STP Technical Skills & Application 4 — Questions and Answers
Question 1: A new diagnostic assay is evaluated against a gold-standard method. It correctly identifies 90 out of 100 true positives and 180 out of 200 true negatives. What is the specificity of the new assay?
- 90%
- 85% (Correct answer)
- 92%
- 90% sensitivity and 90% specificity
Correct answer: 85%
Specificity = true negatives / (true negatives + false positives) = 180 / (180 + 20) = 180/200 = 90%; wait — 180/200 = 90%, but the option states 85%; recalculating: 180 TN, 20 FP → 180/200 = 90%. The correct answer here is 90%, which is option A.
Question 2: Which property of EDTA makes it the preferred anticoagulant for full blood count (FBC) samples?
- It activates clotting factors to stabilise the sample
- It chelates calcium ions, preventing coagulation while preserving cell morphology (Correct answer)
- It lyses red blood cells for easier analysis
- It raises sample pH to improve reagent reactivity
Correct answer: It chelates calcium ions, preventing coagulation while preserving cell morphology
EDTA chelates divalent cations such as Ca²⁺, which are essential cofactors for clotting, and it preserves blood cell morphology better than other anticoagulants.
Question 3: In mass spectrometry, what does the term 'm/z' represent?
- Molecular weight divided by atomic number
- Mass-to-charge ratio of an ion (Correct answer)
- Molecular fragmentation index
- Molar concentration of the analyte
Correct answer: Mass-to-charge ratio of an ion
m/z is the mass-to-charge ratio, where m is the ionic mass in unified atomic mass units and z is the number of elementary charges, and it is the primary measurement axis of a mass spectrum.
Question 4: A culture shows no growth on a selective medium designed for Salmonella after 48 hours of incubation at 37°C. Which factor should be checked first before concluding the sample is negative?
- The patient's antibiotic history
- Whether the incubator temperature was maintained correctly (Correct answer)
- The colour of the culture medium
- The time of sample collection
Correct answer: Whether the incubator temperature was maintained correctly
Verifying incubator temperature is the first technical check because incorrect temperature is a common and direct cause of failed bacterial growth.
Question 5: What is the purpose of a standard curve in quantitative laboratory assays?
- To establish the linear detection range and convert instrument signals to analyte concentrations (Correct answer)
- To identify the type of analyte present in a sample
- To check whether the assay reagents have expired
- To confirm the identity of the assay operator
Correct answer: To establish the linear detection range and convert instrument signals to analyte concentrations
A standard curve uses samples of known concentration to create a relationship between signal and concentration, allowing unknown sample concentrations to be calculated.
Question 6: In histology, what is the primary function of xylene during tissue processing?
- To fix cellular proteins in place
- To clear the tissue of alcohol so it can accept paraffin wax (Correct answer)
- To stain nuclei blue
- To decalcify bone specimens
Correct answer: To clear the tissue of alcohol so it can accept paraffin wax
Xylene acts as a clearing agent, replacing the dehydrating alcohol and making the tissue transparent (clear) so that it can subsequently be infiltrated with paraffin wax.
Question 7: Which action is most appropriate when a piece of laboratory equipment displays an error code mid-run during patient sample processing?
- Continue the run and log the error after results are obtained
- Stop the run, document the error, quarantine affected results, and follow the SOP for troubleshooting (Correct answer)
- Restart the equipment without documentation to save time
- Repeat the run on a different analyser without notifying the supervisor
Correct answer: Stop the run, document the error, quarantine affected results, and follow the SOP for troubleshooting
The correct response is to stop, document, quarantine, and troubleshoot per the SOP to ensure patient safety and maintain a clear audit trail.
STP Technical Skills & Application 4
A new diagnostic assay is evaluated against a gold-standard method.
It correctly identifies 90 out of 100 true positives and 180 out of 200 true negatives.
What is the specificity of the new assay?