A normal, or negative, Mantoux test result is generally indicated by an induration (a raised, hard area) of less than 10 millimeters (mm) at the injection site. This interpretation is based on the traditional cutoff for a positive result.
Understanding the Mantoux Test
The Mantoux test, also known as the PPD (Purified Protein Derivative) skin test, is a common method used to screen for tuberculosis (TB) infection. A small amount of PPD is injected just under the skin. The reaction (or lack thereof) at the injection site is then measured, typically 48 to 72 hours later, to determine if a person has been exposed to the TB bacteria. The measurement focuses on the diameter of the induration, not the redness.
Defining "Normal" (Negative) Test Results
According to the provided reference, the Mantoux test is considered positive if the induration measures 10 mm or more. Therefore, a "normal" or negative result signifies that the induration is below this threshold, suggesting no significant reaction to the PPD and typically indicating that the individual has not been exposed to TB or does not have a latent TB infection detectable by this test at the given threshold.
It's crucial to note that this specific 10 mm cutoff was historically recommended when using a 1 TU PPD RT23 (Tuberculin Units of Purified Protein Derivative, RT23 strain).
Impact of PPD Strengths on Interpretation
While the 10 mm cutoff was established for 1 TU PPD RT23, current laboratory practices often involve different strengths of PPD, which can influence interpretation.
- 1 TU PPD RT23: This strength was the basis for the 10 mm positive cutoff. A result of less than 10 mm would be considered normal/negative.
- 5 TU PPD (RT23 equivalent): Laboratories currently use this strength more often.
- Other Higher Strengths or Types of PPD: Sometimes, other stronger PPDs may be used.
The reference states that the 10 mm or more induration is considered positive, specifically for the 1 TU PPD RT23. For tests conducted with 5 TU PPD or other strengths, while the reference does not provide different cutoffs, clinical guidelines often adjust interpretation thresholds based on the patient's risk factors and the specific PPD strength used. However, strictly based on the information provided, a result of less than 10 mm is what constitutes "normal" or negative, relative to the 10 mm positive threshold defined for the 1 TU PPD RT23.
Interpreting Mantoux Test Results
To summarize the interpretation based on the given reference:
| Induration Size (mm) | Interpretation (Based on 1 TU PPD RT23) |
|---|---|
| Less than 10 mm | Normal (Negative) |
| 10 mm or more | Positive |
It is important to remember that these interpretations provide a general guideline. Healthcare professionals consider various factors, including the type of PPD used, individual risk factors, and clinical history, for a complete assessment of the Mantoux test result.
