Quality control (QC) is an important attribute of any laboratory’s quality assurance (QA) program. Without QC, there is no confidence in the results of analytical tests. As described in Part 1000, essential QC measures include method calibration, reagent standardization, assessment of each analyst’s capabilities, analysis of blind check samples, determination of the method’s sensitivity [method detection level (MDL) or quantification limit], and regular evaluation of bias, precision, and the presence of laboratory contamination or other analytical interference. The details of these procedures, their performance frequency, and expected ranges of results should be formalized in a written QA manual and standard operating procedures. In addition, it is the laboratory’s responsibility to qualify and report data values not meeting QC or other method-defined requirements with sufficient information so the client or end user can determine the usability of the qualified data.
While general information on QC procedures is provided in Part 1000 and specific procedures are typically outlined in individual methods, some of the methods in Part 2000 are not amenable to standard QC procedures; they have procedures considered unique to the method that do not necessarily apply to other more conventional analytical methods. For some methods, such as oxygen-consumption rate, bias is not applicable. Several methods in this part do not have acceptance-criteria guidance for either precision or bias of test results. This does not, however, relieve analysts of the responsibility for evaluating the test’s accuracy and precision. Laboratories should generate method-specific acceptance criteria for precision or bias (or both) using control-charting techniques.
Evaluate precision by analyzing duplicate samples. However, if these results are “nondetect” or “invalidated,” precision cannot be calculated. Laboratory-fortified matrices (LFMs) are not applicable to methods currently in Part 2000, so Table 2020:2 has no entry in the LFM column.
Evaluate bias by analyzing standards or samples with known or certifiable parameter values. If a known or certifiable standard analyte cannot be prepared or is otherwise unavailable, then bias cannot be calculated.
To help verify the accuracy of calibration standards and overall method performance, participate in an annual or preferably semi-annual program of analysis of single-blind QC check samples (QCS)—ideally provided by an external entity. Such programs are sometimes called proficiency testing (PT)/performance evaluation (PE) studies. An unacceptable result on a PT sample is often a strong indication that a test protocol is not being followed successfully. Investigate circumstances fully to find the cause. In many jurisdictions, participation in PT studies is a required part of laboratory certification and accreditation.
Laboratories may save time and money by purchasing premade standards, titrants, and reagents, but they still must perform the QC checks on these materials required by the analytical methods.