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Reading a Levey-Jennings chart

14 min

  • Recognize a shift, trend, or random scatter on a Levey-Jennings chart
  • Tell a 1-2s warning from a 1-3s rejection
  • Tell a 2-2s violation from a 1-3s violation
  • Tell a 2-2s violation from a 4-1s violation

Read the full reference

Try first

Try first

A calcium level 2 control has a mean of 11.6 mg/dL and an SD of 0.2 mg/dL. These are its last eight runs: Run12345678Result11.5 mg/dL11.7 mg/dL11.5 mg/dL11.6 mg/dL11.9 mg/dL11.8 mg/dL11.9 mg/dL11.9 mg/dL What pattern does the chart show?

The next section explains it.

Right. The next section explains why.

The next section explains it.

The next section explains it.

Get the idea

The chart

A Levey-Jennings chart plots each control result against the run in which it was measured.

  • Lines mark the mean and 1, 2 and 3 SD on either side of it, drawn from that control lot's own mean and SD.
  • Each control level has its own chart, so a result is read as its distance from its own mean in SD.1

Random and systematic error

Two kinds of error show on the chart:1

  • Random error scatters results unpredictably to both sides, as a bubble or an uneven pipette delivery would.
  • Systematic error pushes results one way. It appears as a shift, a sudden move to a new level that stays, or a trend, a steady drift in one direction. Failing reagent, a calibration problem or a temperature change can cause either one.

The Westgard rules

The Westgard rules turn those patterns into decisions. Every selected rule is checked with each new result, together with the earlier results the rule needs.2,3

RuleMet whenUsual meaning
1-2sOne result beyond 2 SDWarning: check the other rules
1-3sOne result beyond 3 SDReject: random or systematic error
2-2sTwo consecutive results beyond the same 2 SD limitReject: systematic error
4-1sFour consecutive results beyond the same 1 SD limitReject: small systematic error

Three pairs are easy to mix up:2,3

  • The 1-2s and 1-3s rules. A result between 2 and 3 SD is a warning. A result beyond 3 SD rejects the run whatever the other level shows.
  • The 2-2s and 1-3s rules. A 2-2s violation needs two results beyond the same 2 SD limit, on the same side, and neither has to reach 3 SD. The two results can be both levels in one run or one level in two runs in a row.
  • The 4-1s and 2-2s rules. A 4-1s violation needs four results beyond the same 1 SD limit, and none has to reach 2 SD, so a run can meet 4-1s with no warning at all.

When a rejection rule is met, patient results from the run stay pending release while the cause is found and corrected.3

References
  1. Clinical and Laboratory Standards Institute. Statistical Quality Control for Quantitative Measurement Procedures: Principles and Definitions. 4th ed. CLSI guideline C24Ed4E. Clinical and Laboratory Standards Institute; 2016.
  2. Westgard JO. Westgard rules and multirules. Westgard QC. Accessed September 23, 2026.
  3. Westgard JO. QC: the multirule interpretation. Westgard QC. Accessed September 23, 2026.

Watch one

Glucose runs two control levels. Level 1 has a mean of 95 mg/dL and an SD of 3 mg/dL. Level 2 has a mean of 285 mg/dL and an SD of 8 mg/dL. Both levels were near their means in the last run. Today's run gives these results:

ControlMeanSDToday
Level 195 mg/dL3 mg/dL101.9 mg/dL
Level 2285 mg/dL8 mg/dL303 mg/dL

Do you accept the run?

  1. Convert each result to SD from its mean: (101.9 − 95) ÷ 3 = +2.3 SD and (303 − 285) ÷ 8 = +2.25 SD.

    Each level is read against its own mean and SD, so the two can be compared on one scale.

  2. Check 1-3s: neither result goes beyond 3 SD, so 1-3s is not met.

    A result beyond 3 SD rejects the run on its own, so it is looked for first.

  3. Check 1-2s: both results are beyond +2 SD, so the warning is raised.

    A result beyond 2 SD is a warning that sends you to the other rules.

