STP Technical Skills & Application 3 — Questions and Answers
Question 1: A laboratory result shows an unexpectedly high potassium level of 7.2 mmol/L from a routine blood sample. What is the first pre-analytical variable to investigate?
- Reagent expiry date
- Haemolysis of the sample (Correct answer)
- Calibration of the analyser
- Patient fasting status
Correct answer: Haemolysis of the sample
Haemolysis releases intracellular potassium from red blood cells, causing falsely elevated plasma potassium readings.
Question 2: In flow cytometry, what property of cells does forward scatter (FSC) primarily measure?
- Granularity (internal complexity)
- Cell size (relative volume) (Correct answer)
- Surface antigen expression
- Mitochondrial membrane potential
Correct answer: Cell size (relative volume)
Forward scatter correlates with cell size because larger cells diffract more laser light in the forward direction.
Question 3: Which technique is used to separate proteins based solely on their isoelectric point (pI)?
- SDS-PAGE
- Isoelectric focusing (IEF) (Correct answer)
- Size exclusion chromatography
- Native PAGE
Correct answer: Isoelectric focusing (IEF)
In IEF, proteins migrate through a pH gradient until they reach the pH at which their net charge is zero (their pI), effectively separating them by pI.
Question 4: A scientist records duplicate OD readings of 0.452 and 0.468 for the same sample. What is the percentage difference between these readings?
- 1.6%
- 3.5% (Correct answer)
- 5.0%
- 16%
Correct answer: 3.5%
Percentage difference = |0.468 − 0.452| / ((0.468 + 0.452)/2) × 100 = 0.016/0.46 × 100 ≈ 3.5%.
Question 5: In ELISA, what is the function of the enzyme-conjugated secondary antibody?
- It directly captures the antigen from the sample
- It amplifies the signal by reacting with a substrate to produce a detectable product (Correct answer)
- It blocks non-specific binding sites
- It serves as a negative control
Correct answer: It amplifies the signal by reacting with a substrate to produce a detectable product
The enzyme (e.g., HRP) conjugated to the secondary antibody converts a substrate into a coloured, fluorescent, or luminescent product, amplifying the detection signal.
Question 6: When should internal quality control (IQC) samples be run in a clinical laboratory?
- Once per week before patient samples
- At the start of each analytical run alongside patient samples (Correct answer)
- Only when a new reagent lot is opened
- After each batch of patient samples is completed
Correct answer: At the start of each analytical run alongside patient samples
IQC samples are run within every analytical batch to monitor the performance of the assay in real time and detect errors before patient results are released.
Question 7: Which centrifugation principle is used to separate subcellular organelles by their density rather than their size?
- Differential centrifugation
- Isopycnic (equilibrium density-gradient) centrifugation (Correct answer)
- Rate-zonal centrifugation
- Microcentrifugation
Correct answer: Isopycnic (equilibrium density-gradient) centrifugation
In isopycnic centrifugation, particles migrate until they reach the point in the gradient where their buoyant density equals that of the surrounding medium, separating by density.
STP Technical Skills & Application 3
A laboratory result shows an unexpectedly high potassium level of 7.2 mmol/L from a routine blood sample.
What is the first pre-analytical variable to investigate?