Question

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : If the device is left out on the worktable "faint ghost band" appears at the test region.
Reason (R) : The unbound conjugate deposited on the test band results in appearance of a faint ghost band.
In the light of the above statements, choose the most appropriate answer from the options given below :

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A)
• C. (A) is correct but (R) is not correct
• D. (A) is not correct but (R) is correct

Hint:

Never interpret a lateral flow assay outside the manufacturer's recommended time window! Evaporation lines (ghost bands) are the most common cause of false-positive interpretations in at-home testing.

Answer 1

The Correct Option is A

Step 1: Concept:
The question tests practical laboratory knowledge regarding the troubleshooting and interpretation of rapid diagnostic lateral flow assay (LFA) devices.

Step 2: Step-by-step Explanation:

• Analyzing Assertion (A): In lateral flow tests (like rapid antigen tests or pregnancy strips), instructions explicitly state to read the results within a specific time window (usually 15-30 minutes). If the device is left out on the worktable for hours, a faint line often appears at the test region regardless of whether the sample was actually positive. This non-specific line is known as an evaporation line or "ghost band". Therefore, Assertion (A) is correct.

• Analyzing Reason (R): The mechanism behind this phenomenon relies on fluid dynamics. During a test, colored antibody conjugates (usually gold nanoparticles) sweep across the nitrocellulose membrane. In a negative test, they do not bind and continue flowing into the absorbent pad. However, as the device is left out to completely dry, the capillary flow stops, and any residual fluid remaining in the membrane evaporates.

• This evaporation causes whatever unbound conjugate remains suspended over the test line to precipitate and physically deposit non-specifically onto the nitrocellulose membrane. This creates the optical illusion of a faint positive line (the ghost band).

• Therefore, Reason (R) is correct, and it perfectly explains the biochemical/physical mechanism causing the event described in Assertion (A).

Step 3: Final Answer:
Both statements are true, and R correctly explains A.