Ensuring test quality

Ensuring test quality in any laboratory is a compulsory and urgent requirement. A test in any field must be committed for its accuracy and trueness.

In the field of production, the test result of products’ quality plays an important role in assessing whether the products meet the technical requirements of authorized departments, market and the quality standards. Based on such assessments, users are protected from low quality goods.
In clinical and medical areas, diagnosis results are the base for doctors in making decision on what the problem is and which treatment is appropriate. So, the need of an accurate diagnosis highly arises in order to reduce risks for patients.
For managing and protecting environment activities, results of onsite analysis shall be ensured for its objectivity in reflecting the environmental conditions at the time of analysis. Those results shall be considered as warnings for unexpected development or environments’ pollution and degradation.
Recently, there have been a huge development in both quantity and aspect of laboratories in Vietnam. On the trend of economic globalization, these laboratories have been taken step by step the standardization of their activities by applying quality management standards. In the scale of laboratory, ISO/IEC 17025 is the most popular standard in Vietnam in particular and also all over the world. Ensuring test quality is one of the mandatory requirements given by ISO/IEC 17025. A question arose when applying ISO/IEC 17025 is that most laboratories take difficulties in choosing and developing methods for ensuring their test quality.
To answer the question of how to control the objectivity, accuracy as well as the quality of a test, two parallel activities shall be performed, that are: quality assurance and quality control.
QUALITY ASSURANCE (abbreviated as QA) is all planned and oriented activities that are performing in a quality management system. QA is a broad and crucial sector of any laboratory. It is related to many aspects such as policy, organization, training activities, facility, equipment, method selection, SOPs, sample treatment, report, and competence assessment.
QUALITY CONTROL (abbreviated as QC) focuses on implementing quality requirements related to technical issue. For a quality management system, in general, and testing field, in particular, QC is a part of QA that find out and solve the errors appeared in a test. There are two types of QC: internal QC and external QC.
Internal QC is a tool for daily quality assessment of a laboratory and taken by a lab’s staff in order to continuously evaluate factors that might affect the results’ quality, from what the accuracy of the test shall be determined before reporting.
The aim of IQC are: 

  • To detect and to classify errors (random error, systematic error);
  • To Study the errors’ causes and propose proper solutions to prevent the occurrence of systematic errors;
  • To evaluate the reliability of the test result;
  • To evaluate methods, equipment, chemicals, reagents and staffs’ skills. 
IQC methods:  IQC method shall be different depending on test method and include, but not limited to, the following types: 
  • The use of standards: standards sample are samples with known concentration and used along with other samples from the beginning to the end.
  • The test repetition or method comparison: repeat the test at least 2 times for each parameter and each sample or use another suitable method to compare the results.
  • The stored sample test: After the test, sample shall be stored at reasonable condition that could not affect the sample characteristics, then, such stored sample shall be retest to evaluate the repeatability and reproducibility.
  • The use of blank sample, reagent, and media (negative control): use reagent or media as a blank sample (negative control) along with real sample from the beginning to the end of the test.
  • The use of spiked sample: use real sample with a spiked amount of standard analyte, test the spiked sample along with the real non-spiked sample.
  • Evaluating the staff competence: use criteria such as repeatability, reproducibility, trueness, etc. to evaluate staff competence. Depends on parameter to choose appropriate evaluation method.
  • Tracking the result and analyzing the trend: tracking results, building diagrams and analyzing the tendency of the test procedure. To find out the causes, propose proper preventive and corrective actions. 
Control charts: there are two types of charts that have been widely used in a laboratory, they are: Mean chart for QC sample and range chart.
Mean chart: as also known as the chart to evaluate the trueness of a method. It is initiated from the mean and standard deviation of QC sample. This chart includes an upper warning limit (UWL), a lower warning limit (LWL), an upper control limit (UCL) and a lower control limit (LCL) of the mean concentration. Usually, these limits are associated with ± 2s and ± 3s rule (s is the standard deviation). If the concentration varies within a wide range, the recovery chart shall be applied.
During the analysis, if the concentration in QC sample is out of the range limited by UWL and LWL, the result of unknown sample shall not be accepted. This chart is also used for annual evaluation to neglect the method’s disadvantages and continuously improve the advantage.

