Which Westgard rule is commonly considered a warning when a single control measurement exceeds the 2 standard deviation limit?

• A. 2-2s
• B. 1-2s
• C. R-4s
• D. 4-1s

Answer: (B)

Westgard rules guide when a control result signals a potential problem versus just normal variation. A single control result that lies beyond the ±2 standard deviation boundary is treated as a warning. This threshold is used because one excursion past 2 SD could simply be random fluctuation or a minor, transient issue. It prompts a check of the assay, instrument, or sample and, if needed, a repeat measurement, without automatically discarding the run.

The other patterns require more sustained evidence and are treated as stronger indications of a real issue: two consecutive results beyond ±2 SD on the same side point to a real shift; ranges spanning more than 4 SD between adjacent results suggest random error across the run; and four consecutive results beyond ±1 SD on the same side imply bias. These are more definitive signals than a single outlier.