Internal Quality Controls in the Medical Laboratory: A Narrative Review of the Basic Principles of an Appropriate Quality Control Plan

Abstract

To ensure the quality of their analyses, medical laboratories carry out internal quality control (IQC) on a daily basis. IQC involves control samples with known target values for all parameters used by a laboratory in clinical practice. The use of IQC enables the laboratory to monitor the accuracy and precision of laboratory results. The use of appropriate IQC strategies has been accepted in medical laboratories for decades, and IQC has been included in international recommendations and guidelines. The term "IQC strategy" (also termed a quality control plan) refers to the types of IQC materials to be measured, the frequency of IQC events, the number of concentration levels in each IQC event, and the IQC rules to be used. A scientifically sound IQC strategy must follow two principles, namely, (1) statistical follow-up on the IQC results generated in the laboratory by means of Levey-Jennings control charts and Westgard rules (i.e., quality control by means of statistical procedures) and (2) the determination of limits on the basis of medical considerations and the definition of analytical goals (quality control on the basis of medical relevance). In this narrative review, we describe the fundamental principles of an adequate IQC strategy for laboratorians and nonlaboratorians.

Keywords: bias (MeSH ID: D015982); diagnostic errors (MeSH ID: D003951); laboratories, clinical (MeSH ID: D000090464); quality control (MeSH ID: D011786); risk assessment (MeSH ID: D018570).

Figure 1

A Levey–Jennings control chart. Figure 1 shows an example of a laboratory parameter over a period of 4 weeks with an IQC on weekdays from Monday to Friday (fictitious data). The abscissa indicates the time course. The left ordinate refers to the concentration of IQC values expressed in ng/L. The right ordinate shows the mean of the IQC values measured in these 4 weeks (i.e., “0” and the corresponding horizontal solid red line) and the standard deviation of the measured IQC values or its multiples (i.e., “−1” to “+1” corresponds to the range of ± one standard deviation, and “−3” to “+3” corresponds to the range of ± three standard deviations; horizontal dotted red lines). The blue dots indicate the measured IQC values.

Figure 2

Levey–Jennings control charts for total protein at two concentration levels. Figure 2 shows an example of a Levey–Jennings control chart for total protein over a period of two months at two different IQC levels (actual data from the Voecklabruck laboratory). The abscissa shows the time course. The left ordinate refers to the two mean values of the IQC values measured during these eight weeks, expressed in g/L (“lab mean”), and shows the corresponding 1 s ranges, 2 s ranges and 3 s ranges at the two concentration levels (gray horizontal stripes). The right-hand ordinate shows the target value (mean value of the peer group, i.e., the group mean) with an associated red line and the TEa defined by laboratory management (upper and lower limits, referred to as “UL” and “LL” in the figure). Based on a floating mean, the blue and green dots indicate the IQC values without rule violation, the red dots indicate those IQC values that were rejected by the Westgard rules, and the red triangles indicate those IQC values that were outside the TEa. The gray vertical lines indicate the month break and the red vertical line the time point of a recalculation of the lab mean.