Single-use disposable tests, such as visually read tests used for pregnancy or drug testing, often contain a built-in control. These controls are sometimes referred to as process or procedural controls. The following tips will help you understand how these controls function and their limitations.
- Built-in controls may not monitor the entire test system. There is significant variability among different devices in terms of how procedural controls work and the functions they serve. It is recommended that you read the package insert carefully, and pay particular attention to the description of the chemical or immunological reaction that takes place at the control line in the device being used. Use this information to determine what aspects of the test are monitored by the control and what variables are not monitored.
- Built-in controls are sometimes not a good indicator of the viability of test reagents. This is because the reactions occurring at control lines may be more robust than reactions occurring at the test line. For some immunochromatographic tests, the presence of a control line merely indicates that an adequate volume of sample was added and that the membrane strip is intact.
- You should select external control materials that have concentrations of the targeted analyte that are near to the cutoff concentration or medical decision level of the test. This will optimize your ability to detect early stages of reagent degradation.
- External controls are often the only way to monitor the entire test procedure, including pre-analytical steps, operator technique, procedural steps, and reagent viability.
- If the control line does not appear within the read time identified in the package insert, or if the appearance of the line or the background varies from the description provided by the manufacturer, e.g., the line is speckled instead of solid, you should not report patient results.
- When determining when and how often external controls should be run all laboratories should follow local, state, and federal regulatory requirements. Decisions about control frequency and type should be made in the context of the particular operating environment for the laboratory taking into account operator proficiency, the stability of the environment, the volume of testing, and other relevant factors.