2. What is the most likely cause?
A. The column pressure increased while sample 2 was being analyzed
B. Less recovery from sample 2 occurred in the extraction step
C. The pH of the mobile phase increased during chromatography of sample 2
D. There was more procainamide in sample 1 than sample 2

Answer 21

B. Less recovery from sample 2 occurred in the extraction step

Question 22

After staining a silica gel plate to determine the lecithin:sphingomyelin (L/S) ratio, the medical laboratory scientist notes that the lipid standards both migrated 1 cm faster than usual. What is the best course of action?
A. Repeat the separation on a new silica gel plate
B. Check the pH of the developing solvent
C. Prepare fresh developing solvent and repeat the assay
D. Reduce solvent migration time for all subsequent runs

Answer 22

C. Prepare fresh developing solvent and repeat the assay

Question 23

A quantitative urine glucose was determined to be 160 mg/dL by using the Trinder glucose oxidase method. The sample was refrigerated overnight. The next day, the glucose was repeated and found to be 240 mg/dL by using a polarographic method. What is the most likely cause of this discrepancy?
A. Poor precision when performing one of the methods
B. Contamination resulting from overnight storage
C. High levels of reducing substances interfering with the Trinder reaction
D. Positive interference in the polarographic method caused by hematuria

Answer 23

C. High levels of reducing substances interfering with the Trinder reaction

Question 24

SITUATION: Results of an iron profile are:
Serum Fe = 40 μg/dL TIBC = 400 μg/dL
Ferritin = 40μg/L Transferrin = 300 mg/dL
(reference range 15–200) (reference range 200–360 mg/dL)
These results indicate:
A. Error in calculation of TIBC
B. Serum iron falls before ferritin in iron deficiency
C. A defect in iron transport and not iron deficiency
D. Iron deficiency but increased release of ferritin

Answer 24

D. Iron deficiency but increased release of ferritin

Question 25

SITUATION: Results of an iron profile are:
Serum Fe = 40 μg/dL TIBC = 400 μg/dL
Ferritin = 50 μg/L
All of the following tests are useful in establishing a diagnosis of iron deficiency except:
A. Reticulocyte Hgb content
B. Erythrocyte zinc protoporphyrin
C. Serum transferrin
D. Hgb electrophoresis

Answer 25

D. Hgb electrophoresis

Question 26

Serum protein and immunofixation electrophoresis are ordered. The former is performed, but there is no evidence of a monoclonal protein. Select the best course of action.
A. Perform quantitative IgG, IgA, and IgM
B. Perform the IFE on the serum
C. Report the result; request a urine sample for protein electrophoresis
D. Perform IFE on the serum and request a urine sample for IFE

Answer 26

C. Report the result; request a urine sample for protein electrophoresis

Question 27

SITUATION: Hgb electrophoresis is performed and all of the Hgbs have greater anodal mobility than usual. A fast Hgb (Hgb H) is at the edge of the gel, and the bands are blurred. The voltage is set correctly, but the current reading on the ammeter is too low. Select the course of action that would correct this problem.
A. Reduce the voltage
B. Dilute the buffer and adjust the pH
C. Prepare fresh buffer and repeat the test
D. Reduce the running time

Answer 27

C. Prepare fresh buffer and repeat the test

Question 28

A physician asks the laboratory to use the serum from a clot tube left over from a chemistry profile run at 8 a.m. for a stat ionized calcium (Cai) at 11 a.m. What is the best course of action?
A. Perform the assay on the 8 a.m. sample
B. Perform the test only if the serum container was tightly capped
C. Perform the assay on the 8 a.m. sample only if it was refrigerated
D. Request a new sample

