Tuberculosis (TB) is an infectious disease that is caused by mycobacterium tuberculosis. It affects the lungs mainly but can cause lesions in other body systems. What is the meaning of test positive for tuberculosis? Does it mean to have the infection or are there false-positive TB test?
Manifestations of tuberculosis
Infection doesn’t always mean to have the disease… What??
Yes, there are two patterns of tuberculosis after infection:
The patient is infected already, but he has no manifestations of tuberculosis.
On the contrary to the above type, here the patient shows evident symptoms and signs of tuberculosis disease.
Diagnosis of tuberculosis
It needs complete checkup investigations in order to verify its diagnosis and the organs affected.
- TB test in the skin or the blood.
- Chest X-ray.
- Sputum examination.
- Investigations for other body systems.
Test positive for tuberculosis
It means a positive test for infection with tuberculosis bacteria. The test has two types:
- TB skin test.
- TB blood test.
Physicians recommend using both the skin and blood tests.
Test positive for tuberculosis means that the person has the infection but not necessarily mean to have the disease. In addition, it may indicate a latent (non-active) infection.
This means that a test positive for tuberculosis needs further workup investigations to evaluate the state of infection.
Mantoux tuberculin skin test is the tuberculosis skin test. In involves injecting a tuberculin fluid inside the skin layer.
The physician will ask the person to return 48-72 hours after this injection in order to read the test.
He will estimate the size of the swelling as well as its firmness.
If the physician wants to repeat the test, it will be better to use the other arm.
This test gives the best result in children less than 5 years old.
Interpretation of the tuberculin test
In general, if the size of the firm skin elevation at the injection site is less than 5 mm, it is a negative result.
The size of 15 mm or more is a positive test.
10 mm size swelling means having the infection in the following cases:
- Age less than 4 years.
- If the individual is living in a high-risk area.
- If the person has just traveled from an area of a high number of tuberculosis cases.
- A patient working in a health care facility who is in contact with patients or in a laboratory.
It gives accurate results even in people who received BCG vaccine.
False-positive TB test
It means having a positive result for the tuberculin skin test while the patient is not actually infected with the bacteria
This false result commonly occurs in the following cases:
- Previous vaccination by BCG vaccine for TB.
- The improper procedure of the test.
- Wrong reading and interpretation of the test.
False-negative TB test
It means having a negative tuberculin test while there is an infection. It can happen in patients who had organ transplantation surgery.