Which of these is a pre-analytical error?

Prolonged tourniquet application

Wrong wavelength on the photometer

Typing error in the final report

Internal quality control in a biochemistry laboratory is done mainly by:

Running control sera with each batch

Testing every patient sample twice

Sending samples to an outside scheme

Servicing the analyser every year

Quality control

24 Kerala PSC Lab Technician Gr II practice questions on Quality control, part of Biochemistry. The first 24 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 · Medium
Which of these is a pre-analytical error?
1. AWrong wavelength on the photometer
2. BProlonged tourniquet application
3. CUse of an expired reagent
4. DTyping error in the final report
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Answer: B · Prolonged tourniquet application
Pre-analytical errors occur before the sample is tested: wrong patient identification, wrong tube, delay, hemolysis and poor collection technique. Leaving the tourniquet on too long causes hemoconcentration, raising proteins, calcium and other protein-bound analytes. A wrong wavelength and an expired reagent are analytical errors, and a typing error in the report is post-analytical. Most laboratory errors arise in the pre-analytical phase.
- A: A wrong wavelength is an analytical error.
- C: An expired reagent causes an analytical error.
- D: A typing error in the report is a post-analytical error.
Question 2 · Hard
A glucose control gave 98, 102, 100, 104 and 96 mg/dL in five runs. Using the sample SD (n - 1), what is the coefficient of variation?
1. A3.2%
2. B2.8%
3. C8.0%
4. D10.0%
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Answer: A · 3.2%
The mean is 500 / 5 = 100 mg/dL. The deviations are -2, +2, 0, +4 and -4, whose squares add up to 40; 40 / (5 - 1) = 10, so SD = √10 = 3.16 mg/dL. CV = SD / mean x 100 = 3.16 / 100 x 100 = 3.2%. Dividing by n instead of n - 1 gives 2.8%, and forgetting the square root gives 10.0%.
- B: 2.8% comes from dividing by n (5) instead of n - 1.
- C: 8.0% is the range (104 - 96) divided by the mean, not the CV.
- D: 10.0% comes from using the variance instead of the SD.
Question 3 · Medium
Repeated measurements of a control agree closely with each other but are all well above its target value. The method is:
1. ABoth accurate and precise
2. BAccurate but imprecise
3. CNeither accurate nor precise
4. DPrecise but inaccurate
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Answer: D · Precise but inaccurate
Close agreement between repeated results means good precision, with little random error. A consistent shift away from the target value means poor accuracy, caused by a systematic error (bias). Common causes are a wrong calibrator value, a deteriorated standard or a wrong blank, and recalibration usually corrects it. Random error has other causes, such as pipetting faults or bubbles.
- A: The results miss the target, so the method is not accurate, even though it is precise.
- B: This is the reverse: the results agree closely but miss the target.
- C: The results agree closely, so the method is precise.
Question 4 · Easy
Internal quality control in a biochemistry laboratory is done mainly by:
1. ATesting every patient sample twice
2. BSending samples to an outside scheme
3. CServicing the analyser every year
4. DRunning control sera with each batch
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Answer: D · Running control sera with each batch
Internal quality control means running control materials of known range, usually at normal and abnormal levels, along with patient samples in every run. The values are plotted on Levey-Jennings charts and judged by rules such as the Westgard rules before patient results are released. It monitors day-to-day performance and detects errors as they happen. Sending samples to an outside scheme is external quality assessment, which checks accuracy against other laboratories.
- A: Duplicate testing of every sample is wasteful and does not detect a common bias.
- B: An outside scheme is external quality assessment.
- C: Servicing is preventive maintenance, not quality control.
Question 5 · Easy
The closeness of a measured value to the true value is called:
1. AAnalytical sensitivity
2. BAccuracy
3. CPrecision
4. DReproducibility
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Answer: B · Accuracy
Accuracy describes how close a result is to the true (target) value, and inaccuracy is expressed as bias. Precision describes how close repeated results are to each other, whatever the true value. Reproducibility is precision under changed conditions, such as different days or operators. Accuracy is checked with reference materials and external quality assessment.
- A: Analytical sensitivity is the smallest amount that can be detected.
- C: Precision is the agreement between repeated measurements.
- D: Reproducibility is a form of precision under changed conditions.
Question 6 · Easy
A lyophilised control serum should be reconstituted with:
1. AThe patient's serum instead of water
2. BSaline added by eye up to the label
3. CAny convenient volume of tap water
4. DThe stated volume, using a volumetric pipette