Answer 28

D. Request a new sample

Question 29

SITUATION: A patient’s biochemistry results are:
Na = 125 mmol/L Cl = 106 mmol/L K = 4.5 mmol/L
TCO2 = 19 mmol/L Cholesterol = 240 mg/dL
Triglyceride = 640 mg/dL Glucose = 107 mg/dL
AST = 16 IU/L ALT = 11 IU/L Amylase = 200 U/L
Select the most likely cause of these results.
A. The sample is hemolyzed
B. Serum was not separated from cells in sufficient time
C. Lipemia is causing in vitro interference
D. The specimen is contaminated

Answer 29

C. Lipemia is causing in vitro interference

Question 30

A gastric fluid from a patient suspected of having taken an overdose of amphetamine is sent to the laboratory for analysis. The only test for amphetamines performed by the laboratory is EMIT. What is the best course of action?
A. Perform an EMIT assay for amphetamine
B. Refuse the sample and request urine
C. Dilute 1:10 with H2O and filter; perform the test
D. Titrate to pH 7.0, then follow procedure for measuring amphetamine in urine

Answer 30

C. Dilute 1:10 with H2O and filter; perform the test

Question 31

SITUATION: Results of biochemistry tests are:
Na = 138 mmol/L K = 4.2 mmol/L
Cl = 94 mmol/L TCO2 = 20 mmol/L
Glucose = 100 mg/dL Total bilirubin = 1.2 mg/dL
BUN = 6.8 mg/dL Creatinine= 1.0 mg/dL
Albumin = 4.8 g/dL Total protein = 5.1 g/dL
What should be done next?
A. Request a new specimen
B. Repeat the total protein
C. Repeat all tests
D. Perform a protein electrophoresis

Answer 31

B. Repeat the total protein

Question 32

The following chart compares the monthly total bilirubin mean of Laboratory A to the monthly mean of Laboratory B, which uses the same control materials, analyzer, and method:
Level 1 Control Level 2 Control
Mean CV Mean CV
Lab A 1.1 mg/dL 2.1% 6.7 mg/dL 3.2%
Lab B 1.4 mg/dL 2.2% 7.0 mg/dL 3.6%
Both laboratories performed controls at the beginning of each shift using commercially prepared liquid QC sera stored at –20°C. Which of the following conditions would explain these differences?
A. Improper handling of the control material by Laboratory A resulted in loss of bilirubin because of photodegradation
B. The laboratories used a different source of bilirubin calibrator
C. Laboratory B obtained higher results because its precision was poorer
D. Carryover from another reagent falsely elevated the results of Laboratory B

Answer 32

B. The laboratories used a different source of bilirubin calibrator

Question 33

After installing a new analyzer and reviewing the results of patients for 1 month, the lead laboratory scientist notices a greater frequency of patients with abnormally high triglyceride results. Analysis of all chemistry profiles run the next day indicated that triglyceride results are abnormal whenever the test is run immediately after any sample that is measured for lipase. These observations point to which type of error?
A. Specificity of the triglyceride reagents
B. Precision in pipetting of lipemic samples
C. Bias caused by sequence of analysis
D. Reagent carryover

Answer 33

D. Reagent carryover

Question 34

SITUATION: The digoxin level for a stable patient with a normal ECG result was reported as 7.4 ng/mL (URL 2.6 ng/mL) by using a particle-enhanced immunoturbidimetric inhibition method. Renal function test results were normal, and the patient was not taking any other medications. The assay was repeated, and the results were the same. The sample was frozen and sent to a reference laboratory for confirmation. The result was 1.6 ng/mL measured by a competitive chemiluminescent procedure. Which of the following best explains the discrepancy in results?
A. The immunoturbidimetric inhibition method was performed improperly
B. Digoxin was lower by the chemiluminescent method because it is less sensitive
C. An interfering substance was present that cross-reacted with the antibody in the immunoturbidimetric inhibition assay
D. Freezing the specimen caused lower results by converting the digoxin to an inactive metabolite

