Which factor can contribute to a false negative result in an initial immunoassay screen?

Which factor can contribute to a false negative result in an initial immunoassay screen?

• A. High cutoff levels that miss low concentrations
• B. Cross-reactivity causing false positives
• C. Instrument having too much noise
• D. Reagent expiration causing false positives

Answer: (A)

The key idea is how the assay’s cutoff shapes sensitivity. An initial immunoassay screen uses a threshold to decide if a result is positive. If that cutoff is set too high, samples with the drug present at low concentrations may not reach the threshold, so the test reads negative even though the drug is there. This is a false negative because a true positive is missed due to insufficient sensitivity at that cutoff. In screening tests, lowering the cutoff increases sensitivity (catching more true positives) but can raise false positives; raising it reduces sensitivity and can miss real positives. Cross-reactivity, which causes false positives, isn’t about missing true positives, and instrument noise or expired reagents can cause various errors but don’t explain missing low-concentration positives the way a high cutoff does.

The key idea is how the assay’s cutoff shapes sensitivity. An initial immunoassay screen uses a threshold to decide if a result is positive. If that cutoff is set too high, samples with the drug present at low concentrations may not reach the threshold, so the test reads negative even though the drug is there. This is a false negative because a true positive is missed due to insufficient sensitivity at that cutoff. In screening tests, lowering the cutoff increases sensitivity (catching more true positives) but can raise false positives; raising it reduces sensitivity and can miss real positives. Cross-reactivity, which causes false positives, isn’t about missing true positives, and instrument noise or expired reagents can cause various errors but don’t explain missing low-concentration positives the way a high cutoff does.