Tuberculin Skin Test (also known asMantoux Test, PPD Test or Mendel-Mantoux Test or Pirquet test) is a screening test done on the skin for diagnosis of Tuberculosis (TB) using Purified Protein Derivative (PPD). A Positive Tuberculin skin test is an example of type 4 hypersensitivity reaction (A delayed hypersensitivity Skin reaction) that is, it is cell mediated immunity; which is based on the immune response of the body to the Purified Protein Derivative (PPD). A tuberculin skin test can only be done about 2 to 8 weeks from the time of infection to when it can be detected using the tuberculin skin test.
Mantoux test Procedure (Tuberculin Skin Testing Procedure)
- Inject 0.1ml of 5 units of purified protein derivative (P.P.D.) intradermally (intracutaneously) and the reaction is read and interpreted after 24 hrs to 72 hrs.
- Circle the site and patient should not touch nor wash the area
Mantoux Test Results in low risk individuals
- Reading of 0 4 mm shows it is a negative result, this means the diagnosis is unlikely to be diagnose.
- Reading of 5 9mm shows it the result is indeterminate
- A reading of 10 14 mm is suggestive that the affected person may be positive for Tuberculosis
- A reading of more than 15 mm is positive of TB
Mantoux Test Reading and Interpretation
Interpretation of the Tuberculin skin test must be done with care because you may interpret positive for negative and vice versa. The Difference between positive and negative TB skin test can only be known by taking into accounts the medical history, current illnesses and past illnesses of the individual.
Mantoux test interpretation of Positive result of 5mm or more
A patient is said to be positive when the skin reaction is 5mm or more in immunocompromised patients such as in:
- HIV/AIDS positive person
- Anyone who has had recent contact with someone with active tuberculosis
- Anyone whose Chest X-ray findings suggest previous TB infections such as pulmonary nodules
- Patients on immunosuppressants or on cytotoxic drugs such as those with cancer or those who have had organ transplant.
- Prolong or chronic use of systemic corticosteroids lasting for more than 6 weeks or those on a dose of prednisone of more than or equal to 15 mg/day or equivalent.
- Patients having end stage renal disease
Conditions for Positive Mantoux Test at 10mm or more
A person is said to be positive when the Tuberculin skin test is 10mm or more under the following conditions:
- Laboratory scientist working in the lab for tuberculosis specimens
- Intravenous drug users
- When a patient had recently travelled (less than 5 years) to a country with high prevalence of tuberculosis (less than five years)
- Working or living in congested places such as prisons or nursing homes
- Presence of associated medical conditions that place patient at high risk of having Tuberculosis due to immune suppression such as in diabetes, prolonged use of steroids, leukemia, chronic malabsorption syndromes, end-stage renal disease, low body weight, protein energy malnutrition such as in Kwashiorkor and Marasmus
- Infants, children, or adolescents exposed to anyone at high-risk of having TB
Conditions for Positive Tuberculin Test (Mantoux Test) of more than 15 millimeters or more
- Low risk individuals : a skin reaction of 15 mm is positive in anyone including those with no known risk of tuberculosis
Reasons for False Positive Mantoux Test result
There may be some people who may show a skin reaction that may seem to be positive but they are not actually positive; such an instance is called a false positive result (that is, a positive result that is actually false). These conditions include:
- Any Infection with different species of mycobacteria that is not Mycobacterium tuberculosis (that is, a non tuberculous mycobacterium)
- Previous BCG vaccination
- Error due to administration of Purified Protein Derivative during the Tuberculin Skin Test Procedure
- Error in interpreting the Mantoux test result
- Error due to incorrect bottle of antigen used
How do you know if a TB skin test is Positive?
A TB skin test is positive when the area of skin reaction is measured with a tape or ruler and is found to be more than 15 mm in diameter. In some illnesses such as Diabetes, a reading of 10 mm can be said to be positive.
Reasons for False Negative Tuberculin Skin Test for TB
Sometimes even when someone has tuberculosis, the Mantoux test may not be positive. The Mantoux test works based on the ability of the body immune system to mount an immunologic respond to the purified protein derivative. When the body immune system is intact and someone is infected with Tuberculosis, it will mount a response and a skin reaction greater 15mm is seen after the tuberculin test; when the bodys immune system is down, it means the body cannot mount an attack and less or no skin reaction is seen. There are various factors that can reduce the immune function of the body and may give a false TB result even when someone has TB and are mentioned below.
Factors that can affect Mantoux test and give false result even when patient is actually positive
- Malnutrition: this affects the bodys function and the immune system cannot trigger skin reaction
- Severe illnesses such as HIV, Tuberculosis, and Measles: this is similar to malnutrition.
- Immunocompromised patients such as in Diabetes mellitus, long use of steroids, HIV and others
- Young age such as in infants less than 6 months old
- Recent live-virus vaccination such as measles or smallpox vaccinations
- Error due to the method of Tuberculin skin test administration
- Incorrect interpretation of mantoux skin test reaction
- Old age
- Insufficient dose of PPD and inadvertent subcutaneous injection
Matoux Test Side effects
- Swelling and redness of injected site especially in those who have had TB
- Regional lymphangitis
- Regional adenitis
These tuberculin skin test side effects are rare.