What is a Proficiency Testing Sample (PT sample)?

• A. A sample whose composition is unknown to the environmental laboratory and the analyst.
• B. A sample with known concentration.
• C. A calibration sample.
• D. A production sample.

Answer: (A)

Proficiency testing is a form of external quality assessment where the sample is designed so its true value is unknown to the laboratory and the analyst. The lab analyzes it just like any routine sample, and the result is then compared to a target value set by the PT provider and to results from other laboratories. This unknown-for-both-analyst aspect ensures the assessment measures actual measurement performance, not bias from knowing what to expect, and helps identify biases, systematic errors, or skill gaps in the laboratory’s procedures.

If the sample had a known concentration, it wouldn’t test the lab’s ability to determine an unknown; calibration samples are used to adjust instruments with known values, and production samples are real-world samples from processes, not external performance assessments.