Medical tests provide crucial information for healthcare decisions. Ensuring the accuracy of these diagnostic results is fundamental to patient care. To achieve this, tests often incorporate built-in checks that confirm their proper function.
What Are Internal Controls?
An internal control is a mechanism integrated directly into a test system to confirm its operational status. These controls are not separate tests, but components within the test itself. They operate using either the sample or a specific reaction inherent to the test’s design, verifying reagent activity and correct procedure.
The “internal” aspect means the control is processed alongside the actual sample, under the same conditions. This ensures that any observed results are due to the sample’s properties and the test’s correct performance. For example, some tests include a substance expected to react in a specific way if the test is working, indicating proper system function.
The Role of a Positive Internal Control in Urine Testing
In urine testing, a positive internal control confirms the test device and its reagents are reacting as expected, and the urine sample is suitable for analysis. Many rapid urine tests, such as dipsticks or immunoassay drug tests, feature a control line that must appear for the test to be considered valid. This line indicates the test strip has absorbed sufficient urine and its chemical reagents are active.
Beyond visual indicators, urine tests use internal controls to verify sample integrity. Parameters like specific gravity and pH are often assessed. A specific gravity within the expected range (1.002 to 1.035) confirms the urine is not overly diluted or concentrated, which could affect other measurements. Similarly, urine pH (usually 4.5 to 8.0) is monitored, indicating the sample’s chemical environment is appropriate for test reactions. When these controls show expected reactions or fall within normal ranges, it assures the test system is functioning correctly for reliable results.
Why Internal Controls Are Essential for Reliable Results
Internal controls are essential for ensuring the accuracy and trustworthiness of any test result. They provide confidence that the test equipment and reagents are performing as designed. Without these built-in checks, it would be difficult to determine if a test result is genuinely accurate.
These controls help guard against misinterpretations from a malfunctioning test. For example, an internal control can prevent a false negative, where a test incorrectly indicates a substance’s absence because the test is not working. They also provide a check against false positives, though their primary function is to confirm basic operational integrity. If the internal control does not perform as expected, the result cannot be relied upon, requiring re-evaluation.
What Happens When Internal Controls Don’t Work?
If an internal control does not show the expected positive result, the test is invalid or inconclusive, meaning the results cannot be trusted. Common reasons for failure include expired test reagents, which may no longer be chemically active. Improper sample collection or storage, such as contamination or extreme temperatures, can also compromise the sample’s integrity, leading to a control failure.
Insufficient sample volume is another frequent cause of an invalid result. Additionally, some test strips may not absorb the sample correctly due to manufacturing defects or improper handling. When an internal control fails, the standard procedure is to retest the sample, often with a new test kit, to obtain a reliable result.