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Read the full lessonQuality Control, Method Evaluation, and Quality Management

Laboratory Operations, quick reference 2 of 10

02 / 10 · Laboratory Operations

Common QC rule violations

Six statistical rules and the limits of their interpretation

Sources checked Reviewed by Alexan Berberian, MS, MLS(ASCP) ·

In quality control (QC), each control result is compared with its established mean and standard deviation (SD), a measure of usual variation. Random error widens the scatter around the mean. Systematic error shifts results to one side of the mean. The patterns below suggest which type to investigate.1

Vertical position = (result − mean) ÷ SD, using each level's own target. Lines: mean (0), ±1, ±2, ±3 SD. A square marks each result that meets the named rule.

12s

One result beyond ±2 SD.

1-2s example−3−2−10+1+2+3SD1234Run number →

In the classic scheme this is a warning. Check the selected rejection rules.

13s

One result beyond ±3 SD.

1-3s example−3−2−10+1+2+3SD1234Run number →

Reject the run and investigate random error first.

22s

Two in a row beyond +2 SD, or two in a row beyond −2 SD.

2-2s example−3−2−10+1+2+3SD1234Run number →

Reject the run and investigate possible systematic error.

R4s

One > +2 SD, one < −2 SD, in one run.

R-4s example−3−2−10+1+2+3SDABLevel (one run)

The two levels are 4.8 SD apart. Reject the run. This pattern suggests random error.

41s

Four in a row beyond +1 SD, or four in a row beyond −1 SD.

4-1s example−3−2−10+1+2+3SD1234Run number →

Reject the run and investigate possible systematic error.

10x

Ten consecutive on one side of the mean.

10-x example−3−2−10+1+2+3SD12345678910Run number →

Reject the run and investigate possible systematic error.

Examples use one level across runs, except R4s (two levels, one run). The 22s rule also applies across levels within a run. The 41s and 10x rules can span levels and runs as the QC plan specifies.2

Warning and rejection

In the classic Westgard scheme, 12s is a warning to check the rejection rules. Plans that evaluate every rule directly can detect the 41s and 10x examples here even though no result exceeds 2 SD.1,2

Respond to a rejection

Stop reporting affected patient results. Use the adopted QC plan to investigate controls, reagents, calibration, and instrument performance. After correction and acceptable QC, assess which patient results need review or repeat testing. An acceptable repeat control result does not identify the cause of the original failure.2

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