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 45

^{0}line suggest a proportional analytical error. Points falling away from the center but not on the 45^{0}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.