Which Westgard rule acts as a warning rule to trigger careful inspection of the following control results?

• A. 10x Westgard Rules
• B. 2(2s) Westgard Rules
• C. 1(3s) Westgard Rules
• D. 1(2s) Westgard Rules

Answer: (D)

In Westgard QC, warning rules prompt a careful look at the upcoming results, while rejection rules require action to stop or halt the run. The rule that serves as a warning is the one that flags a single control result falling outside the ±2 SD limit on one control chart. This signals that something may be drifting or biased and deserves a cautious review of the current run and the immediate follow-up results to see if the issue persists. The other options describe stronger or more persistent signals: a single point beyond ±3 SD is a rejection rule, two consecutive points beyond ±2 SD on the same side is a stronger signal that can also lead to rejection, and ten consecutive results on the same side indicate a systematic shift. Therefore, the single out-of-control result at the 2-sigma level on one chart is the warning rule.