What is the upper control limit and lower control limit in testing?

In testing and quality control, the Upper Control Limit (UCL) and Lower Control Limit (LCL) are statistical limits that define the range of acceptable variation in a process. These limits are commonly used in control charts to monitor and assess the stability and performance of a process over time.

The Upper Control Limit (UCL) is the highest value that a process is allowed to achieve without being considered out of control. It helps identify if there is any significant increase in variation or a potential issue that needs investigation.

The Lower Control Limit (LCL) is the lowest value that a process is allowed to achieve without being considered out of control. It helps identify if there is any significant decrease in variation or if the process is not meeting the desired standards.

Here’s an example to illustrate the concept of Upper Control Limit (UCL) and Lower Control Limit (LCL) in a medical laboratory setting :

Example: Haemoglobin Testing

Let’s consider a medical laboratory that performs haemoglobin testing using a particular method. The laboratory sets up a control chart to monitor the accuracy and precision of this test over time.

1. Upper Control Limit (UCL): Suppose the laboratory establishes an Upper Control Limit of 16.0 g/dL for haemoglobin testing. This means that if any test result exceeds 16.0 g/dL, it would be considered out of control, and the laboratory would investigate the reason for the higher value.
2. Lower Control Limit (LCL): For this example, the laboratory sets a Lower Control Limit of 12.0 g/dL for haemoglobin testing. If any test result falls below 12.0 g/dL, it would be considered out of control, and further investigation would be required.
3. During routine testing, the laboratory will continuously plot new test results on the control chart. As long as the test results remain within the UCL and LCL boundaries, the process is considered stable and in control. If any test result falls outside these limits, it signals a potential issue with the test method, instrument, or process, and the laboratory would take corrective actions to investigate and resolve the problem.
4. Control charts help laboratories ensure the accuracy and reliability of their test results by providing a visual representation of process variation over time and enabling timely detection of potential issues.

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About the author

Dr. Sambhu Chakraborty is a distinguished consultant in quality accreditation for laboratories and hospitals. With a leadership portfolio that includes directorial roles in two laboratory organizations and a consulting firm, as well as chairmanship in a prominent laboratory organization, Dr. Chakraborty is a respected voice in the field. For further engagement or inquiries, Dr. Chakraborty can be contacted through email at director@iaqmconsultants.com and info@sambhuchakraborty.com. Additional resourcesand contact information are available on his websites, https://www.quality-pathshala.com and https://www.sambhuchakraborty.com, or via WhatsApp at +919830051583