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The 13s rule identifies unacceptable random error or possibly the beginning of a large systematic error. Any QC result outside ±3SD violates this rule. Since only 0.3% or 3 out of 1000 points will fall outside the ±3SD limit, any value outside of ±3SD is usually considered to be associated with a significant error condition.
While a value outside ±3SD may be statistically significant, it may not be biologically or medically relevant due to the fact that modern laboratory instruments are often more precise than what is needed medically.
The following figure shows a Levey-Jennings Chart with a data point outside the ±3SD limit.
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