Understanding Lab Tests for False Positive Results
Written by Private MD Labs Medical Writing Team • Last Updated: September 8, 2025
A false positive result occurs when a blood test incorrectly indicates the presence of a condition, infection, or antibody when it is actually absent. While blood tests are highly accurate diagnostic tools, false positives can occasionally occur due to various factors including cross-reactivity, autoimmune conditions, vaccinations, or technical issues.
Top Recommended Test
HIV 1/2 Antigen and Antibodies, Fourth Generation
Occasionally, this test may yield a false positive result due to the presence of conditions that cause the immune system to produce antibodies that cross-react with HIV antigens, such as autoimmune diseases. While rare, this underscores the critical need for confirmatory testing.
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Why False Positive Results Occur
False positive results can happen when the immune system produces antibodies that cross-react with test components, when previous vaccinations interfere with test interpretation, or when certain medical conditions affect the body's immune response. Understanding which tests are more susceptible to false positives helps healthcare providers make informed decisions about confirmatory testing.
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Hep B Core Antibody, IgM
In rare cases, this test can result in a false positive, indicating a Hepatitis B infection when there is none. This can occur due to cross-reactivity with other viral infections or due to technical issues with the test, making confirmatory testing important for accurate diagnosis.
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Cytomegalovirus (CMV) Antibodies, IgM, Quantitative
The presence of CMV IgM antibodies can sometimes be falsely positive, particularly in cases of autoimmune diseases or during pregnancy. This means the test may show the presence of antibodies even though there is no current CMV infection, requiring additional testing for confirmation.
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A positive result can occur in individuals who have been vaccinated with the BCG vaccine or who have been infected with non-tuberculosis mycobacteria. This can lead to false positive results for tuberculosis infection, necessitating clinical correlation and additional testing.
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The Importance of Confirmatory Testing
When a test result is unexpectedly positive or doesn't align with clinical symptoms, healthcare providers typically recommend confirmatory testing using different methodologies. This approach helps distinguish between true positive results and false positives, ensuring accurate diagnosis and appropriate treatment decisions.
Important Considerations
The information here is for educational purposes only and is not a substitute for professional medical advice. If you receive an unexpected positive test result, consult with a healthcare provider who can interpret the results in the context of your medical history and symptoms, and recommend appropriate follow-up testing if needed.
