QC - THE LEVEY-JENNINGS CONTROL CHART

Patricia L. Barry, BS, MT(ASCP)

• Answers for this exercise
• Interpretation of control results
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 Answers for this exercise

Interpretation of control results - with 1_2s and 1_3s rules

Use of a 1_2s rule as a strict rejection rule would result in rejecting runs on days 5, 6, 8, 11, 13, 14, 17, 25, and 27, for a total of 9 runs, as shown by the check marks in the column for 1_2s rule violations.

Use of a 1_3s rejection rule would lead to rejection of only one run on day 5, as shown by the single check mark in the column for 1_3s rule violations.

It makes a big difference what control rule is being applied -- 9 rejections vs 1 rejection!
 

What if different control rules were used?

Given that the 1_2s rule is known to cause a high level of false alarms or false rejections, it might be better to interpret the data more carefully, in effect applying additional control rules, such as the 2_2s and R_4s rules:

• 2_2s indicates a rejection when two consecutive control values exceed the same mean +2s limit or the same mean -2s control limit; this rule is sensitive to shifts in the mean of the distribution, therefore it is a good indicator of increases in systematic error or changes in the accuracy of the method.
• R_4s indicates a rejection when one control measurement in a run exceeds a +2s control limit and another exceeds a -2s control limit. This "range" rule is sensitive to changes in the width of the distribution, therefore it is a good indicate of increases in random error or changes in the precision of the method.

Use of the 1_3s rule together with the 2_2s and R_4s rules leads to a multirule QC procedure in which multiple decision criteria are applied simultaneously. If any single control rule is violated, the run is rejected. Here's how the 1_3s/2_2s/R_4s multirule procedure would be interpreted for this example set of control results:

• Day 5.  The value for Control 1 exceeds a -3s control limit, which is a good indication that there is a problem with the method.  Stop, reject the run, trouble-shoot the method, fix the cause of the problem, then restart the method and reanalyze the patient specimens.
• Day 6.  The value for Control 2 exceeds a +2s control limit, but doesn't exceed a 3s limit.  There might be a problem, but this might also be a false rejection.  If a 1_2s rule were strictly applied, the run would be rejected.  However, because the value for Control 1 is okay, it is likely that this is a false rejection.  Accept the run.
• Day 8.  Both the  values for Control 1 and Control 2 exceed their respective +2s control limits. It is rare to see two values in a row exceed the same +2s limit, therefore this occurrence indicates a problem with the method.  Note that this interpretation applies the 2_2s control rule, i.e., 2 values in a row exceeding the same control limit.  Since both controls are out in the same direction, it is likely there is a systematic error (or problem with the accuracy of the method).  Stop, reject the run, trouble-shoot the method, fix the cause of the problem, then restart the method and reanalyze the patient specimens.
• Day 11.  Both control values exceed 2s control limits, but one is positive and one is negative.  It is a rare occurrence and most likely there is a problem with the method.  Since the two controls are out in opposite directions, it is likely that there is a random error (or problem with the precision of the method).  Note that this interpretation applies the R_4s rule, i.e., the range of the control values exceeds 4s. Stop, reject the run, trouble- shoot the method, fix the cause of the problem, then restart the method and reanalyze the patient specimens.
• Day 13.  The value for Control 2 is outside the low end of the 2s range.  There is a warning of a possible problem, but this might also be a false rejection.  Accept this run because none of the rejection rules are violated.
• Day 14.  The value for Control 2 is again outside the low end of the 2s range.  This makes 2 days or 2 runs in a row, which is unusual. Since both values for Control 2 are out in the same direction, it is likely there is a systematic error (or problem with the accuracy of the method).  Stop, reject the run, trouble-shoot the method, fix the cause of the problem, then restart the method and reanalyze the patient specimens.
• Day 17.  Control 1 exceeds the +2s control limit.  There might be a problem, but this might also be a false rejection.  If a 1_2s rule were strictly applied, the run would be rejected.  However, because the value for Control 2 is okay, it is likely that this is a false rejection.  Accept the run.
• Day 25.  Control 1 exceeds the -2s control limit. There might be a  problem, but this might also be a false rejection.  If a 1_2s rule were strictly applied, the run would be rejected.  However, because the value for Control 2 is okay, it is likely that this is a false rejection. Accept the run.
• Day 27.  Control 1 exceeds the -2s control limit. There might be a  problem, but this might also be a false rejection.  If a 1_2s rule were strickly applied, the run would be rejected.  However, because the value for Control 2 is okay, it is likely that this is a false rejection. Accept the run.