Answer and explanation in LearnFree account
Question 7 · Easy
A material of known value used to set or adjust an analyser's response is called a:
1. APatient pool
2. BCalibrator
3. CReagent blank
4. DControl serum
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Question 8 · Medium
Why are two levels of control serum, normal and abnormal, usually run with each batch?
1. ATo replace external quality assessment
2. BTo avoid having to calibrate the method each day
3. CTo double the number of patients reported
4. DTo check performance at low and high values
Answer and explanation in LearnFree account
Question 9 · Medium
To compare the precision of a glucose control at 60 mg/dL with one at 300 mg/dL, the CV is used rather than the SD because the CV:
1. ADoes not depend on the number of results
2. BExpresses the SD as a percentage of the mean
3. CIs always smaller than the SD
4. DAlso measures accuracy against the target
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Question 10 · Hard
A new rapid test was evaluated on 200 people: 80 with the disease and 120 without. It was positive in 68 of those with the disease and in 6 of those without. What is its specificity?
1. A95%
2. B91.9%
3. C85%
4. D90.5%
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Question 11 · Easy
The QC chart on which each day's control value is plotted against lines for the mean and ±1, 2 and 3 SD is the:
1. AReceiver operating curve
2. BLevey-Jennings chart
3. CYouden plot
4. DCusum chart
Answer and explanation in LearnFree account
Question 12 · Medium
Immediately after a new reagent lot is started, both control levels move to a new level above the mean and stay there. This indicates:
1. AExpected chance variation
2. BAn improvement in precision
3. CA random error
4. DA systematic error (shift)
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Question 13 · Hard
A glucose control has mean 100 mg/dL and SD 2. The last ten runs gave 101, 102, 103, 102, 101, 103, 102, 101, 102 and 103 mg/dL. What is the correct interpretation?
1. A1-3s rule broken; random error
2. BIn control; no rule is broken
3. CR-4s rule broken; random error
4. D10x rule broken; systematic error
Answer and explanation in LearnFree account
Question 14 · Medium
Why does in-vitro hemolysis make a serum potassium result unreliable?
1. AFree hemoglobin binds potassium ions
2. BHemolysis dilutes the serum potassium
3. CRed cells contain far more potassium than serum
4. DHemoglobin inhibits the potassium electrode
Answer and explanation in LearnFree account
Question 15 · Easy
A laboratory tests unknown samples sent by an outside agency and its results are compared with other laboratories. This is:
1. AInternal quality control
2. BExternal quality assessment
3. CMethod calibration
4. DReference interval study
Answer and explanation in LearnFree account
Question 16 · Easy
Leaving clotted blood unseparated for several hours before testing falsely lowers which result?
1. APhosphate
2. BLactate dehydrogenase
3. CGlucose
4. DPotassium
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Question 17 · Medium
In one run, the normal control is +2.5 SD and the abnormal control is -2.0 SD. Which rule is broken?
1. A10x, showing systematic error
2. B2-2s, showing systematic error
3. C4-1s, showing systematic error
4. DR-4s, showing random error
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Question 18 · Easy
In a Gaussian distribution of control values, about what percentage fall within the mean ± 2 SD?
1. A50%
2. B99.7%
3. C68%
4. D95%
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Question 19 · Hard
A well outpatient has serum potassium 9.8 mmol/L, calcium 3.6 mg/dL (8.5-10.5) and ALP 15 U/L (40-140). The sample is not hemolysed, and the purple-top tube was filled before the plain tube. What is the most likely cause?
1. AEDTA contamination of the serum
2. BAcute kidney injury
3. CHemolysis during collection
4. DHypoparathyroidism
Answer and explanation in LearnFree account
Question 20 · Easy
The ability of a diagnostic test to give a negative result in people who do not have the disease is its:
1. APositive predictive value
2. BPrecision
3. CSpecificity
4. DSensitivity
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Question 21 · Medium
Control values that rise slowly but steadily over several days, without crossing the 3 SD line, show a:
1. AShift
2. BRandom error
3. CTrend
4. DOutlier
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Question 22 · Medium
Two consecutive results of the same control are both above +2 SD. Which rule is broken, and what type of error is suggested?
1. A1-2s; no error
2. B2-2s; systematic error
3. CR-4s; random error
4. D1-3s; random error
Answer and explanation in LearnFree account
Question 23 · Hard
A cholesterol control has mean 120 mg/dL and SD 5. Today it reads 136 mg/dL. What is its SD index, and what should be done?
1. A+2.2 SD; warning only
2. B+1.6 SD; accept the run
3. C+3.2 SD; reject the run
4. D+3.2 SD; accept the run
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Question 24 · Easy
A single control result beyond the mean ± 3 SD violates which Westgard rule?
1. A1-2s
2. B1-3s
3. C10x
4. D2-2s
Answer and explanation in LearnFree account

