Control line

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Control Line
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Specialty	Laboratory Medicine
Synonyms
Pronunciation	Phonetic spelling or audio file
Definition	Definition of the medical term
Causes	Common causes or associated conditions
Diagnosis	Methods for identifying the term in clinical practice
Treatment	Treatment or management options
Related terms	Other related medical terms
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A control line is a critical component in various diagnostic tests, particularly in lateral flow assays (LFAs), which are commonly used for rapid testing in medical diagnostics. The control line serves as an internal check to ensure that the test is functioning correctly.

Function

In a typical lateral flow test, such as a pregnancy test or a COVID-19 rapid antigen test, the control line appears on the test strip to confirm that the test has been performed correctly. It indicates that the sample has flowed through the test strip as intended and that the reagents are working properly.

The control line is usually located downstream of the test line, which is the line that indicates the presence or absence of the target analyte. The appearance of the control line is independent of the test result; it should appear whether the test result is positive or negative.

Mechanism

The control line typically contains antibodies or antigens that bind to a labeled conjugate present in the sample pad. When the sample is applied, it migrates along the strip by capillary action. As the sample passes over the control line, the labeled conjugate binds to the immobilized antibodies or antigens, resulting in a visible line.

The presence of the control line confirms that the sample has migrated properly and that the reagents are active. If the control line does not appear, the test is considered invalid, and the results cannot be interpreted.

Importance

The control line is essential for the reliability and accuracy of diagnostic tests. It provides a built-in quality control measure that helps prevent false negatives or false positives due to test malfunction. Without a visible control line, the test results are not trustworthy, and the test should be repeated with a new device.

Applications

Control lines are used in a variety of diagnostic tests, including:

• Pregnancy tests
• Infectious disease tests (e.g., HIV, COVID-19)
• Drug tests
• Allergy tests

Also see

• Lateral flow test
• Diagnostic test
• Quality control
• False positive
• False negative