Which Westgard rule is violated when two consecutive control values exceed the mean ±2SD?

• A. 1(2s) Westgard Rules
• B. 1(3s) Westgard Rules
• C. 2(2s) Westgard Rules
• D. R(4s) Westgard Rules

Answer: (C)

Westgard rules are statistical criteria used to decide when QC results indicate a problem with the measurement process. When two consecutive control values both fall beyond the same limit of ±2 standard deviations from the mean, this pattern signals a systematic issue such as a shift or bias that is not just random noise. Requiring two in-a-row outliers on the same side helps avoid overreacting to a single excursion and flags the run for rejection or investigation because the process appears to be consistently off.

If you see only one value outside ±2 SD, that would be a warning under a different rule. A single value outside ±3 SD would trigger a separate rule, as would four consecutive values outside ±1 SD on the same side or a pair of consecutive results with a range exceeding 4 SD (R4s). These patterns describe different scenarios than two consecutive points beyond the same ±2 SD limit, which is exactly what the two-consecutive 2 SD rule captures.