Answer 34

C. An interfering substance was present that cross-reacted with the antibody in the immunoturbidimetric inhibition assay

Question 35

The following results are reported on an adult male patient being evaluated for chest pain:
Troponin I (Cutoff = 0.018 μg/L) CK-MB (Cutoff =4 μg/L)
Admission 0.21 μg/L 18 μg/L
1-hour post admission 0.015 μg/L 3 μg/L
2-hours post admission 0.24 μg/L 20 μg/L
What is the most likely cause of these results?
A. The wrong patient was drawn for the 1-hour post-admission sample
B. The patient did not suffer an MI until after admission
C. Hemolysis caused interference with the 1-hour sample
D. The patient is experiencing unstable angina

Answer 35

A. The wrong patient was drawn for the 1-hour post-admission sample

Question 36

Analysis of normal and abnormal QCs performed at the beginning of the evening shift revealed a 22s error across levels for triglyceride. Both controls were within the 3s limit. The controls were assayed again, and one control was within the acceptable range, and the other was slightly above the 2s limit. No further action was taken, and the results that were part of the run were reported. Which statement best describes this situation?
A. Appropriate operating procedures were followed
B. Remedial evaluation should have been taken, but otherwise, the actions were appropriate
C. Corrective action should have been taken before the controls were repeated
D. The controls should have been run twice before reporting results

Answer 36

C. Corrective action should have been taken before the controls were repeated

Question 37

A biochemical profile routinely performed bimonthly on a patient receiving renal dialysis showed decreased serum calcium and decreased PTH levels. Such a laboratory result may be explained by which of the following circumstances?
A. Malignancy
B. Aluminum toxicity
C. Hypervitaminosis D
D. Acidosis

Answer 37

B. Aluminum toxicity

Question 38

SITUATION: The laboratory reports a very high cardiac troponin level, and the attending physician questions the result because it does not correlate with the patient’s condition. The laboratory scientist suspects a false-positive result caused by a heterophile antibody. Which course of action might confirm this?
A. Have the sample assayed on a different platform
B. Dilute the specimen with saline and repeat the test
C. Call for a new specimen, and compare results
D. Ultracentrifuge the sample, and perform the test on the supernatant

Answer 38

A. Have the sample assayed on a different platform

Question 39

Hemoglobin electrophoresis performed on agarose at pH 8.8 gives the following results:
A2 Position S Position F Position A Position
35% 30% 5% 30%
All components of the Hgb C, S, F, A control hemolysate were within the acceptable range. What is the most likely cause of this patient’s result?
B. HgbLepore
B. Hgb S-β-thalassemia (Hgb S/β+)
C. Hgb SC disease after transfusion
D. Specimen contamination

Answer 39

C. Hgb SC disease after transfusion

Question 40

Two consecutive serum samples give the results shown in the table above for a metabolic function profile. The instrument is a random-access analyzer that uses two sample probes. The first probe aspirates a variable amount of serum for the spectrophotometric chemistry tests, and the second probe makes a 1:50 dilution of serum for electrolyte measurements. What is the most likely cause of these results?
A. Both patients have renal failure
B. There is an insufficient amount of sample in both serum tubes
C. There is a fibrin strand in the probe used for the spectrophotometric chemistry tests
D. The same patient’s sample was accidentally run twice

Answer 40

C. There is a fibrin strand in the probe used for the spectrophotometric chemistry tests

Question 41

SITUATION: A blood sample in a red-top tube is delivered to the laboratory for electrolytes, calcium, and phosphorus. The tube is approximately half full and is accompanied by a purple-top tube for a CBC that is approximately three quarters full. The chemistry results are as follows:
Na K Cl HCO3 Ca Pi
135 11.2 103 14 2.6 3.8
mmol/L mmol/L mmol/L mmol/L mg/dL mg/dL
What is the most likely explanation of these serum calcium results?
A. Severe hemolysis during sample collection
B. Laboratory error in the calcium measurement
C. The wrong order of draw was used for vacuum tube collection
D. Some anticoagulated blood was added to the red-top tube