STP Technical Skills & Application 4

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?

Accepted Answer

85%

Answer

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?

Accepted Answer

It chelates calcium ions, preventing coagulation while preserving cell morphology

Answer

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?

Accepted Answer

Mass-to-charge ratio of an ion

Answer

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?

Accepted Answer

Whether the incubator temperature was maintained correctly

Answer

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?

Accepted Answer

To establish the linear detection range and convert instrument signals to analyte concentrations

Answer

A standard curve uses samples of known concentration to create a relationship between signal and concentration, allowing unknown sample concentrations to be calculated.

(STP) Scientist Training Programme Practice Test

(STP) Scientist Training Programme Practice Test

STP Technical Skills & Application 4