Whether using assayed controls or unassayed controls, acceptable ranges for each lot of control materials will need to be calculated. Sufficient data points must be collected to guarantee that the calculations are valid and provide assurance that patient test results are as accurate as possible.
For the data to reflect future system performance, the measurements must be made under conditions representative of the normal variation encountered during daily testing.
Possible variations in testing include:
- Changing calibrator lots
- Performing calibrations
- Changing reagent lots
- Changing bottles of control material
- Changing operators
- Data collected at different times of the day
- Changes in sample probes or pipettes
- Post-maintenance changes
Important: Data should
not be collected during abnormal or out-of-control conditions.
All valid data points should be used in calculations or the control limits may become too restrictive. This would not reflect the actual variability of the test system. Including quality control data from afternoon and night shifts makes sure you cover all results and all employees in your quality control program.
- Quality control results that exceed + 2 standard deviations (SD) should not be deleted when you are establishing QC limits as this action can distort or skew the limits.
- However, if more than two data points appear to be significantly higher or lower than the rest of the data, this could be an indication of a problem. In this case, you should resolve the problem and recollect the data.
Using control data from the previous month can help you predict what may occur this month, thus helping with calculations.
Using target values suggested by the manufacturer provides an example of the likely values that will be encountered.
