SELECTION OF QC PROCEDURE

SELECTION OF QC PROCEDURE

Levy-Jennings Control Chart

Control charts were first introduced into the clinical chemistry laboratory by Levy and Jennings in 1950. Here single control values are plotted directly. To use a Levey-Jennings control chart, follow these steps:

1.      Analyze control sample at least 20 different days. Calculate the mean and SD for those results.

2.   Construct a control chart either manually on graph paper or using computer. In the y-axis put control values along with mean ± 4SD which covers about 95% or 99.7% of the measurements. Draw horizontal lines for the mean and upper and lower control limits. Set the control limits as the mean ± 3SD when the number of control observations, n is 2 or greater. When n is 1, control limits may be set as mean ± 2SD. Label x axis in terms of time, using day, run number, control observations number.

3.     Introduce control specimen into each analytical run, record the values and plot each value on the control chart.

4.      When the control value fall within the control limits (±3s), interpret the run as being in control and report the patient results. When a single control value exceeds the control limits, stop the method; do not report patient results. Inspect method to determine the cause for the errors. Resolve the problem, then repeat the entire run specimen and control samples.

In practice 1_2s rule has been used for rejection of test when n = 1. Whenever control value exceeds 2s limit repetitive measurement of control and patient sample should be done. When a second or repeated control value is outside 2s control limit, there must be true rejection and problem solving procedure should be started.

Westgard Multirule chart

The Multirule procedure developed by Westgard and associates uses multiple control rules for interpreting control data. The procedures require a chart having lines for control limits drawn at the mean ± 1s, 2s and 3s. This chart drawing is similar to levey-Jennings chart.

      

      The following control rules are used. 

  

Rule	Description
12s	One control observation exceeding the mean ± 2s Warning sign, control data should be tested by other control rules, do not report the result
13s	One control observation exceeding the mean ± 3s Rejection, due to random error
22s	Two consecutive control data exceeding  same mean  plus 2s or mean minus 2s Rejection, systematic error
R4s	One observation exceeding mean plus 2s and another exceeding mean minus 2s Rejection, random error. Note: this rule applies only within a run not between runs.
41s	Four consecutive observations exceeding mean plus 1s or mean minus 1s Rejection, systematic error
10x	Ten consecutive control data falling on one side of mean (above or below) Rejection, systematic error