control limits

Control limits are boundaries used to interpret results from quality-control materials. In quantitative systems they are often derived from a control mean and standard deviation, although the decision scheme may use additional rules or other statistical approaches. Their purpose is to identify potentially important changes in analytical behavior, not to describe the distribution of results in healthy patients.

Establish limits using suitable data and a documented process. Manufacturer ranges can help during initial setup but may not represent local precision or provide sufficient error detection. Review new control lots, changes in method performance, and the adequacy of the data supporting assigned values. Do not broaden limits simply to accommodate unexplained failures. A limit violation is a signal for defined action, not a diagnosis of the underlying cause.

Accreditation review considers whether control limits are justified, approved, and used consistently. Staff should distinguish them from reference intervals, reportable ranges, and allowable-error specifications. Retain records of limit changes with reasons and effective dates, so historical control results remain interpretable and the laboratory can demonstrate that changes did not conceal deteriorating performance.