The concentration of the low calibration verification standard must be within which range?

• A. Within the central 50% of the curve.
• B. Within the lower 20% of the calibration curve and not more than five times the lowest quantitation level.
• C. Within the upper 20% of the curve.
• D. Exactly equal to the lowest quantitation level.

Answer: (B)

The important idea here is testing the assay’s performance where it’s most vulnerable—the low end of the calibration range. A low calibration verification standard is used to confirm that the method can accurately quantify near the limit of quantitation (LOQ) and that the response remains reliable as you approach the bottom of the curve. Placing this standard in the lower 20% of the curve ensures you’re evaluating accuracy and precision where signal is weakest and potential issues like poor sensitivity or higher relative error are most likely to occur. Adding the cap that it should not exceed five times the lowest quantitation level keeps the verification firmly within the low end of the curve, preventing the test from drifting into higher, less challenging regions of the range.

Why the other positions aren’t as appropriate: testing a mid-range (central 50%) shifts focus away from the low end where quantitation is hardest and can miss problems near LOQ; testing the upper end (upper 20%) checks high-concentration performance rather than low-end capability; and requiring the verification concentration to be exactly at LOQ doesn’t allow a little margin to assess consistency around that threshold. The combination in this approach best ensures reliable performance where it matters most for quantitative accuracy.