  4. Check 2-2s: both levels are above +2 SD in the same run, so 2-2s is met.

    Two results beyond the same 2 SD limit, both levels in one run, meet 2-2s even though neither is extreme.

  5. Name the error type: 2-2s signals systematic error, so the investigation looks at reagent, calibration and temperature.

    Both levels moved the same way, which points to something every specimen in the run shares.

Reject the run for 2-2s and keep its patient results pending release while the systematic error is found.

Your turn

Problem 1 of 3

Potassium level 1 has a mean of 3.20 mmol/L and an SD of 0.08 mmol/L. Level 2 has a mean of 6.20 mmol/L and an SD of 0.14 mmol/L. Today's run: ControlMeanSDTodayLevel 13.20 mmol/L0.08 mmol/L3.48 mmol/LLevel 26.20 mmol/L0.14 mmol/L6.25 mmol/L What does the run show?

Level 1 is (3.48 − 3.20) ÷ 0.08 = +3.5 SD from its mean, beyond the 3 SD limit. One result there rejects the run, whatever level 2 shows.

The warning covers results between 2 and 3 SD. Level 1 is +3.5 SD, so the run is rejected even though level 2 sits near its mean.

Treated a control beyond 3 SD as a warning

One control beyond 3 SD meets 1-3s and rejects the run, whatever the other level shows. The 1-2s warning covers results between 2 and 3 SD. Treating a 3 SD excursion as a warning releases patient results from a run with large random or systematic error.

2-2s needs two results beyond the same 2 SD limit. Level 2 is only +0.4 SD, so only one result is out, and it is beyond 3 SD.

Hint
  1. Work out how many SD each result sits from its own mean.
  2. Level 1 is (3.48 − 3.20) ÷ 0.08 SD from its mean.
  3. Compare that distance with the 2 SD and 3 SD limits.

Review Multirule interpretation

Problem 2 of 3

A chloride level 2 control has a mean of 116 mmol/L and an SD of 1.5 mmol/L. Its last four runs: Run1234Result117.8 mmol/L118.0 mmol/L117.7 mmol/L118.1 mmol/L Which rule, if any, is met?

2-2s needs results beyond 2 SD. The highest here is +1.4 SD.

Four results in a row are all beyond +1 SD. That meets 4-1s even though none goes beyond 2 SD.

Ignored a sequence because every control was inside 2 SD

Four consecutive results beyond the same 1 SD limit on one side meet 4-1s even when none exceeds 2 SD, as with four sodium controls between +1.33 and +1.67 SD. Waiting for a 1-2s warning misses this small systematic shift.

No result goes beyond 2 SD, so there is no warning. The sequence meets a rule on its own.

The results are +1.2, +1.33, +1.13 and +1.4 SD: four in a row beyond the same +1 SD limit. That meets 4-1s and points to a small systematic shift.

Hint
  1. Convert each result to SD from the mean.
  2. Every result is between +1.1 and +1.4 SD.
  3. Look for a rule that counts results beyond 1 SD.

Review Multirule interpretation

Problem 3 of 3

A creatinine level 2 control has a mean of 5.20 mg/dL and an SD of 0.18 mg/dL. Level 1 has been near its mean in both runs. RunLevel 2 resultYesterday, 15:005.60 mg/dLToday, 07:005.63 mg/dL Which rule is met?

Both results are beyond +2 SD, at +2.2 and +2.4 SD, in two runs in a row. That meets 2-2s, which does not wait for a result beyond 3 SD.

Looked only for a single value beyond 3 SD

Two consecutive results beyond the same 2 SD limit, in one run across levels or across two runs for one level, meet 2-2s even when neither exceeds 3 SD. Waiting for a 1-3s value lets a systematic error through.

(5.60 − 5.20) ÷ 0.18 = +2.2 SD and (5.63 − 5.20) ÷ 0.18 = +2.4 SD. One level beyond the same 2 SD limit in two consecutive runs meets 2-2s and points to systematic error.

Neither result goes beyond 3 SD. The highest is +2.4 SD.

Review Multirule interpretation

Use it

Try this at the QC workstationPendingQC run due07:00READY TO REVIEWCase 06Review QC

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