Quality control

Which of these is a pre-analytical error?

A glucose control gave 98, 102, 100, 104 and 96 mg/dL in five runs. Using the sample SD (n - 1), what is the coefficient of variation?

Repeated measurements of a control agree closely with each other but are all well above its target value. The method is:

Internal quality control in a biochemistry laboratory is done mainly by:

The closeness of a measured value to the true value is called:

A lyophilised control serum should be reconstituted with:

A material of known value used to set or adjust an analyser's response is called a:

Why are two levels of control serum, normal and abnormal, usually run with each batch?

To compare the precision of a glucose control at 60 mg/dL with one at 300 mg/dL, the CV is used rather than the SD because the CV:

A new rapid test was evaluated on 200 people: 80 with the disease and 120 without. It was positive in 68 of those with the disease and in 6 of those without. What is its specificity?

The QC chart on which each day's control value is plotted against lines for the mean and ±1, 2 and 3 SD is the:

Immediately after a new reagent lot is started, both control levels move to a new level above the mean and stay there. This indicates:

A glucose control has mean 100 mg/dL and SD 2. The last ten runs gave 101, 102, 103, 102, 101, 103, 102, 101, 102 and 103 mg/dL. What is the correct interpretation?

Why does in-vitro hemolysis make a serum potassium result unreliable?

A laboratory tests unknown samples sent by an outside agency and its results are compared with other laboratories. This is:

Leaving clotted blood unseparated for several hours before testing falsely lowers which result?

In one run, the normal control is +2.5 SD and the abnormal control is -2.0 SD. Which rule is broken?

In a Gaussian distribution of control values, about what percentage fall within the mean ± 2 SD?

A well outpatient has serum potassium 9.8 mmol/L, calcium 3.6 mg/dL (8.5-10.5) and ALP 15 U/L (40-140). The sample is not hemolysed, and the purple-top tube was filled before the plain tube. What is the most likely cause?

The ability of a diagnostic test to give a negative result in people who do not have the disease is its:

Control values that rise slowly but steadily over several days, without crossing the 3 SD line, show a:

Two consecutive results of the same control are both above +2 SD. Which rule is broken, and what type of error is suggested?

A cholesterol control has mean 120 mg/dL and SD 5. Today it reads 136 mg/dL. What is its SD index, and what should be done?

A single control result beyond the mean ± 3 SD violates which Westgard rule?