 Figure 1. Mean chart of calcium content in a QC sample
Range chart: is also known as chart for accuracy evaluation of a method. It is also built based on the mean and standard deviation of QC sample but differs from mean chart, the range chart represents the deviation of repeated analysis or iterative analysis over time. If the results collected from repeated analyses, the base line shall be considered to be zero. So, only Upper Warning Limit (UWL) and Upper Control Limit (UCL) are meaningful.

Figure 2: Range chart that represents the deviation between 2 repeated analyses of Calcium content in QC sample. (Each point is the deviation between 2 paralleled analyses in a day).
If the standard deviation is known, standard deviation shall be converted into limitation by using a conversion factor.
Average limitation: 
Where D2 is the conversion factor from standard deviation into limitation; D2 = 1.128 if the number of repetition is 2, D2 = 1.693 if the number of repetition is 2; SR is the standard deviation of the limit
Warning limit: 
Control limit: 
Correction method:
Statistical parameter Situation Action
Upper Control Limit 1.   A data point is over the UCL
2a. repeated analyses < UCL
2b. Repeated analyses > UCL
1.   Redo the analysis
2a. Continue to analyze
2b. Stop and recalibrate
Upper warning limit 1.   2 to 3 data points are over the UWL
2a. The next data point < UWL
2b. The next data point > UWL
1.   Analyze another sample
2a. Continue to analyze
2b. Stop and recalibrate
Standard deviation 1.   4 to 5 data points > 1 SD
2a. The next data point < 1 SD
2b. The next data point > 1 SD
1.    Analyze another sample
2a. Continue to analyze
2b. Stop and recalibrate
Centerline (CTL) 1.   A data point is on the CTL
2a. The next data point is under the CTL
2b. The next data point is above the CTL
2.    Analyze another sample
2a. Continue to analyze
2b. Stop and recalibrate
 External Quality Assessment (EQA)
In the past, there were two different terms “External Quality Control” (EQC) and “External Quality Assurance” (EQA) to name the external quality evaluation. Later on, to emphasize the objectivity of the external evaluation that must be taken by an independent organization, the term “External Quality Assessment” (EQA) have been being used and become a recognized term all over the world.
The aim of EQA: EQA is an important tool that helps to monitor the test quality, in details: 
  • Evaluate and monitor continuously the operation of the test of participating laboratories;
  • Define the risks that might affect the test quality, thence propose proper preventive action in order to enhance the test quality;
  • Compare the test results among participating laboratories and among methods;
  • Prove the reliability of the test result to customers;
  • Estimate the uncertainty of the test result;
  • Evaluate the method’s characteristics;
  •  Supply documents for the continuous training activities aimed at laboratory’s staff in order to meet the regulation of authorized agencies, accreditation bodies and laboratory’s desires.

EQA methods: there are three EQA methods: rechecking/retesting, On-site evaluation, and Proficiency Testing.
Rechecking/retesting: the host laboratory selects sample(s) randomly and send to other laboratories or reference laboratory to recheck and re-evaluate the results obtained by the host laboratory.
On-site evaluation: Authorized agencies or accreditation bodies shall establish an assessment team for the periodical or sudden assessment. Such assessments require to be done by experts that have technical and experience competence on quality management. So the mainspring in applying this method does not belong to laboratories. 
Proficiency Testing (PT): is to assess the performances on the same homogenous samples by laboratories in given conditions.
PT provider shall distribute PT sample(s) to participants. Participants are responsible for analyzing given sample and reporting the result to the provider. The statistical data analysis shall be done by a statistical expert of the provider to evaluate the competence on doing such analyses. The reports then shall be sent to participating laboratories, based on the statistical results, a laboratory shall review and propose appropriate corrective actions (if any).
 The PT method is much more effective from the view of ensuring test quality. The requirement of PT is that the provider must be an independent organization holding enough competence to organize and operate the program. Such competence shall be evaluated by clarifying whether the operation of that PT provider meets the requirements of ISO/IEC 17043:2010 and whether it is accredited by an accreditation body.
In recent years, the PT demand stays high but the number of PT providers is limited, and the number of accredited providers is even fewer. 

Author: VOVNews/VNA

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