ATS Pulmonary Function Laboratory Manual

chapter  5

4. Thereafter with subsequent testing, the laboratory should use Westgard’s rules to enact quality assurance responses (7). 4.1. When one observation exceeds the mean ±2 SD, a “warning” condition exists. 4.2. When one observation exceeds the mean ±3 SD, an “out of control” condition exists. 4.3. When two consecutive standard observations exceed ±2 SD, an “out of control” condition exists. 4.4. When four consecutive standard observations exceed the mean ±1 SD, in the same direction an “out of control” condition exists. 4.5. When 10 consecutive standard observations fall on the same side of the mean, an “out of control” condition exists. 4.6. If an out of control situation exists, the instrument should not be used for patient testing until it has been evaluated to assure it is good working order. Once this evaluation is complete and the instrument consid- ered to be working properly, repeat testing of the control should be done to verify. 5. A reasonable way to record these measurements is with control charts (Levey Jennings Plot). 5.1. Control charts indicate the mean of the distribution and an upper limit (UL) and lower limit (LL), which can be fixed or vary according to the SD. A common approach is to use the mean ± 2SD to represent UL and LL boundaries (see section 2.12 and 4.1 above). This is often termed the 95% confidence interval (95% CI). An alternative approach is to use a fixed percent of the mean (e.g., 95%–105% or 97%–103%) to describe the UL and LL boundaries. 5.2. Figure 5.1 is an example of a control chart using 2 SDs for the UL and LL of data from a biologic control performing the single-breath carbon monoxide diffusing capacity (Dl CO ) test on one PF device each week.

Figure 5.1  An example of a control chart of data collected on a biologic control performing the Dl CO values collected each week. In this example, the UL is 30.4 units and the LL is 25.9 units. These limits represent ±2 SD. This control chart is set up for a 6-month time window, but yearly windows can also be used. test. The data show Dl CO

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