#Negative tb test skin#

Therefore a person who has received BCG vaccine and tests positive for Tb skin test is considered to have latent Tuberculosis.

According to US department of health, BCG vaccine does not give immunity against military tuberculosis and tuberculous meningitis.

Persons who have received BCG vaccine previously show a false-positive result for several years.

In the above mentioned conditions, the reading should be considered as negative unless proved otherwise from some other tests to diagnose Tb.

Hypersensitivity or an allergic reaction to Tuberculin.

Touching the skin over injected area causing itching and swelling is also a possible cause for a false-positive result.

Fault in the administration of Tuberculin.

Infection with a non-tuberculosis causing Mycobacterium.

False positive results for Tb skin test can be seen in Once a person has been tested positive for Tuberculosis, he/she may have to undergo further investigations to have a confirmatory diagnosis of Tuberculosis. A negative skin test result on the other hand can be divided into false-positive and false-negative for the purpose of understanding False Positive Test ResultsĪ false positive test result means a Tb skin test appears positive even though the person is not infected with Mycobacterium Tuberculosis. Previous history of receiving BCG vaccine.Īn induration of more than 15 mm is considered positive even in persons with no known risk factors for Tuberculosis. Persons who have had gastrectomy or jejuna bypass done. Persons who have diabetes, silicosis or malignancies Persons who are on suppressive therapy for about one month or more. Infants, children and adolescents exposed to high-risk patients Lab personnel coming in contact with tubercle bacillus People working at high-risk areas (hospitals, shelters, nursing homes, etc.)

Recent arrival from a country which has a high prevalence of Tb

Persons who are on suppressive therapy for one month or more. Persons who have had organ transplant done. Patients who are receiving long-term corticosteroid therapy Difference Between Positive and Negative Tb Skin TestĪ positive skin test can be seen and interpreted according to diameter of induration. Induration of skin less than 5 mm is considered negative for Tuberculosis. What is a Negative Tb Skin Test?Ī negative Tb skin test means that either a person is not infected with the bacillus or the infection is too recent and not too much time has elapsed for the body to react to a Mantoux test. What is a Positive Tb Skin Test?Īn induration (swelling) of more than 5 mm is considered to be positive under certain circumstances. A table later in the article will help you understand the difference between a positive and negative Tb skin test.

In elderly individuals, however, results may take more than 72 hours to appear.Īnalyzing the difference between a positive and negative Tb skin test is quite difficult for a lay-person.

Redness or eythema is not considered as a parameter to diagnose Tb using a Tuberculin test.

The reaction is estimated by measuring the diameter of induration in millimeters.

The result of this test is observed between 48-72 hours.A standard dose of 5 units of Tuberculin is injected intradermally (into the skin) over the arm right below the elbow in a palm-facing-above position.The Tuberculin test kit includes a vial, a disposable syringe for administering the dose and a ruler to measure induration is additionally used.The procedure for performing a Tuberculin or Mantoux test is It is performed not only to diagnose Tb but also for the screening of people who frequently come in contact with Tb patients such as health care workers, residents of a location where Tb is rampant or foreign-born people from countries who have a high incidence of Tb. A Tb skin test is also called the Mantoux test or a Tuberculin test.

Negative tb test