2-2s

The 2-2s rule detects systematic error only. The 2-2s rule is violated when two consecutive QC results are:

• Greater than 2SD.
• On the same side of the mean.

The rule is applied both within a run and across runs:

• Within run violation

  Two control values in the same run are >2SD on the same side of the mean. Violation of the within run application indicates systematic error is present and potentially affecting the entire analytical curve.

• Across run violation

  In one run a single level of control is >2SD from the mean. In the next run, the same level of control is >2SD on the same side of the mean. Violation of the across run application indicates systematic error is present but affects only a single portion of the analytical curve.

The following figures show a Levey-Jennings Chart with a data point violating the 2-2s rule within and across a run.