Which QA sample is designed to be indistinguishable from production samples but known to be free of the analyte?

• A. Proficiency testing sample.
• B. Blank spike.
• C. Method blank.
• D. Reference sample.

Answer: (C)

In QA testing, you want to verify that what you measure in real samples isn’t coming from the process itself. The key idea is to have a sample that looks and is treated exactly like a production sample, but contains no analyte. This is a method blank. By processing it through all steps—reagents, tubes, instruments, and procedures—you expose any background signals, contamination, or carryover introduced by the method. If the method blank shows a signal, you know the lab’s workflow or reagents are contributing something other than the real sample, so you can account for it or fix it before interpreting production results. The other options serve different QA purposes: proficiency testing checks overall performance against external standards; a blank spike is used to study recovery by adding known analyte to a blank; a reference sample provides a known concentration for calibration or verification, not to monitor process contamination.