Sunday, November 11, 2012


All the procedures described earlier have focused on monitoring a single laboratory. These are called internal quality control. In this EQA there is comparison of performance of different laboratories called external quality assessment. The internal QC is necessary for daily monitoring the precision and accuracy of the analytical method, and EQA being important for maintaining the long-term accuracy of the analytical methods.

External Quality Assessment:

These are sponsored by professional societies and manufacturers of control materials. The basic operation of these programs involves having all the participating laboratories analyze the same lot of control material, usually daily as part of internal QC activities. The results are tabulated monthly and sent to the sponsoring group for the data analysis. Summary reports are prepared by the program sponsor and distributed to all participating laboratories. 

These programs are useful only for monthly reviews and period problem solving activities. But due to development in information system real-time external QC is possible. The mean of all results or the mean of results from peer laboratories (those performing the test with similar methods) is taken as the target value and is used for comparison with the individual laboratory’s result.

Statistical significance of any difference between an individual laboratory’s observed result and the group mean can be tested by t-test. When the difference is significant, the laboratory is alerted that its results are biased compared with the results of other laboratories. Another approach is to divide the difference by the overall standard deviation of the group, and then to express the difference in terms of the number of SDs

SDI = (Laboratory result – group mean)/Group SD
Where SDI is SD interval or index and Group s is the SD for group of subsets. Differences greater than 2 or 3 indicate that a laboratory is not in agreement with the rest of the laboratories in the program. The SDI is a statistical calculation that is used for peer group comparisons. Participation in interlaboratory QC comparison programs provides peer group QC data based on the same methodology and the same instrument for the same lot number of control material. The SDI can be used to assess the validity of individual laboratory results as compared to a peer group QC result or proficiency sample by calculating the probability of the result obtained. It relates the bias, or difference from the true result, as compared with the SD expected. This calculation is used to assess PT results and other aspects of QA.

Fig. EQAS Cycle  (Source: Tietz clinical chemistry, 4th edition)                 
Additional information about the nature of systematic error is obtained when there are two different control materials analyzed by each laboratory. For example the laboratory observed mean for material A is plotted on y-axis versus its observed mean for material B on the x-axis; these graphs are called Youden plots. Ideally the point for a laboratory should fall at the center of the plot. Points falling away from the center but on the 450 line suggest a proportional analytical error. Points falling away from the center but not on the 450 line suggest either an error that is constant for both materials or an error that occurs either in just one material.

The inner square of the plot (yellow) represents one standard deviation (1SD). The next larger square (green) represents 2SD, and the outer square (blue) represents 3SD. A horizontal median line is drawn parallel to the X-axis and a second median line is drawn parallel to the Y-axis. The intersection of the two median lines is called the Manhattan Median. One or two 45-degree lines are drawn through the Manhattan Median. The results of at least two different levels of controls (e.g. Level 1/Level 2 or Normal/Abnormal) are then plotted on the chart as X-axis versus Y-axis.

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