Clinical and Laboratory Standards Institute guideline C24—Statistical Quality Control for Quantitative Measurement Procedures: Principles and Definitions discusses the principles of statistical QC, with particular attention to the planning of a QC strategy and the application of statistical QC in a medical laboratory. Although these principles are of interest to manufacturers, this guideline is intended for use by medical laboratory personnel in order to provide a QC strategy that uses control materials that are external to a reagent kit, instrument, or measuring system and that are intended to simulate the measurement of a patient specimen.

This guideline replaces the previous edition of the approved guideline, C24-A3, published in 2006. The fourth edition maintains the focus on principles and approaches to laboratory QC design, implementation, and assessment that reflect the realities of the modern medical laboratory and its role within the health care enterprise. Several changes were made in this edition, including: 

• The alignment of principles and definitions to be consistent with and to supplement the general patient risk model described in CLSI document EP23™2 

• The introduction of additional performance measures useful for evaluating the performance characteristics of a QC strategy (see Chapter 5) • Expanded guidance on setting target values and SDs for QC materials (see Subchapter 5.3) 

• A greater focus on QC frequency and QC schedules as a critical part of a QC strategy (see Subchapter 5.5) 

• A substantive chapter on recovering from an out-of-control condition (see Chapter 6), including sections on: 

– Responding to an out-of-control QC event 

– Responding to an out-of-control condition 

– Identifying and correcting reported erroneous patient results

This guideline explains the purpose of statistical QC for quantitative measurement procedures, describes an approach for planning a QC strategy for a particular measurement procedure, describes the use of QC material and QC data, and provides examples that demonstrate a practical QC planning process for medical laboratories. 

The recommendations for establishing and maintaining a statistical QC strategy are applicable to quantitative laboratory measurement procedures in all fields of laboratory medicine for which stable control materials can be measured in the same manner as patient specimens. The intended users of this guideline include those responsible for designing, implementing, and using QC, ie, medical laboratory scientists. 

This guideline does not: 

• Describe built-in control mechanisms that might be part of a measuring system, or qualitative or semiquantitative measurement procedures. • Define specific QC strategies that are appropriate for an individual device or technology. 

• Describe alternatives to statistical process control, eg, real-time patient-based QC.

• Consider specific legal requirements that may impose different philosophies or procedures on QC practices (eg, a specific approach for defining quality requirements, specific values for quality requirements, a specific procedure for determining target values for control materials, or a frequency and number of QC measurements) defined by government regulation in a specific country or region. 

Additionally, there are types of random errors that may affect measurements performed on individual specimens, rather than a whole group of specimens, and those errors are not detected by a statistical QC strategy. Such errors may be due to the specific design of a measuring system (eg, effect of specimen viscosity, carryover from a previous specimen, or specimen-specific interferences) or possible operator errors that affect individual specimens, as well as preexamination errors of specimen preparation, storage, and transportation. Special QC strategies may be needed to monitor known special vulnerabilities that relate to a particular device or system design.