A tuberculosis (TB) skin test, also known as the Mantoux test or PPD test, is a crucial diagnostic tool for identifying TB infection. Understanding the difference between a positive and negative test result is essential for proper diagnosis and treatment. This comprehensive guide will help you understand what to look for and what different test reactions mean.
How TB Skin Tests Work
The TB skin test involves injecting a small amount of purified protein derivative (PPD) under the skin of your forearm. Your healthcare provider will then evaluate your body's reaction to this substance after 48-72 hours by examining and measuring any bump (induration) that forms at the injection site.
Identifying a Positive TB Test Result
A positive TB test result is characterized by a raised, firm bump (induration) at the injection site. The size of this bump is crucial for interpretation, as different measurements indicate positive results for different risk groups.
Measuring the TB Test Bump
- They measure the induration (firm, raised area) width in millimeters
- Only the firm, hardened area is measured, not the surrounding redness
- Measurements are taken across the forearm, perpendicular to the long axis
Positive Result Criteria
- 5mm or larger: HIV-positive individuals, recent TB contacts, organ transplant recipients
- 10mm or larger: Healthcare workers, recent immigrants, injection drug users
- 15mm or larger: Persons with no known risk factors
Understanding a Negative TB Test Result
- No raised area or bump
- Possibly slight redness or irritation
- No firm or hard area when touching the site
The Role of Redness in TB Test Interpretation
It's important to understand that redness (erythema) around the test site is not used to determine whether the test is positive or negative. Only the firm, raised area (induration) is measured and considered in the interpretation of results.
Next Steps After Your TB Test
- For negative results: Usually no further action is needed unless you have ongoing risk factors
- For positive results: Additional testing, including chest X-rays and possible sputum tests, will be required to determine if you have active TB disease
Frequently Asked Questions
What is the difference between a positive and negative TB test bump?
A positive TB test shows a firm, measurable raised area (induration), while a negative test shows little to no raised area at the injection site.
How is the size of the TB test bump measured and what sizes indicate a positive result?
The bump is measured across the width of the forearm, considering only the firm, raised area. Positive results are determined by specific size thresholds: 5mm, 10mm, or 15mm, depending on individual risk factors.
Why does redness around the TB test site not affect the test result?
Redness is not considered because it can occur from simple skin irritation and doesn't reliably indicate an immune response to TB. Only the firm, raised area (induration) is a reliable indicator of TB infection.
What do different TB test bump sizes mean based on individual risk factors?
Different size thresholds indicate positive results based on risk: 5mm or larger for high-risk individuals (HIV+, recent contacts), 10mm for medium-risk (healthcare workers, recent immigrants), and 15mm for those with no risk factors.
What are the next steps after receiving a positive TB skin test result?
After a positive result, you'll need additional testing, including chest X-rays and possibly sputum tests, to determine if you have active TB disease. Your healthcare provider will develop an appropriate treatment plan based on these results.
