Which Westgard rule is described by two consecutive control values where the first exceeds the mean +2SD and the following exceeds the mean −2SD?

• A. 1(3s) Westgard Rules
• B. 2(2s) Westgard Rules
• C. 10x Westgard Rules
• D. R(4s) Westgard Rules

Answer: (D)

The pattern being tested is two consecutive control measurements that straddle the mean by 2 standard deviations in opposite directions—the first above +2 SD and the next below −2 SD. That creates a gap of more than 4 SD between the two values, signaling a potential shift or instability between successive runs. This exact scenario is what the R 4s rule is designed to detect: two consecutive results that are more than 4 SD apart because they sit on opposite sides of the mean. The other rules describe different patterns (a single value beyond 3 SD, two consecutive values beyond 2 SD on the same side, many consecutive results on one side, etc.), which don’t match this crossing pattern.